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Subject: Volume ray marching with motion blur?



How can I get motion blur when volume rendering through ray marching?
The volume I'm rendering is a voxel grid. The grid doesn't move, but
the values in the grid change over time based on a navier-stokes fluid
simulation. I can think of 3 possible methods to get motion blur with
ray marching:

(1) Interpolate the grid values at times t and t+1 to get inbetween
grid values. Then render at many fractional time steps and average the
results. However, this will only reflect change that occurs in place,
not motion.

(2) Run the simulation that generates the grid data at a small time
step (less than the frame rate), thereby getting sub-frame data. Then
render all grids that temporally fall within a given frame and average
the results. However, this would obviously be slow (more simulation
iterations).

(3) Average the fluid velocity in the grid along each ray. Then
assign the average velocity to the pixel the ray was shot through.
This should allow for 2D (pixel-based) motion blur in software such as
RenderMan, I think. However, the motion blur would be a gross
approximation. It would be nice to be able to do 3D motion blur.

Can anyone think of any other approaches I could try? Or are there
known techniques that I'm now aware of?

Thanks in advance for any info!

~Jon


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