FA-51577 / Raster clipping / Member archive
Liang barsky parametric rect: entry classified by distance · case 02
Boundaries are classified as entering or leaving by the sign of the boundary distance instead of the direction term, so interior starts treat every plane as leaving.
Case contract
Each command is [x0,y0,x1,y1,rect] with integer coordinates and an inclusive integer rectangle [left,top,right,bottom] that is first intersected with the pixel-center raster [0,size-1]x[0,size-1]. An empty rectangle returns None. Liang-Barsky tests left, right, top and bottom inequalities with exact rational parameters: parallel segments outside a boundary are rejected, points on a boundary are inside, entering parameters raise t0 and leaving parameters lower t1. The result is [t0,t1] as exact rational strings or None.
Why this case matters
Raster clipping decides which pixels a primitive may touch; a single wrong clip decision writes outside the target or scissor, or silently drops visible coverage.
One recorded failure
Sample boundary fixtureThis sample comes from the broken implementation of a controlled reproducer.
| Boundary fixture | Actual | Expected | Outcome |
|---|---|---|---|
| explicit case 0 | [null] | [["1/5", "4/5"]] | Failed |
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