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Subject: Re: field of view math

Daniel wrote:
> Hello All,
> I'm using a 3d renderer that does not allow for motion bluring FOV
> changes (prman). I'd like to simulate this with a 2D filter, I think it
> should be possible.
> Right now I'm trying to construct a vector image where each pixel
> describes a dir and mag of where that point will move to in screen
> space.. then use this vector image with a vector blur operation in a
> compositing package to smear my final image.

Some problems:

1) Trajectories of objects are not straight segments; I don't know how
bad the approximation looks in practical applications.
2) Parallax of objects projected to the same screen position from
different scene positions can cause errors; the vectors you are trying
to compute can easily intersect one another, and objects can move
between the same projected directions along different paths.
3) Transparent objects cause colour leaks: the blur of the foreground
object picks up some background colour.
4) Linear interpolation of lighting isn't correct. During object
movement, incidence angles change.

The obvious, easy, correct and common approach to rendering motion
blur, especially for offline rendering as in your case, is temporal
oversampling; can't you afford to render more than 1 or 2 images per
frame of animation?
Oversampling can be adaptive, rendering more samples in the frames
and/or composited layers and/or image regions that contain fast-moving
objects, and proportionally to movement speed.

Lorenzo Gatti


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