FA-51689 / Raster clipping / Member archive

Scanline polygon span clip: row sample top · case 04

Rows are sampled at their top edge instead of their pixel-center line.

Member previewVariant 4 · 3 implementations · 13 checks per implementation

Case contract

Each command is [polygon,scissor]. Rows y in the scissor rows [top,bottom) intersected with [0,size) are sampled at y+1/2. Each polygon edge is oriented bottom to top and contributes a crossing when top<=yc<bottom (half-open in y), interpolated exactly. Crossings are sorted and paired even-odd. A pair covers pixels whose centers x+1/2 satisfy xa<=x+1/2<xb, i.e. [ceil(xa-1/2),ceil(xb-1/2)), then clipped to the scissor columns [left,right) intersected with [0,size). Non-empty spans are returned as [row,start,end].

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 fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
explicit case 1	`[[[1, 1, 2], [1, 4, 5], [2, 1, 3], [2, 3, 5], [3, 1, 5], [4, 1, 5]]]`	`[[[1, 1, 2], [1, 4, 5], [2, 1, 5], [3, 1, 5], [4, 1, 5]]]`	Failed

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