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Author Topic: My very own Minecart Education Thread. Ten Lessons, now complete.  (Read 15261 times)

Larix

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My very own Minecart Education Thread. Ten Lessons, now complete.
« on: September 30, 2014, 06:54:20 pm »

Oh, i know. Dwarfs are moody drunks, elves are stupid hippies and Friendship is Magma. Why bother experimenting with stuff trying to find how it works when you can murder a dozen dwarfs, set fire to their remains and get a sound round of applause for your ‼SCIENCE‼ ?

Still, i think i found out some stuff that's worth sharing. So let me tell you about trees!

No?

Come on, trees are awesome!

...

Alright, then i'll tell you about minecarts.

Contents:

This post: Lesson One: Track on flat floor, Lesson Two: Ramps, basics

Lesson Three: rollers and guided carts
Lesson Four: flight


Lesson Five: diagonal movement and how to fix it
Lesson Six: False ramps


Lesson Seven: Pathing across levels
Lesson Eight: Meet the checkpoint bug


Lesson Nine: Practical applications of the checkpoint bug
Lesson Ten: Understanding corner ramps


The wiki article contains a lot of varied information, but i've delved quite a bit into minecart pathing; i.e. where a minecart goes when you let it run free, how its paths change and so on. There's almost no in-depth information on all this stuff on the wiki and it seems to me that much of it isn't well understood.

I encourage all readers to replicate my designs and experiments and offer corrections or alternative interpretations. In order to properly trace what's going on, you will need to look at events closely and that means (unless you have an infallible hack script to do it for you) you'll need to pause the game, advance by single steps and count the steps exactly. You can't just eyeball the speed as "pretty fast" or "sort of sluggish", you'll have to e.g. count out a hundred steps and look how far a cart travels in that time, so you can definitely tell whether a cart moves at 45000 or 55000 speed.

Let's start simple.

Lesson One: Track on flat floor

In short, the only type of track that matters to a free-running cart are corners. All other types are irrelevant.

A sweeping statement, sure, but i found it to be perfectly true. The basic rule is that a minecart will move in a straight line. The only exception is when it encounters a track corner (the two-connection type, not T-junctions) that's connected to the direction the cart is coming from. Let's take an example:

Code: [Select]
a═══════╗   b═════╝
Cart gets pushed east from a/b, moves east until the corner and turns south/north there.


Code: [Select]
a║╚╔╣╩╦╠╗    b╬╥╨╞╡╝
Cart gets pushed and behaves exactly the same as above.

The cart incidentally also behaves like that when the route before the corner is entirely non-tracked floor, it'll path just the same, it will only slow down more thanks to higher friction.

Why is this so? As far as i can tell, the game doesn't calculate any sort of "heading" for the cart, it just keeps track of the velocity, probably split between x- and y- axis. When the cart moves over flat floor, all that'll happen on "direction-neutral" track is deceleration. When the cart is re-pathed by a legal corner, the whole input speed is taken and turned into speed in the exit direction of the corner.

"Straight" tracks have no pathing power over minecarts, they don't keep them "on track", because carts don't consider themselves "on track" in the first place. They're in contact with the floor and react to corners/constructions, or they're in flight and don't. That's it.


Lesson Two: Ramps, basics

Of course, everyone who works with carts for a while will probably get to love ramps. They allow carts to climb levels, they can provide speed, they even allow perpetual motion.

First of all, what makes a ramp tick?

A ramp is only fully traversible for carts and can only provide acceleration if it's properly connected by track. There are two arguments that get checked, and they concern track connections and nothing else. It doesn't matter where the cart comes from, whether it changes level or not, it's all about how the ramp is built.

* Requirement one: the ramp must have track connection to wall. One, two or three connections are all acceptable.

* Requirement two: the ramp must have exactly one track connection to a non-wall tile. This can be an adjacent ramp on the same level, flat floor or a hole (e.g. containing a down ramp).

Examples of functional track ramps

Code: [Select]
#    #    #    #     #     #
║    ║    ║    ╚+   #╩+   #╬#
+    ▲    ▼                +
All shown track engraved on up ramp.
▲ - up ramp
▼ - down ramp (i.e. hole containing a ramp)
+ - flat floor

Examples of non-functional track ramps

Code: [Select]
###   ###   ##
+═+   +╩+   #╝

First example: no connection to wall
second: more than one connection to floor
third: no connection to floor

When a ramp is properly connected, it provides acceleration towards its "down" direction; ~5000 speed units for every step a cart moves across it. Ten steps of acceleration give as much speed as a highest-speed roller, but you'll need multiple ramps for that.

The important part is that the game only checks if the ramp is properly connected, it doesn't check where the cart's coming from. This is the foundation of the fabled impulse ramp - a cart entering this ramp

Code: [Select]
###
+╚+

from the west will be accelerated towards the east, the same as a cart going down a level down such a ramp:

Code: [Select]
#═+
(cart's coming from track or a ramp on the level above). Impulse ramps thus grant speed without needing to sacrifice height, no more. They do not provide more or a different acceleration, just the exact same amount (which is quite a lot considering it means perpetual motion at practically any speed up to ~250.000 that you desire).

If a cart moves onto a ramp _from_ the ramp's down direction, it'll be accelerated in the direction it was coming from, i.e. it decelerates (at the normal ramp rate). Excepting a rather powerful bug that'll come up later, this deceleration will stop carts that are moving at the speed of a medium-speed roller or less before they reach a ramp's top, whereupon they'll roll back down from the place they've reached. The resulting speed when leaving the ramp again will be less than the speed the cart entered with, a cart bouncing between two ramps separated by one tile of level floor
Code: [Select]
#####       #####       #####
#▲═▲#       #═══#       #╚═╝#
ramps     track variants
will after about a dozen bounces stop on the flat middle tile. There's no observable difference between the two ramp layouts.


I built a fifteen-level straight ramp slope to measure the speeds different numbers of ramps will give to a cart. While the speeds found were just as expected and only of minor practical use (as reference to "generate" carts of specific speeds), the experiment provided a few valuable pointers, stuff that has been worked out by others before but doesn't seem to be widely known:

- a dropped cart (i always dropped it off a hatch) will land in the middle of the tile below and only roll down _half_ the ramp it lands on.
- the speed rises with the number of turns the cart spends on the ramps (just under 5000 speed for every turn, ~130 000 for a cart sent down a fifteen-level ramp), but one turn is subtracted from the count; i.e. the cart is charged ~5000 speed for leaving the ramps in the end.
- the "length" of a ramp is bigger than that of a flat tile. Since i only had full steps to calculate with, the numbers aren't super-precise, but it appears to be sqrt 2  times the length of a flat tile (the "lost" acceleration step mentioned above actually is needed for the length calculation to best fit the results).



