[yorwba]

Why do conflation artifacts always exist with a box filter? AFAIK conflation artifacts are a product of the compositing process, not the filtering process.

If you have two non-overlapping shapes of the same color covering the plane and use a box filter on the first shape to sample a pixel on the boundary, and then use the same box filter on the second shape, and then composit them with alpha blending, you get a conflation artifact along the boundary where the background bleeds through.

But if you use the fact that the shapes are non-overlapping and sum their contributions instead, the artifact disappears, while still using the same box filter.

[dahart]

It’s because sampling artifacts never disappear with Box. The reason is the high frequency aliasing is introduced by the filtering. It’s because the Box itself has infinite frequency response that you cannot eliminate the artifacts, it’s not possible. This is why all other, better filters fade their weight smoothly to zero at the support boundary.

You can see this with a single sharp edge, it doesn’t need to involve multiple polygons, nor even vector rendering, it happens when downsampling images too.

[raphlinus]

These are sampling artifacts, but I believe yorbwa is correct in distinguishing these from conflation artifacts, as defined in Kilgard & Bolz. I think of the latter as compositing not commuting exactly with antialiasing (sampling). You only get conflation artifacts when compositing multiple shapes (or rendering a single shape using analytical area when the winding number is not everywhere 0 or 1), while you definitely see aliasing when rendering a single shape, say a Siemens star.

[dahart]

Okay, that’s fair. I’m misusing the term ‘conflation’ in that sense. I was trying to make the point that compositing two wrong answers yields a wrong answer, but I stand corrected that it’s not the compositing that’s introducing error, it is the sampling + box-filtering.