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Posterize: round_away decided half-way by subtracting, and drivers may fuse the product that feeds it into one fma (AMD does), skipping the product's rounding. A straight value of 5/6 at level 4 gives 5/6 * 3 = 2.5 exactly in f32 on the CPU but 2.49999994 fused, so the GPU rounded a level down. round_away now compares x with t +/- 0.5 (exact for integer t), which no driver can fuse, and the level divide uses an fma-corrected division (div_rn) like the CPU's. CC Grid Wipe: the cell lookup floor(q / cs) used WGSL's division, which may be off by 2.5 ULP; at q = 7, cs = 3.5 AMD and Intel return just under 2 and pick the neighbouring cell. It uses div_rn too. New test: posterize matches the CPU exactly on every 8 bpc premultiplied colour and alpha at three levels (59715 of 65280 pairs differed before at level 4). RX 6900 XT, Vulkan: effectcraft-gpu 222 passed; the one failure left is CC Hex Tile, fixed by #1776. DX12 + FXC (on #1571): both tests pass. Co-authored-by: 00xJ0 <56056764+00xJ0@users.noreply.github.com> Co-authored-by: Claude <noreply@anthropic.com> Co-authored-by: FredApps <153173854+FredApps@users.noreply.github.com>