Enough for now, more to follow.
« Last Edit: December 26, 2014, 02:31:46 am by Larix »
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gunpowdertea

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Re: My very own Minecart Education Thread
« Reply #1 on: October 01, 2014, 03:24:06 am »

That is an amazing overview! I have only started playing with mine carts, they take a long time to set up to do the stuff I am interested in, so I never got that far. Thanks for putting that list together.

Also: The results re: length of a ramp mean that tiles are cubes (as high as wide). Don't know if it has been suggested elsewhere (I believe it has...) but more evidence to support it is always welcome.
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Larix

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Re: My very own Minecart Education Thread
« Reply #2 on: October 01, 2014, 08:51:08 am »

As a sum of lesson two, i'd offer:

Ramps' main parameter is the direction they accelerate to. The exact track engraved on them and where a cart enters the ramp is secondary. If there's a corner engraved on the ramp and the cart actually moves following that corner, the corner is respected (and things get weird), otherwise exact track is just as irrelevant as on level floor.

Lesson Three: rollers and guided carts

Rollers are the powered means of providing speed to a cart. As has been widely observed

* for practical purposes, it's easiest to assume that rollers simply set the cart's speed to a fixed value.
Spoiler (click to show/hide)
* rollers will not slow down a cart moving faster than the roller's set speed; they will "brake" and turn around a cart moving in the opposite direction
* rollers working laterally to a cart's current movement direction result in diagonal movement
* rollers only affect free-running carts (not guided carts) and only when they are on top of track. Rollers on ordinary floor are ignored.
* rollers which are not powered are completely ignored, you just get the effects of the tile underneath.

The "braking" power of rollers is impressive: each step spent on an opposing roller slows down a cart by 100 000 speed units. The strongest track stops slow a cart by 50.000 per step. You need a very speedy cart to keep moving past a single-tile roller. A cart moving more than one tile per step will, however, not be affected by the friction of tiles it "skips" over during its turns; only the tiles it's in at the end of a step do count. Two rollers (of any speed) in the correct spots are enough to stop and turn around a cart moving at maximum ramp speed (~270.000, that's eight tiles in three steps on average).

Since non-corner track doesn't matter, it also doesn't matter much what kind of track you build a roller on. A west-pushing roller on an E-W track tile works the same way as a west-pushing roller on N-S track, an E "track end" or a NSE t-junction.

Corners do affect the way rollers work, however:

a) a roller pushing "from" a connection of a track corner results in movement towards the corner's exit direction, not towards the roller's push direction.
b) a roller on a track corner pushing from a direction the corner does not connect to pushes to its normal direction (of necessity one of the track corner's connections) and may cause diagonal movement.

a)
Code: [Select]
.║      ║
═╗═    ═╢═
 ║      ║
╢ - Roller pushing east

No matter where the cart comes from, it exits to the south as long as the roller is powered (and the cart isn't super-fast). The most evidently useful application is with a cart coming from the east, because that allows a simple powered switch: when the roller is off, the cart moves off to the west, if the roller is on, the cart goes south.
Still, it's interesting to see that carts coming from the north or south are not thrown into a diagonal, although the roller's nominal push direction is lateral to their movement. It looks like the corner sort of turns the roller's effect around.

b)
Code: [Select]
.║      ║
═╗═    ═╟═
 ║      ║
╟ - Roller pushing west

Results: when the cart comes from the east or west, it moves west.
Carts coming from the north go on a diagonal heading southwest.
Carts coming from the south go on a diagonal heading northwest.

Since the roller's "push from" direction is not in line with the corner, it keeps its "push to" direction and causes the cart coming from the south to ignore the corner track. The latter isn't some kind of cumulative speed derailing, it happens when combining two low-speed effects which even theoretically can't add up to more than 40.000 (50.000+ is the derail threshold). The only rationale i can find for it is that the roller indeed "overrules" the corner when active.

* More likely, however, the laterally-working roller just "adds" its movement speed to the cart's velocity and leaves it to the corner to sort things out. The observation remains valid that a roller pushing "into" a corner is less likely to cause wild diagonal movement even when working laterally, while a roller working towards a corner's exit often causes trouble. On the whole, however, cart motion is most predictable and controllable when working with rollers in line or opposed to cart direction, not with lateral rollers. Corners appear to apply at the end of a turn, after all speed changes on the tile are done with, so in the example above, the "bend the cart to the south" effect of the corner happens after the last "set speed towards east" effect of the roller and the leaving cart goes off with southward-only speed and no eastward component.

A cart encountering a laterally-working roller which does not sit on a corner will generally be thrown off onto a diagonal trajectory. Diagonally-moving carts are great fun, because (Lesson One) only corners matter, so the carts'll merrily barrel all across your carefully laid-out track, smacking into walls and stopping or going places you don't want them (most cases of unexplicably-stopping carts are due to diagonal movement and wall collisions). Unless you manage to thread them through track corners, that is, because a cart properly taking a corner will move precisely in its exit direction and will not retain any diagonal movement component. More on that later.

Late PS: rather confusing results of a recent roller-based device show that rollers indeed accelerate carts, at 100.000 subtiles/stepē in their given direction, but capped at roller's set speed. When a cart moves from roller to roller, this won't matter: since the highest speed that can be imparted by a roller is 50.000, the 100.000 acceleration is enough to neutralise the speed of a cart incoming at max. roller speed and impart max speed, all in a single step. However, if the cart moves at higher speeds, one step of acceleration may only change it from, say, -70.000 to +30.000 and when the cart leaves the roller's tile on the following turn, it will move at the received 30.000 speed, even if it was affected by a highest-speed roller with a set speed of 50.000. In addition, the cart actually calculates the distances it moves on the roller's tile, so the right combination of cart speed and "offset" can result in very irregular speeds. A rather bare-bones test allowed achieving different non-max speeds from a highest-speed roller by slightly varying input speed - from 12.000 to 22.000, both from a "highest" roller.


