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20161023 - Sphere Tracing AA
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General idea for anti-aliasing with sphere tracing. The end result of this would be a very analog film feeling image with grain...
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Ray Directions - Shoot one ray per pixel and use a good temporally changing low discrepancy sampling for ray direction.
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First Hit - Trace forward until 2D projected distance to surface is within pixel diameter.
+I'm using diameter here because one wants a thin feature to show up as partial coverage across at least 2 pixels (to get good AA).
+Likely this distance needs a tuning knob, best might be half between pixel radius and diameter, depends on visual preference.
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Coverage - Compute pixel coverage and store coverage with ray output.
+This part I'm speculating on and would need to try to see what works.
+Initial idea: step one more time through the surface,
+then take the last 2 distance samplings and try to estimate the actual surface,
+sample the distance function at the estimated surface,
+take this final distance as "coverage"
+(on surface = full coverage, reduce coverage as distance increases).
+Exact coverage isn't as important has having coverage approach zero as ray passes a surface without getting near enough.
+Error on surfaces will just show up as temporal grain, which is visually pleasing.
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Reconstruction - For each pixel take nearest 11 rays.
+Take a weighted sum of ray colors where weight is
F(DistanceFromRayToPixelCenter) * Coverage then divide by sum of weight.
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F() function is the filter kernel, where fastest (but not highest quality) is the Gaussian,
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exp2(-scale*dst*dst).
+Could also do disk average (return 1.0 under a given radius, and some smooth fall-off outside that radius).
+Note
DistanceFromRayToPixelCenter is constantly changing temporally due to low discrepancy sampling.
+Also nearest 11 rays is a fixed pattern,
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+ XXX
+ XXOXX <-- 11 sample pattern
+ XXX
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Grain - After reconstruction add some grain to help mask the temporally changing sampling pattern.
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