Motion Blur doesnt work with transparent Halo Material

Hello artists!

I’ve got a scene with fast moving particles that have a halo material assigned to them. The alpha value is animated with an IPO curve.

When I activate motion blur it works perfectly as long as the alpha value of the particle’s material is 1, but as soon as the alpha value decreases the motion blur is completely gone.

Do I have to change a setting here?
Is it a bug?

Please let me know, if the .blend file is useful for you, I’ll upload it if you want me to.

This is a known issue (for some time more than a year now) that happens with FSA also. It apparently isn’t a priority with the development team so you’ll just have to learn to live with it or fix the code and submit it as a patch.

Work-around:

  • Render nothing but a pass with the motion vector data, using Alpha of 1 for the whole thing. Save those motion passes as RGB image sequence.
  • Go back, do your decreasing alpha thing, and instead of using the “live” motion data, bring in the image sequence.

This gives you motion blur data from the first pass, which can be used to generate the vector blur on your “beauty” render.

Since I’m not good at writing code I really appreciate your workaround.

Thanks a lot, I’ll try it this afternoon.