I'll make guided carts short, because there's not much about them: guided carts ignore special track buildings like rollers or track stops, the pushing dwarf just moves them at their walking speed, much like a wheelbarrow. Track must be "connected" for dwarfs to actually guide a cart. If they find no connection, they'll lug the cart by hand, which ranges from much slower to abysmally slow. "Connectivity" is quite lenient, however - in most cases, a tile only needs one track connection in the correct direction, and bridges are accepted as track, too. It's best to just engrave/build an identifiable unbroken track, though. Guided track can go up/down ramps without trouble, all at the normal dwarven walking speed.

Lesson Four: flight

Carts can be sent over ramps or over the lips of cliffs, and the game will trace a ballistic trajectory. Carts in flight are not subject to air friction (according to hack scripts), but they are subject to gravity. Someone did the calculation, i forget. Anyway, observation tells us that a free-falling cart takes as long to reach the bottom of a shaft as one rolling down a flight of ramps. Thus, the acceleration is the same - corrected for the greater length of ramps (sqrt 2 times length of a flat tile), we get something just under 0,035 zlevels/stepē. (Which shows that dwarven physics are screwy, acceleration on a ramp should be lower than free-fall acceleration.)

A cart released by a hatch takes six steps before it's displayed on the next level down, which suggests - hm, that the cart is considered to start falling about 2/3 up the current level? There's some tricky stuff going on with the decision whether a cart's actually in contact with the floor (and thus subject to corners, rollers and track stops): carts can make small jumps in some cases which don't move them to a different level, and in those cases it seems to take ~those six steps before they start registering as "on floor" again.

A cart pushed off a cliff follows an ordinary downward curve. It keeps its horizontal velocity and will keep moving at the same speed when it lands, while vertical speed will build up during the fall and will completely disappear when it hits the ground.

If a cart is sent over an upward ramp into the open sky, it can go up several levels, depending on its speed. A highest-speed roller will barely manage a hop, the cart won't even reach the level above the ramp, but it'll be in flight for a few steps. A cart accelerated by a long downward slope or an impulse ramp array can go over the ramp at much higher speeds and can reach heights of up to 26 z-levels (or more with added trickery). The "launch ramp" converts the horizontal speed of the incoming cart into ramped-upward velocity, and the upward component will grant height while gravity nibbles away at it.

Counting steps and trying to calculate out the results, my best estimation for ramp launches is as follows:
The baseline is the speed on horizontal track. This speed is converted into speed calculated for ramps. When released, the cart moves vertically at 1/2 the original speed and horizontally at ~70% of the original speed. Assuming this is all ramp stuff, it's likely sqrt 1/2 the original speed horizontally. As per usual, vertical speed disappears upon landing and if the cart is launched off a ramp again, its horizontal speed will be 1/2 the original, vertical speed sqrt 1/8 (ca. 35%) and the height reached will only be about one half of what the first jump achieved (a bit less because of ramping speed costs).

Standard design for a launch ramp:
Code: [Select]
.
____/#

Fast cart comes from the west, goes over the ramp, flight happens. It must be a proper track ramp :P

Carts that fail to enter a hole in the floor "jump" over it, and this also seems to count as flight: speedy carts will not follow a track going down a ramp when coming from level track, and they will ignore corners directly behind the hole because they haven't touched floor again.


Bodycount: 6 dogs (+1 since last update), one mangled dwarf (survived and is fine, but keeps cleaning himself).
« Last Edit: December 26, 2014, 02:30:08 am by Larix »
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Xinael

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Re: My very own Minecart Education Thread
« Reply #3 on: October 01, 2014, 10:41:37 am »

Please try to update the wiki pages or create new ones with your findings, too. This thread may get buried one day, but hopefully the wiki will live on!
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khearn

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Re: My very own Minecart Education Thread
« Reply #4 on: October 01, 2014, 11:21:33 am »

Great information, very clearly stated. Lots of stuff here that I didn't know before. Thanks for all the effort of putting this together!

   Keith
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Th4DwArfY1

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Re: My very own Minecart Education Thread
« Reply #5 on: October 01, 2014, 11:23:12 am »

Nice work. I've been looking to get into minecarts for a while.
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TheHossofMoss

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Re: My very own Minecart Education Thread
« Reply #6 on: October 01, 2014, 12:50:30 pm »

THANK YOU for posting this!
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Baffler

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Re: My very own Minecart Education Thread
« Reply #7 on: October 01, 2014, 01:58:37 pm »

Aye. This could be such a huge thing, but I've never had the heart to use them for anything beyond short-barreled shotguns.
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suokko

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Re: My very own Minecart Education Thread
« Reply #8 on: October 01, 2014, 02:05:05 pm »

- the speed rises with the number of turns the cart spends on the ramps (just under 5000 speed for every turn, ~130 000 for a cart sent down a fifteen-level ramp), but one turn is subtracted from the count; i.e. the cart is charged ~5000 speed for leaving the ramps in the end.

Do you apply acceleration change before or after velocity change for a tick?

That order would explain the 500 subtraction if you apply acceleration change after velocity change. At least I get velocity of 39120 when coming out of single ramp starting from zero after hitting wall. Single tick in up ramp subtracts 5k and the next corner another 1k leading to very close 1/3 tile per tick speed. After second corner I see first 4 tick tile making me suspect that out speed from z-1 double ramp starter is 34210 like my math says.

Btw, I suppose single tick move after ramp is because of sqrt(2) length for ramp tile making overflow calculation complex. So engine simple puts the cart to end of next tile insteas of correct overflow of pos-100k*sqt(2).
« Last Edit: October 01, 2014, 02:12:11 pm by suokko »
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gchristopher

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Re: My very own Minecart Education Thread
« Reply #9 on: October 01, 2014, 02:35:19 pm »

Why is this so? As far as i can tell, the game doesn't calculate any sort of "heading" for the cart, it just keeps track of the velocity, probably split between x- and y- axis. When the cart moves over flat floor, all that'll happen on "direction-neutral" track is deceleration. When the cart is re-pathed by a legal corner, the whole input speed is taken and turned into speed in the exit direction of the corner.

"Straight" tracks have no pathing power over minecarts, they don't keep them "on track", because carts don't consider themselves "on track" in the first place. They're in contact with the floor and react to corners/constructions, or they're in flight and don't. That's it.
I have sometimes had a hard time with the implications of this, because a fast-moving cart can be derailed (maybe the wrong word?) and acquire a small velocity perpendicular to a line of straight tracks. The cart appears to follow straight tracks for several tiles, then goes off to the side, leaving the track or hitting a wall.

