FA-51725 / Raster clipping / Member archive

Scanline polygon span clip: scissor rows ignored · case 05

All raster rows are scanned regardless of the scissor top and bottom.

Member previewVariant 5 · 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 5	`[[[0, 2, 4], [1, 2, 4], [2, 2, 4], [3, 2, 4], [4, 2, 4], [5, 2, 4], [6, 2, 4], [7, 2, 4], [8, 2, 4], [9, 2, 4]]]`	`[[[1, 2, 4], [2, 2, 4]]]`	Failed

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