I have a scene with motion blur enabled, and some particles emitted from a volume. The particles only move very slowly; they're basically static (no new particles over time). I've baked everything, and when I render all the frames look fine except the first one, which has long motion-blur trails. I use motion blur with 50% shutter and "centered on frame". If I switch to "starts on frame" everything is fine. Is this a bug, or am I doing things wrong? Blender 2.91 nightly, but also happens on 2.83.

Test blendfile here. Load it, render frame 1, you'll see long motion blur trails. Render frame 2, it's OK. Then go back to frame 1, set Scene > Motion Blur > Position to Starts On Frame, and re-render frame 1: all OK!

  • $\begingroup$ share the blend file. $\endgroup$
    – Sanbaldo
    Jul 25 '20 at 15:21
  • $\begingroup$ @Sanbaldo: edited the post to include test file. $\endgroup$
    – GaryO
    Jul 26 '20 at 1:01

Start to render from frame 2 and you are ok (I can't find the "starts on frame" setting)

"Start on frame" means that the shutter opens at the start of the frame and closes after the % of 1 specified in the "shutter" attribute.

The problem is that there are no particles at Frame 1.. first emission particles show up at 1,00000001 frame.

that's why you have blur on first video frame.

Don't know why the first static render is ok.

  • $\begingroup$ I can't paste an image here, but the "starts on frame" setting is here: pasteall.org/pic/5db0b424e1d1425eac2ef1cf8977c82f (and yes I know I can ignore the first frame, but I'm trying to build an automated render pipeline, and anyway it shouldn't have those streaks.) $\endgroup$
    – GaryO
    Jul 27 '20 at 15:31
  • $\begingroup$ Ah, sorry -- "Starts on Frame" is under Render Properties (camera icon). I was confused by the section header that says "Scene" there. $\endgroup$
    – GaryO
    Jul 27 '20 at 15:33

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