I'd like to hear what you find about the conditions where this can happen. I never got a complete understanding of it when building squirt guns. (I don't know if water is still a deadly projectile in .40.x)



A worthwhile addition to your section on falling carts might be to mention the implications of how a falling cart is a projectile object, not a solid minecart, and collision is only checked with solid objects. (Again, my terminology might be wrong.)

In .34.x, if you drop several carts down a shaft in succession, the first one will land on the bottom, and the others will all stop one tile above the bottom. This is a somewhat finicky way of quantum stockpiling minecarts for a cart system that requires multiple carts.

If the bottom cart moves out from that tile, then one cart from the quantum stack above it will fall and cease to be a projectile. (I think.) IIRC, this works as long as there is any number of carts in the quantum stack other than two. If there are two carts waiting to fall, both will fall simultaneously and become solid in the same tile and something weird happens with collision physics. I never fully understood it, I just avoided the scenario by keeping either less or more than two carts in the stack waiting to fall.

The rate at which carts are moved from the bottom of that stack can be used as the rate-limiting mechanism to keep other parts of a cart system from being overwhelmed. One common application of this setup would be bringing magma up without using a pump stack.
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Larix

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Re: My very own Minecart Education Thread
« Reply #10 on: October 01, 2014, 06:31:58 pm »

(I'll try to mention possible answers to questions raised where they fit, but for now i'm putting priority on getting through the curriculum i've laid out. There are still a few lessons to come.)

Lesson Five: diagonal movement and how to fix it

Diagonal movement, on the face of it just means that a cart is not moving in a cardinal direction and will eventually move off the "straight line" or bump into a wall, stopping dead.

I admit that this is just interpretation, but i'm reasonably certain that diagonal movement is not handled as a "heading" like "fifteen marks east off north" but rather as a combination of movement on the two flat axes.
Spoiler: laborious example (click to show/hide)

Diagonal movement commonly occurs when a cart moves up a corner ramp. Since minecarts don't care about flat-floor track apart from corners, a long straight track line will do nothing to rule in a diagonally-moving cart, it'll just move along and take its sideways step when it's time[1]. And if there's a wall next to the track (e.g. because you're trying to keep accelerating the cart via impulse ramps) it'll just hit the wall and stop, at least temporarily. If it stops on flat track, it'll stop for good, if it stops on a ramp, it'll start moving again, but it may lose its load. As far as i can tell, that was the problem encountered in this http://mkv25.net/dfma/movie-2507-strange water gun design. Thanks to uncorrected truetype font turning all text into garbage, i can only guess (and you better ramp speed up to 1000+ and "step" the thing yourself by hitting forward/pause repeatedly).

[1] Note: in my experience, a cart always gets one ramp-step's speed (i.e. about 5000, 1/20 tile/step) to the "outside" of the curve on the corner ramp. It will step off the straight path on the eleventh step after the corner, i.e. after this lateral speed component has accumulated half a tile of distance. This holds both for a cart propelled by a highest-speed roller (50.000 speed) and a maximum-speed cyclotron (265.000); both will stop/go off the straight path after ten steps.

I've re-built WanderingKid's impulse/something elevator and found the problem he faced (reported here) was also nothing fancier than diagonal movement: sending the output of a corner ramp onto a straight (i.e. inconsequential) track. In my re-build, the cart would move off the straight line on the eleventh step after the corner.

So how to avoid diagonal-movement troubles?

The easiest option is not to generate diagonal movement in the first place: don't use corner ramps to move carts up levels. For moving carts up levels, straight ramps work just as well as corner ramps; better in fact, since they don't cause the added 1000 speed loss from the corner (and don't cause diagonal movement). There are some special cases of upward movement over multiple levels which require corner ramps, but if you only want to go up a single level, just use a straight ramp.

The other option, when corner ramps are used, is to use the one track type carts care about: corners.

If a cart tries to leave a corner tile, the game checks whether the border the cart tries to leave over is "blocked" by the corner: on a NW corner, those will be the E and S borders. If a cart tries to leave to the south, it's treated as coming from the north, and it leaves towards the west. This rule appears to only care for the tile border the cart tries to leave over. A diagonally-moving cart is also subject to these checks: let's assume a cart moving from the northwest towards the southeast: if the tile the cart'd leave to would be the one directly south of the corner, the cart will turn around to the west and will move west only. Notably, the resulting speed is the cart's previous N->S velocity, the W->E velocity will disappear. If the cart would have left to the eastern tile, it'll turn north (moving at the previous W->E velocity). If the cart's go-to tile is the exact southeastern one, the corner will not affect it. Which of the two axial speeds is higher doesn't matter.
A cart moving from northeast to southwest will only be affected by the corner if its go-to tile is the southern one. If it tries to leave to the western (or southwestern) tile, it'll stay on its diagonal course, because the border over which it attempts to leave isn't blocked.

My standard approach to the output of corner ramps is to just put a corner on the tile immediately behind the ramp, like this:

Code: [Select]
z+0       z+1
####      ══╗
══▲#      ++▼

####
══╝#
track on ramp

I've yet to see a case where this doesn't work (if necessary propped up by a wall behind the corner above when working with fast carts).

PS: my best interpretation is that a corner "sets" the cart's speed in the exit direction to its previous value in the "input" direction. Since the diagonal component is actually velocity on the corner's exit axis, that part of the cart's movement speed just gets overwritten. Result in any case: successfully rounded corners fix diagonal movement.


Lesson Six: False ramps

In the ramps section, i mentioned ramps which don't accelerate carts. Those may seem kind of pointless for building tracks, but the lack of acceleration can actually be a benefit.

If a ramp connecting levels doesn't cause friction, you can change level without losing/gaining speed (apart from ordinary floor friction). It's decidedly weird - my constructions only work when the cart enters at very low speed - around that of a dwarven push - but a single push can move a cart up/down 40+ levels without notably changing the cart's speed. Example: http://mkv25.net/dfma/movie-2653-minecartescalator (o hey, it was 47 z. You can safely speed past the end, i just showed that each ramp was a non-functional E-only one.) It's of course also possible to do this without dwarven labour, you just need sufficiently regulated cart speeds from proper ramps or rollers, if needed combined with a few track stops. A super-low-tech and low-risk way of lifting a cart up a huge number of levels.

