Mastodawn

LR Oct 4, 2023

mh. stairstep effects in in shadow mapping are just a symptom of inaccurate z-buffer rasterization. the z-value chosen per pixel should be the maximum depth of the intersection of frustum pixel and triangle, not the average or center value. then the stairstep effects will vanish. applying a bias instead is a terrible solution.

you can see below how the error is caused by taking the midpoint as depth; if the max depth were taken (red lines) we'd have no issue.

Show thread

LR Oct 4, 2023

if we had normal information for each pixel in the shadow map, then taking the center depth value would probably be sufficient, as the plane at that point can be reconstructed from the normal, and we can test points against per-pixel planes. then a much smaller bias can be applied.

also, from the same plane, the max depth value can be reconstructed.

Show thread

Ben Golus Oct 4, 2023

@lritter But that's what normal biasing is supposed to be for. Of course it's usually applied at the per-vertex level, so it's not super accurate. PolygonOffset / DepthBias also exists which the slope factor could theoretically be abused to fix this, if the implementation was consistent. Or calculate it manually in the fragment shader and use depth writes. But then you're giving up a ton of performance by adding pixel shaders to the shadow map generation.

Show thread

Ben Golus

@lritter I conceptually prefer receiver plane biasing as it doesn't introduce any significant performance penalties... if I could ever get it to work properly.