Yep, that's basically a combination of these two points Clam mentioned:
The terrain added to the level by builders can be used to keep a basher/miner/digger going, by providing them with ground to stand on and/or terrain to satisfy the check to make them continue digging.
Bashers slide down slopes they encounter in the terrain below their feet. This can be exploited by mining or digging at the basher's position to make it bash lower.
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I'll also add the following:
7-builder stack: Let out 7 lemmings at RR88, and have each build as soon as it steps onto the brick laid by the previous one. This creates an impassable wall for the remaining lemmings, and all but 2 builders turn around due to floor checks.
I'll like to add that about 2-3 months ago I've found an improvement to this basic technique, as you might've seen from some of my recently posted replays. Basically, to get a stack of height N pixels, you can use just N-2 lemmings at RR88 and use N-1 builders. You do an N-2 stack just like the above, but then after the last turnaround builder has turned around and has just stepped up onto the highest point of the stack, assign him builder again. He will complete the last 2 pixels of height you need for your stack, and will actually trigger the floor check on one more lemming. So you end up turning around all but 1 builder, and use one less builder than the basic stack technique above. Of course, because the last 2 build bricks are laid by the same builder, it will take slightly longer for the stack to reach the desired height this way.
Note: this works both facing left as well as right, but I think you have to take some care with the builder placement for left due to the asymmetry of brick placement for left builders
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Lemmings walk over bridges one way and under the other way, eg. if you build to the right, then any lemmings walking beneath the bridge will be facing left. This allows you to pick out a lemming facing the right way.
A collorary to this is that if your bridge goes up to the left, does not connect with a wall, and is right underneath an entrance trapdoor, you get a nice setup where you can have a lemming coming out of the trapdoor start building right, and only subsequent lemmings from the trapdoor will follow that bridge, while the rest of the crowd will not, until you release them with another builder.
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Another useful builder technique is to take advantage of their crude ceiling check. Basically only a single pixel of ceiling is checked each time a lemming steps up onto the brick he lays, plus the ceiling check is
skipped entirely on the final brick. The ASCII diagrams below depict exactly the check:
(X = ceiling check location, * = where lemming is standing, + = the build brick being laid down, before builder steps up onto brick and checks for ceiling; . = nothing [just annotation to help line you up visually])
10 X
9 .
8 .
7 .
6 .
5 .
4 .
3 .
2 .
1 *+++++
.
10 X
9 .
8 .
7 .
6 .
5 .
4 .
3 .
2 .
1 ++++*+
Some useful techniques deriving from this mechanic:
- Notice that the builder moves forward 2 pixels each time, so with certain ceilings (eg. 1-pixel thin icicle tips), if you plan carefully where you start building, you can "skip" over low points in the ceiling that are only 1-pixel in width.
- It's actually typically the ceiling check that stops the builder when he builds into a wall. However, if the wall you are building into is not that high, then instead of the ceiling check the builder will only be stopped by the floor check, which means he can get a bit closer to the wall before his building is interrupted. A simple example is to compare how many build bricks you can lay when assigning builder to a digger, with a digger's pit of height 10 vs height 11 on flat ground.
- Also notice that this means your builder will completely ignore terrain between roughly from his head to his legs, as such terrain will trigger neither the ceiling nor the floor checks.
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The asymmetry with the placement of the build brick by the left-facing builder means you get a pixel of brick immediately behind where the builder is standing.
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basher/miner "backstroke": the basher removes 9 pixels from the vertical column of pixels at his standing position, while the miner removes 12 pixels from both that column plus the column immediately behind.
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how to plan your basher: by understanding exactly how the basher checks for more terrain to continue bashing, you can better plan where to start bashing to achieve particular outcomes (eg. have the basher automatically stop, or to continue through patches of insufficient terrain):
1) All the pixels being checked are vertically at 6 pixels above the floor level of where the lemming is standing when the bash stroke is made (eg. from the pixel of floor terrain lemming is standing on top of, move up 6 pixels and that's the height we're looking at).
2) At that height, the game checks the pixels from 8 to 11 pixels horizontally away from where the lemming is standing (obviously in the direction he's facing), and will continue bashing if at least one of these 4 pixels are terrain. The basher's full stroke reaches 7 pixels horizontally in front of the lemming, so very roughly speaking, this means the basher continues bashing if there is more terrain immediately following the current end of the bash tunnel. It doesn't even matter if the bash stroke just made didn't take out a single pixel of terrain, as long as there's more just beyond the reach of the stroke (at the right height of course).
3) This check is skipped on every other stroke, specifically the even-numbered strokes (counting from 1). In other words, without falling or steel or other such interruptions, a basher will always stop after making an odd number of strokes, not even.
4) The basher moves forward 5 pixels after each stroke.
Armed with this information, the attached sketching can be used to help plan out your bashers in situations where you need to do so. The red dot represents the floor pixel the lemming is standing on when he starts bashing, and the yellow "dashes" are where terrain is being checked in order to continue bashing. The green dots represents the floor pixels the lemming will be standing on for each subsequent bash stroke after the 1st (skipping over the even strokes), in case the basher drops lower over the course of his bashing.
[edit: include attachment
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