Another application of false ramps is to make the loading of liquids into carts easier: pioneered by flameaway: http://www.bay12forums.com/smf/index.php?topic=120435.msg3873427#msg3873427 i found it to be an impressively fast, fully-automatable loading mechanism for waterguns allowing cadences of up to one shot per ten steps (using multiple carts in one barrel). It works so well because it doesn't accelerate/decelerate the carts. The loader simply consists of a single channelled-out tile containing a track ramp with no actual down direction. Its track connections only go to wall,  therefore it is treated as ordinary flat floor by the game. The cart is never at the "bottom" of the "ramp", because as far as the minecart engine is concerned, there's no ramp here. Thus, the cart also doesn't need to "climb" out of the hole, it just needs enough forward motion to roll to the next tile.

A cart moving slowly enough will pick up water/magma from a 7/7 tile; the speed imparted by a high-speed roller is just low enough. Dwarven pushes have the advantage that they "teleport" the cart to the middle of the first pushed-to tile, which makes them the fastest loading event. They're decidedly less automatable, though. There's no need to engrave a corner into the pond tile, a straight fake ramp works better.

Bodycount: nothing new! Well, one diagonal vs. roller test ended up giving a dog a bruised stomach. Big deal, i don't really count dogs if they don't end up in multiple parts, like the puppy that during the last round teleported its torso through a wall while leaving all its limbs on the other side. The highly irresponsible flying minecart test, however, didn't cause any harm at all.
« Last Edit: January 29, 2015, 09:55:32 am by Larix »
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Larix

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Re: My very own Minecart Education Thread
« Reply #11 on: October 02, 2014, 10:58:06 am »

Lesson Seven: Pathing across levels

Pathing on flat floor is easy enough: only corners matter. It's not quite so easy when minecart paths go to different z-levels, either up or down.

Getting a cart to move upwards is easy enough - just offer it a track ramp. Carts will not go up ramps without engraved track, and they will not reliably go up "false" ramps (i.e. ramps which don't accelerate/decelerate carts). You'll eventually want the cart to stop going up, and there things can go awry. A cart moving up a ramp with no closed ceiling (or building) immediately above the exit tile may get airborne. The speed from a highest-speed roller is enough for this, but high-speed rollers or equivalent speeds like the acceleration from a single down ramp can suffice, too. An airborne cart will not be in contact with the floor underneath it and will thus not care about track corners, rollers or track stops on that tile.

A closed ceiling or building (bridge, hatch cover etc.) above the exit tile will make the cart behave and stick to the floor, regardless of its speed - a high-speed roller cart will be reined in by a ceiling just the same as a highest-ramped-speed cart or a supercharged cart.

If there's open ceiling above the exit tile, a cart can still be ruled in by a functional ramp on the exit tile.

Code: [Select]
z+0              z+1, a)    b)    c)    d)
######                 #     #     #     #
▲▲▲▲▲▲▲#▲══           ▼═▼   ▼▲▼   ▼▲▼   ▼▲▼
.
######                 #     #     #     #
╚╚╚╚╚╚═#═══           ▼═▼   ▼╚▼   ▼║▼   ▼╝▼

Cart comes from the west, accelerated by a series of impulse ramps, then goes over an up ramp.
a) - no ramp (can be smoothed floor instead of straight track): cart goes into flight, several z-levels up.
b), c), d): cart goes down the ramp to the east and follows the track.
Notably, the orientation of the ramp on the top tile doesn't matter, it just needs to be a legal ramp. Carts can be made to "level out" via ramp, but as seen here, they can also be forced down an adjacent ramp this way.

So, if you send a cart up several levels to the surface and don't want it to go flying, put a ramp on the exit tile.

When you want a cart to enter a downward path, there are a few issues and solutions, as well:

A cart coming upon a hole in the ground will by default just jump across it. If the cart moves at a speed of at least 1/5th of a tile per step, it can jump over one tile of open space and continue moving on flat floor on the other side. A dwarven push or low-speed roller are enough for this purpose. A peculiar issue was found with dwarven pushes: a dwarf pushing a cart from right next to a hole in the floor cannot move the cart across. It will collide with the hole's edge and fall down into the pit. This seems to happen because the push "teleports" the cart to the middle of the adjacent tile, without giving it the "lift" gained by a jump. If there's one tile of "buffer" between the dwarf and the hole, the cart jumps just fine.

If there is a ramp in a hole (ordinary floor ramp or track ramp, both are recognised), a cart will treat the hole as an appropriate pathing destination and will directly move into it (i.e. without spending time in the "open space" above the hole) as though it were rounding a "downward" track corner. Carts moving at derail-capable speeds will not enter a downward ramp, they'll jump over the tile and continue beyond it. In addition, the tile before the ramp must be a "track" tile - either engraved track or a bridge. Carts coming from ordinary floor will jump, regardless of their speed.

As noted above, however, a cart coming from a legal track ramp (any orientation!) will enter a downward track ramp just fine. This allows sending very fast carts down ramps simply by putting an impulse ramp before the actual ramp entrance:

Code: [Select]
. #        #
══▲▼     ══╚▼

Other ramp orientations seem to work just the same, as long as they're legal and don't open a diverging path. Ramps will not send a cart into a hole that doesn't contain a ramp.

Lesson Eight: Meet the checkpoint bug

Let's face the possibly most powerful feature/bug of minecarting. Nope, not impulse ramps. For demonstration purpose, let's take two sets of opposed ramps:

Code: [Select]
a)        b)
#▲═▲#     #▲▲#

#═══#     #══#
Offer open floor above and to the sides.

Drop a cart onto one of the ramps via hatch. In each case, the cart will start out by rolling along a ramp for five steps.

In a), the cart will then pass over the flat tile in a single step, spends eight steps on the opposing ramp, rolls across the middle tile in a single step again, spends seven steps on the first-touched ramp, then across in a single step etc., until after a few iterations it sits still in the middle tile.

In b), the cart goes onto the opposing ramp, passes over it in a single step, goes to the tile above and to the side, passes over that tile in a single step again and then moves off at about 1/5 tile per step (~19 000 speed). If you offer no exit, the cart will bounce between the two ramps forever, spending eight steps on each. You can temporarily stop it by blocking the opposite ramp with another minecart, but as soon as one cart is removed, the remaining cart starts bouncing again.

