Random notes for Rambetter, don't expect to understand this:
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Brush 0 is the problem.
Side 0 is the problem (under surf tri).
Side 1 is the +y 4-face.
Side 2 is the -x 4-face.
Side 3 is the -y 4-face.
side 4 is the +z tri.
(6144, 16122) -> (6784, 16241)
x "climb" of side 1 is 6784 - 6144 = 640.
y "climb" of side 1 is 16241 - 16122 = 119.
x/y "climb rate" of side 1 is 640 / 119 = 5.378151261.
After clipping side 0 against side 1, we get
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**** (-262144, -33762.8125) -> (262144, 63722)
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The slope of that is (262144 + 262144) / (63722 + 33762.8125) = 5.378150571.
(-262144, y) -> (6784, 16241)
So (6784 + 262144) / (16241 - y) = 640 / 119
So y = 16241 - ((119 * (6784 + 262144)) / 640) = -33762.8
(6144, 16122) -> (262144, y)
So (262144 - 6144) / (y - 16122) = 640 / 119
So y = 16122 + ((119 * (262144 - 6144)) / 640) = 63722
After clipping side 0 against side 1 should have
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**** (-262144, -33762.8) -> (262144, 63722)
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