I have a shot that needs motion blur. I know that motion blur works, because I accidentally got it working… Once. But, since then I have not been able to recreate how. Can someone please help me out with a simple tutorial?
Because I could not let this go, here is how I spent my saturday.
V-Ray for Blender 7.4 Motion Blur Investigation (macOS, CPU)
I’m trying to determine whether I’ve found a bug, a configuration issue, or if I’m misunderstanding how motion blur is intended to work in V-Ray for Blender.
System
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Blender 5.2
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V-Ray for Blender 7.4
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macOS (iMac)
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CPU Renderer
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Rendering to the V-Ray Frame Buffer (VFB)
Test Scene
To eliminate variables, I created a very simple scene consisting of:
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Default scene
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Suzanne
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Camera
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Default light
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24 FPS
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Animation enabled (not single frame)
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Frame range: 1–24
Animation:
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Frame 1: Suzanne at Y = 0
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Frame 24: Suzanne at Y = -100
No constraints, no Geometry Nodes, no complex materials.
Camera
I tested primarily with a non-Physical Camera using the Motion Blur controls under:
V-Ray → Globals → Motion Blur
I also briefly tested camera movement with Camera Motion Blur enabled, but the results were the same.
Motion Blur Settings Tested
Duration values tested:
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0
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0.5
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1
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5
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10
Rendering Frame 12 in every case.
Observation 1
Changing Duration causes Blender’s current frame to jump forward when rendering begins.
Examples:
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Duration = 1 → frame jumps from 12 to 13
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Duration = 5 → frame jumps from 12 to 15
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Duration = 10 → frame jumps from 12 to 17
This appears to be approximately:
Current Frame + (Duration / 2)
This makes me think V-Ray is evaluating the shutter interval correctly.
Observation 2
Motion blur appears to be calculated.
The V-Ray log changes from:
MB triangles: 0
to
MB triangles: 2
when Motion Blur is enabled.
So V-Ray appears to recognize moving geometry.
Observation 3 (Main Issue)
When Motion Blur is enabled, the render never finishes.
The buckets complete normally.
However:
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VFB never reports Finished
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Blender continues to report the render as 0%
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The render never returns control to Blender
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It appears to hang indefinitely after all buckets are complete
The last lines of the log are:
SD triangles: 15744
MB triangles: 2
Static primitives: 0
Moving primitives: 0
Infinite primitives: 1
Static hair segments: 0
Moving hair segments: 0
Nothing is written after that.
When Motion Blur is disabled, the render completes normally.
The log continues with:
Maximum memory usage for attributes: 0.00 MB
Finished in 0h 1m 28.6s (100.0%)
Additional Tests
Tested both:
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Bucket rendering
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Progressive rendering
Both exhibit exactly the same behavior.
I also tested enabling Camera Motion Blur and animating the camera instead of the object.
No change.
Question
Has anyone seen Motion Blur cause V-Ray for Blender 7.4 on macOS to hang after rendering?
The renderer appears to complete the image successfully, but it never performs the final completion step or returns control to Blender.
Since the log reports moving geometry (MB triangles > 0), it looks like Motion Blur is at least partially functioning, but something prevents the render from finishing.
Hi Brad,
Thanks for the detailed report. You’re not doing anything wrong, and it isn’t a macOS or CPU-specific problem. It’s a bug on our side and I’ve logged it.
1. The frame jumping is expected. To render motion blur we export the scene at several points across the time the shutter is open, so the timeline steps through that interval. With Duration 5 and the default Interval center of 0.5, the interval around frame 12 runs from 10.0 to 15.0, which is exactly the jump to 15 you saw. Nothing wrong there.
2. Also correct. “MB triangles: 2” means the moving geometry is being exported for blur, so motion blur itself is genuinely working.
3. With Output set to Single Frame and motion blur on, we internally switch to animation rendering so we can export those extra sub-frames, but we then ask V-Ray to render your whole frame range (1 to 24) while only ever sending it one frame. V-Ray renders what it has and waits for the rest, which never arrive, so the job never reports Finished and Blender stays locked.
As a workaround for the moment you should be able to set Output to Animation instead of Single Frame. If you only want the one frame, set both the start and end frame to 12 and render as Animation.