What we're seeing is an artefact of the game having to switch distance calculations as soon as ramps get involved. The upshot is that

a) if track changes from flat track to a ramp, the cart must step onto the new ramp tile. No matter how fast the cart is, the tile cannot be skipped. I'll call this a "half checkpoint".
b) if track changes from a type of ramp to anything else, the "changed" tile cannot be skipped and the cart will spend exactly one step on it, regardless of its speed (as long as speed is above zero). Finally, the last speed increment the cart received on the ramp is erased, presumably by applying equivalent acceleration in the opposite direction. I'll call this a "full checkpoint".

"Anything else" notably means that checkpoints happen whenever the cart passes from a ramp to a different ramp, i.e. a ramp with a different slant (accelerate-to direction), and when passing to a non-ramp tile, preferably flat track.

The biggest effect here is that
Checkpoints effectively divorce the rate of movement from internal speed of the cart.

Cart propelled by a single ramp (about 1/3 tile per step) going over checkpoint? Spends exactly one step there. Cart propelled by maximum number of ramps (about 2,5 tiles per step) crossing checkpoint? Spends exactly one step there. In fact, if a cart is moving along a ramp- and corner-heavy track and crosses one tile each step, it's almost a given that you're dealing with chained-up checkpoints.

Simple example:
Code: [Select]
##########      ##########
═▲═▲═▲═▲═▲      ═╚═╚═╚═╚═╚

A cart going in at sufficient speed (must be ~72 000+) will cross this track spending one step on each tile and will come out on the east at almost exactly the speed it went in. This holds both for a 72 000 speed and a 265 000 speed cart, they'll move at the same rate through this track, they'll only lose the speed for normal track friction but the slower cart will also not accelerate. Their actual internal speeds will only again assert themselves after the cart left this track section.

This happens because each impulse ramp is a half and each flat tile a full checkpoint. The slower cart is just fast enough to make it off the ramp in a single step (apparently a cart moves its full movement rate "into" a half-checkpoint (but not past it when moving faster than one full tile per step): a fast-enough cart makes it to just past the half-way point of the ramp upon entering, and just past the tile's "exit" on the very next turn). PS: i haven't checked this exact design, but as long as incoming speed is at least 80.000, this thing should work the same way in both directions - carts going "with" the impulse ramps won't accelerate, and those going "against" them won't slow down.

Let's look at the first example with the double-ramp again and see what happens by checkpoint rules, dropping the cart onto the western ramp:
-cart goes "down" ramp to the east, picks up 25 000 speed.
-cart enters ramp slanting west - checkpoint: accelerate 5000 to the west (compensating for last step of acceleration), go to end of tile
-cart "accelerates" west by 5000 on the west-slanting ramp, has 15 000 speed left to cross the threshold to the next tile, thus reaches flat tile above and to the east - checkpoint: accelerate 5000 east (compensating for westward acceleration), go to end of tile
- cart keeps moving on flat track to the east, now with normal distance calculations so it takes five steps per tile again.

Why the weird "accelerate backwards on the checkpoint" thing? Because in example a), the cart actually stops. It also explains why the highest speed i've got through ramps (measuring actual track covered) is not 270.000 but 265.000.

For a clearer example:
Code: [Select]
#▲+ ═     #═══
+ - lever-operated door

Station a cart on the ramp, then open the door. The cart instantly rolls onto the flat tile and stops there. This is, it picked up speed from the ramp, used that speed to pass over to the flat ground, but had no speed left thereafter (or it'd have moved to the next tile east on the next step). I interpret this so that the cart actually loses its speed after taking the move. Other evidence supports the interpretation.

This bug allows deriving speed from pits in the floor and moving carts up levels with ease. It's the actual power behind the "impulse elevator" shown on the wiki. WanderingKid's elevator uses impulse ramps to gain speed, but checkpoints to go up levels.

I'll leave you with this for now. More to come.

Bodycount: kitty!
Someone's pet cat wandered into the cyclotron. It's the only contraption that has caused any real damage so far, and the only dwarf who was hurt remains the spinner/leatherworker who tried to "clean" puppy blood out of it while it was spinning.
« Last Edit: December 26, 2014, 02:43:34 am by Larix »
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Larix

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Re: My very own Minecart Education Thread
« Reply #12 on: October 02, 2014, 04:17:35 pm »

Lesson Nine: Practical implications of the checkpoint bug

The checkpoint bug affects all manner of minecart constructions, as soon as ramps get involved. For a start, let's look at the lowly single-ramp cyclotron:

Code: [Select]
#####      #####
#╔═╗#      #╔═╗#
#╚▲╝#      #╚╔╝#
#####      #####
Cart cycles counter-clockwise and its speed oscillates somewhere between 70.000 and 80.000.

It won't go any faster, ever, although one step of ramp acceleration gives 4900 speed while four corners and, say, seven steps of movement cost no more than 4070. Evidently, if the cart spends only one step on the ramp, this acceleration is eaten up by the checkpoint compensation when moving off the ramp to level floor. It'll only really pick up speed when it spends at least two steps on the ramp and it must be slower than ~72.000 for this to happen.

Indeed, the cart cycles at an oscillating speed: it goes five rounds at eight steps each (spending two steps on the ramp each time) and seven steps in the sixth round (spending only one step on the ramp).

For speed to keep building up, you need an unbroken stretch of three impulse ramps: due to the greater length of ramp tiles, the maximum speed available through ramps (270.000) is just less than two ramp tiles per step, so a cart will always spend at least two consecutive steps on the three-ramp stretch. Such a three-ramp cyclotron is enough to achieve maximum ramp speed.

***

When moving a cart laterally onto an impulse ramp track, the checkpoint effect can be used to prevent diagonal movement.

Throwing a cart directly into a sideways impulse ramp

Code: [Select]
a)                   b)
####      ####      ####      ####
▲▲▲▲      ╝╝╝╝      ▲▲▲▲      ╝╝╝╝
   ║         ║         ▲#        ╚#

from the south like in a) will have the cart accelerate to the west on top of a pre-existing and lingering northward speed. It'll either bump into the wall and temporarily stop or exit the impulse stretch on a diagonal trajectory. Sending it through an immediately adjacent impulse ramp lets it pass right through the first ramp of the acceleration stretch via checkpoint effect, stopping it against the wall and cancelling the northward speed instantly, so that it can accelerate west on a straight course.

