Subject: Re: Bkg Anti-aliasing
From: Allen D Hastings <[email-masked]>
To: [email-masked]
Date: Tue, 23 Aug 1994 04:05:24 -0700

>> >decreasing the shading rate on motion blurred objects :-)
>
>Shading rate is the frequency at which shading calculations are made for
>a surface. In LightWave and most other renderers, the shading rate is
>essentially the same as the sampling rate. For every pixel (or sub-pxel)
>that a geometry calculation takes place, there is an associated shading
>calculation. However, this need not be true as in the case of Renderman
>where the two rates are handled independantly. By decreasing the shading
>rate of a motion blurred object, you would still be calculating the geometry
>to a fine degree, but the heavily blurred surface color would be sampled
>far less frequently.

Actually, LightWave is already doing something like that!  Usually the
Adaptive Sampling option renders one sample per pixel and then looks for
edges where additional samples are needed (by limiting the extra samples
to known high-contrast areas, the option speeds up rendering).  However,
if Motion Blur is turned on, that method can't be used, since the edges
found after the first pass are only known to be valid for one particular
instant in time.  So in scenes with motion blur, the Adaptive Sampling
option has a different meaning, which is more like what Mark described
above - exact shades are only calculated for every other pixel (the in-
between pixels are interpolated), unless the colors of a pair of pixels
differ by more than the threshold (in which case the in-between pixel
is calculated exactly).  If you crank up the threshold to the max, you
essentially get a halved shading rate.  This doesn't hurt the image much
at all, since even with low (5 pass) antialiasing, pixels are getting an
average of at least 2.5 shading samples.  In fact, the quality is better
than with edge detection (where many pixels only get one sample), bu
the speedup is lower.  This alternate interpretation of the Adaptive
Sampling option is also used when Depth of Field is turned on (because
edge detection would only be valid for one position within the iris of
the camera), and when antialiasing is completely off.

-AH