FA-51823 / Raster clipping / Member archive

Circle span scissor: row sample offset · case 03

Row distance is measured from the row top rather than the pixel center, skewing the disc vertically.

Member previewVariant 3 · 3 implementations · 10 checks per implementation

Case contract

Each command is [cx,cy,r,scissor] for a filled disc centred on the integer pixel-corner point (cx,cy). Pixel (x,y) is covered when its center (x+1/2,y+1/2) lies within the closed disc. Rows of the half-open scissor [left,top,right,bottom) intersected with [0,size) are scanned in order, computing exact integer span bounds with half-pixel doubled coordinates and an integer square root, then clipping to scissor columns. Non-empty spans are [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 0	`[[[2, 1, 5], [3, 1, 5], [4, 1, 5]]]`	`[[[1, 2, 4], [2, 1, 5], [3, 1, 5], [4, 2, 4]]]`	Failed

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