***

Of course, others have, often unknowingly, used checkpoint effects in their constructions. Take the "impulse elevator" on the wiki:

Code: [Select]
####      ##╗#      ####
▼╔╝#      ##╚#      ╔╝▼#
▼###      #▼▼#      ##▼#

All track on ramps, going up from left to right.
Looking at the thing in action, we'll see that the cart moves at a rate of exactly one tile every step until after five levels or so it stops, rolls back from an "up" ramp in eight turns, spends another eight steps on the ramp behind, then starts going at the previous rate for another five levels. Clearly, this means that the cart moves at one ramp-length per step, i.e. 140.000 speed, right?

Haha, of course not. It's checkpoints all the way up. The cart hiccups and stops not because it's too fast, but because it ran all out of speed and had to checkpoint-cheat itself some new steam.

Observe the ramp slants in the example above: E, W, N, S, W, E. Slant changes every tile, thus every tile is a full checkpoint. The checkpoint bug runs the cart up at a rate of one ramp every step, until speed falls to zero. At that point, the cart makes it onto the next tile (and technically all the way "up" on it) but has no more speed to make it to the next tile (up), so it stays on the ramp and accelerates there for the full eight steps. This moves it back to the last (opposing) ramp, which it again fully crosses, but here it bumps against a wall and accelerates all the way forward again. With the shiny new 35.000 speed, it can take the up checkpoint and have speed leftover to keep moving.

It's peculiar that this thing loses speed so quickly - it appears to burn through its store of ~35.000 speed points in five levels, although it should only lose 1.000 speed per level for the corner. It's almost as if there's something fishy with corner ramps that enforces a higher speed loss.

***

Another ramp spiral was invented by WanderingKid and has the advantage of doing without the annoying back-and-forth every few levels. The cart in that design just keeps going. Let's check it out:

Code: [Select]
z+0   z+0, track    z+1   z+1, track    z+2 (z+0 mirrored)
####      ####      ####      ####        ▲▲╗#
▼###      ▼###      ##▲#      ##║#        ##▼#
╚▲▲#      ╚╔╝#      ##▼#      ##▼#        ####
####      ####      ####      ####        ####

This one surprised me at first, because it "somehow" manages to send a cart up two levels, seemingly with a single checkpoint. Spoiler: of course it's two checkpoints.
The east-pointing ramp on z+0 works as a proper speed-granting impulse ramp here, because the cart enters it from flat floor, not from another ramp. When i tried it out, the cart spent two or three steps (repeating pattern of different rates, like in the cyclotron above) on the ramp each time, so there was always speed gained here. The corner up ramp is, unsurprisingly, a checkpoint, the cart passes it in a single step. What i hadn't fully understood yet - the next, straight, ramp is also a checkpoint, because the slant of ramps changed, from west to south. The flat corner is yet another full checkpoint, which doesn't really matter in and of itself, but the fact that it's normal floor and not a ramp saves the following impulse ramp from being a full checkpoint, so it can actually do its impulse work.

***

Let's crack an old puzzle next: the 2x2 ramp spiral. It's a notoriously ill-behaved contraption, carts keep stopping on it for no discernible reason. At the same time, it looks so simple:

Code: [Select]
####        ####        ####        ####        ####
#╔╗#        #▲▼#        #▼##        ####        ##▲#
#╚╝#        ####        #▲##        #▼▲#        ##▼#
####        ####        ####        ####        ####

Spread over four levels, one corner on each level, each leading into the next. Throwing a cart down such a spiral lets the cart start going at one ramp per step, but after five, it stops, starts again, goes another five, stops again etc.

Ho hum. Is it picking up too much speed? I put a few stone blocks into a cart and sent it down there. The blocks staid in the cart. Well, it was moving at one ramp per step, so it was probably checkpoint-hopping again. Makes sense, of course, since ramp slant changes on every tile. So it probably stopped simply because its speed dropped to zero. Still, a cart going down a ramp spiral and losing speed? I revved up a cart in the trusty cyclotron and sent it down a nice long spiral. It kept going and emerged 21 z-levels below - at 130.000 speed. The cart was definitely losing ~6.000 speed on every ramp, a few more tests confirmed this. In fact, a downward spiral slows down a cart exactly as much as an upward spiral.

Inspired by rhesusmacabre's long table:
http://www.bay12forums.com/smf/index.php?topic=125679.msg4223763#msg4223763
i built a few simple test spirals, and yes, i was getting checkpoint-movement up the spirals, over nice large numbers of levels, and my eyeballed speed loss of 6.000 per level seemed to work out.

I definitely needed to crack the puzzle of corner ramps. But first, some light entertainment.

***

Since different-slant ramps work as checkpoints for each other and the compensating speed effects cancel out their acceleration, shouldn't it be possible to send reallllly slow carts along a line of impulse ramps, bouncing one ramp per step until ramps stopped and the actual speed reasserted itself? I built a line of 24 impulse ramps stretching from east to west and with wall to the south, alternating between NS and SW every step, hatch-dropped a minecart on the easternmost (SW) ramp and watched it. Yep, cart rolled down the usual five steps, then went forward at a rate of one ramp every step over the whole line, and once it emerged from the ramp line, it crawled along at the actual ~19.000 speed (five to six steps used for every tile).

But shouldn't the northward acceleration, although it's cancelled instantly, result in a minor northward displacement on every NS ramp that should eventually push the cart past the northern border? I expanded the row to ~40 ramps, and sure enough, after the thirtieth ramp (15th NS ramp) the cart moved off the ramp-line to the north. To make sure it's really displacement and not northward velocity, i covered ten ramps with a bridge so that north-pointing ramps #15 to #19 were obscured. The cart moved over this stretch without diverting, went over the SW ramp directly behind the bridge - and made its step to the north when it checkpoint-passed the NS ramp behind it, the twentieth northward ramp in the line, but this time, the fifteenth touched by the cart.

Fifteen pushes of presumably 4900 distance units give 73500 distance units, just over half the assumed length of a ramp (140.000 or so - i don't know the exact number Toady uses). Enough to move over the border to the next tile when starting in the middle of a tile. Seems that it works out.

