20150718 - Stochastic 1 Sample/Pixel Lit Fog stills
++ +Found a the old stochastic 1 sample/pixel lit fog. +Left the post process grain on, it is smoother in practice. +This could be an algorithm which only looks good in this demo, never really tried adjustments on other content...
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+General idea is to stochastically select a z value/pixel between the eye and the opaque backing z value based on the volume of material in between. +In this demo I just used a very large sphere of volumetric stuff behind the center sphere. +Each {z} point is shaded and "lit" (fake in the demo), and also has some opacity value. +Then there is a separate spatial+temporal filter process which attempts to remove the noise from the extremely sparse volume sampling, +and also correctly manage un-occlusions, etc. +The volume is treated separately from the opaque layer and blended together before the final temporal noise reduction pass (scene is traced). +The demo was running at 120Hz, and didn't ever look right at 60Hz. +These temporal techniques are all about visual masking of artifacts in motion, +so they tent to be highly tuned just to the point of perceptual artifacts at a given target frame rate. +The one takeaway from this little project, +was so weight samples in the filter based on similarity of their backing opaque z value to center backing opaque z value. +This tends to maintain an even gradient based on objects which are at a similar distance from the eye. +Which is what one would expect in general for fog volumes.
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+Something I didn't try but would help here is to decouple volume density sampling (aka alpha value) from shaded color. +Run alpha computation at a higher sampling rate, then mix together later...
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+ + +Algorithm +
+Runs a spatial filter on {color,alpha} with 13 taps in the following pattern,
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+ +. . . . . . . . .
+. . x . . . x . .
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+. . x . . . x . .
+x . . . x . . . x
+. . x . . . x . .
+. . . . x . . . .
+. . x . . . x . .
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+Pixel weights are "gaussian * f(sampleOpaqueBackingZ,centerOpaqueBackingZ)", where f(s,c) decreases weight as opaque z-buffer value becomes non-matching +(filter intent is that fog tends to have similar effect when the opaque backing is at a similar distance away),
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+r = min(c,s)/max(c,s);
+return (r*r)*(r*r);
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+Runs a second spatial filter with 13 taps in the following pattern,
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+ +. . . . . . . . .
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+. . . x x x . . .
+. . x x x x x . .
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+Pixel weights are "gaussian * f(sampleOpaqueBackingZ,centerOpaqueBackingZ)", where f(s,c) does something similar,
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+r = 1.0/(1.0+abs(c-s)/min(s,c));
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+Cannot remember why these two spatial filter passes have different z based weighting functions. +Turns out the temporal filter has another depth weighting function. +They both have some fixes for when depths are zero which I didn't bother to copy in. +The temporal filter reprojects 5 points in a packed + pattern. +Want to use reprojected Z (project reprojected backing Z into the current frame). +Does a neighborhood clamp, +then has reprojection weights based on "gaussian * f(sampleOpaqueBackingZ,centerOpaqueBackingZ)", where f(s,c) does something similar but with a depth bias,
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+r = 1.0/(1.0+abs(c-s)/c);
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