Of course, northward displacement can simply be compensated by southward displacement. I dug out a track all across the embark (normal embark, so just 190ish tiles) and carved out a nice stretch of 160ish alternating track ramps. First ten "forward" ramps interspersed with 10 North-slanted ramps, then (changing the adjacent wall) 20 forward with 20 south-slanted, then another 20/20 stretch forward/north etc.., finally a bit of flat track leading into a little loop at the far end. The cart was dropped in via hatch as usual and moved all across the embark without falling off the row, passing one tile per step as long as it was bouncing over ramps, while the flat track at the end demonstrated its internal speed remained at the original 19.000. The loop itself contained a nice juicy acceleration rail, increasing speed on the route back to ~120.000, and the cart went back all the way, once again at 1 tile/step externally, unfazed by the 80 "opposing" impulse ramps.

Lesson Ten: Corner ramps

Corner ramps had been bugging me for a while now, so i built a simple test rig:

Code: [Select]
above        below
#▼═════      ▲#
             ║

With a SE (╔) track ramp.

First of all, send a cart up the ramp: no matter what i do, when given straight track, the cart will move diagonally and the first step aside happens after 11 steps, adequate for a lateral component of just under 5000 speed, i.e. the acceleration gained by a single step on a ramp. Curiosly, while the corner should convert all south-to-north velocity of the cart into west-to-east velocity and the ramp slants to the south, the aberration was to the north.

Unsurprisingly, the culprit is the checkpoint bug: almost always, a corner ramp passed upward leads to a checkpoint - the ramp slants south and the most sensible connections above are flat track or a west-slanting ramp. Thus, the checkpoint effect is applied: a) the next tile is crossed in a single step. b) compensative acceleration is applied which is opposed to the ramp's slant. That's it - the corner outputs the cart on a pure-eastward path but then the "compensating" speed is applied and gives the much-abhorred diagonality to the cart.

So, putting it in numbers: when a cart checkpoint-hops up a corner ramp, it loses 5000 from its original incoming speed to ramp acceleration, loses another 1000 for the corner, and the checkpoint doesn't "refund" the 5000 speed but rather (since it's applied after the corner turn) applies it as lateral/diagonal speed towards the "outside" of the corner. A cart going up a corner ramp at any speed loses 5000x(time on ramp)+1000(corner penalty) speed, and gains 5000 lateral.

That was the easy part.

Let's send a cart down the ramp now.

If the cart is fast enough (about 45.000 minimum), it takes the corner and continues perfectly straight in the corner's exit direction, with a speed loss of ca. 6000. I tried it with a highest-speed roller, and the cart going through a corner ramp would emerge at 44.000 speed, while a cart going down a straight ramp would gain ca. 5000 and emerge at 55.000.
Once again, we're dealing with checkpoints and a corner, so let's step through it:
On the corner ramp, all acceleration goes to the side, it doesn't accelerate the cart in its original travel direction. Here, we have a cart going west, which is accelerated south. Unsurprisingly, the westward speed isn't increased by this event. At the end of the turn on which the cart wants to leave the tile, the corner comes into play, converts all westward to southward motion overwriting the extant southern vector, the acceleration gained is therefore lost. On the next step, the cart reaches a checkpoint and to compensate, it is "accelerated" 5000 units to the north. Summa: all southward acceleration was ignored because of the corner, but the compensative deceleration still applies, so the cart loses 5000 speed, plus 1000 for the corner. 6000 in total.

What's that about a 45.000 minimum speed? Ah well, losing speed on a down ramp is not the weirdest thing here. A cart moving at lower speeds than that is liable to malfunction even more blatantly. A cart propelled by a dwarven push emerges at a mostly-south-and-slightly-west trajectory, going off the straight line after two tiles. A cart entering the ramp at between 30.000 and 40.000 speed leaves at an almost-45° angle, a very sharp diagonal. It took me quite a while to think up a solution for that one, but i think it works out:

Corners are only checked when a cart tries to leave a tile, and they only check whether the side opposed to the "border" over which the cart is trying to leave is connected. In understandable: if a cart on a southwest heading is trying to leave a tile going over the western border of the tile, the pathing algorithm checks if the tile underneath is a track corner with an eastern connection. If yes, the cart is turned around towards the corner's other connection. If the cart tries to leave over the southern border, the algorithm checks whether the tile is a north-connected corner. If the checked border is not connected or if the tile isn't a corner, the cart leaves normally and its speed(s) is (are) unchanged.

So what happens with these slower carts is this: they move so slowly to the west and thus pick up so much southward speed on the ramp, that the cart's "exit" direction from the tile is south (or SW (??) in the case of the somewhat-slow cart), and thus the corner has no power over them. Consequently, they move off on their screwy diagonal course.

A cart going down a corner ramp, properly taking the corner, loses 5000(checkpoint compensation)+1000 (for the corner)=6000 speed, independent of lingering time on the ramp.
If time on the ramp is too long, the corner starts checking the wrong (unconnected) side of the tile when the cart tries to leave and no longer applies. In that case, the output trajectory is purely diagonal, presumably incoming speed in the incoming direction + 5000x(lingering time minus one) lateral (towards ramp slant), no corner penalty.

Bodycount: nothing new, no new tests required. I just wrote up what i had worked out previously.

This concludes our course on Minecarts. Annotations, corrections, claims of priority will be gracefully accepted and carefully considered. Possibly. I've tried to link to sources and earlier findings. I owe a large debt to other players for their research and inspiring inventions.
« Last Edit: October 03, 2014, 11:38:01 am by Larix »
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gchristopher

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Re: My very own Minecart Education Thread
« Reply #13 on: October 03, 2014, 04:01:54 am »

Of course, others have, often unknowingly, used checkpoint effects in their constructions. Take the "impulse elevator" on the wiki:

Code: [Select]
####      ##╗#      ####
▼╔╝#      ##╚#      ╔╝▼#
▼###      #▼▼#      ##▼#

All track on ramps, going up from left to right.
Looking at the thing in action, we'll see that the cart moves at a rate of exactly one tile every step until after five levels or so it stops, rolls back from an "up" ramp in eight turns, spends another eight steps on the ramp behind, then starts going at the previous rate for another five levels. Clearly, this means that the cart moves at one ramp-length per step, i.e. 140.000 speed, right?

Haha, of course not. It's checkpoints all the way up. The cart hiccups and stops not because it's too fast, but because it ran all out of speed and had to checkpoint-cheat itself some new steam.
That's a great explanation! What a ridiculously convoluted chain of events. Thanks Larix!
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askovdk

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Re: My very own Minecart Education Thread. Ten Lessons, now complete.
« Reply #14 on: October 03, 2014, 05:23:09 pm »

This is very good stuff.  :)  Thank you
I can't wait to come home and experiment in the weekend.
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