Vector motion blur causing alpha to not be fully transparent

[kariomart]

egg_test_v002.c4d
When I turn on Vector Motion Blur, my render does not have a fully transparent alpha. When I turn it off, this issue goes away. How can I render it with motion blur but without this problem? I've also tried rendering with straight alpha turned on and off.

[Dr. Sassi]

Hi kariomart,

Thanks a lot for the file; very helpful. I need to explore this a little bit more.

Yes, the 11.1% gray in float is there as long as the Motion Blur Tag is active. I have to write a report.

Some observations:

The Frame rate is 48, and the Camera exposure is 1/30. If you want to have filmic motion blur, then 1/96, or pure video-ish, then 1/48, would be advised. I left the Shutter Efficiency at 70% for the demo, but that can go higher for video-ish.

Either way, to see what contributes to the milky alpha, switch to 32bit/float and Open EXR while setting the Shutter to a full Second to see what is happening. (8bit/channel, gamma, sRGB, and png are significantly limiting.)
The image below is slightly adjusted for demo purposes and web quality.

Frame 75

Workaround (not ideal!): Render with 100% Luminance material, not set to straight alfa, an "A" pass. Then, with the colors of your materials, a Straight alpha-based "RGB" pass. Keep this in 32bit to avoid any gamma on the "A" pass.

All the best

[Edited dec11 23:51pm]

[kariomart]

Hi Dr. Sassi,
Thanks as always for the help! Is this workaround what needs to be done whenever rendering motion blur with an alpha?

[Dr. Sassi]

Hi kariomart,

No, it should not be needed at all. It should work. Please have a look below.

I have filled out a report and hope it will improve.

I had to stop last night at midnight, as we had our first meeting at 7 am (L.A. time). Now, I try to catch up after a busy, long day.

I explored your file a little bit more. So, here is my suggestion: it might render longer, but it needs a higher Sample radius for this file.

Try "eight" as the start value and measure in the Picture Viewer (CMD+Right mouse down) with the Alpha in the view.

That should work; if not, "ten" seems stable. I have not tested the whole file; I have only a laptop here as the fastest machine.

Sorry for all the waiting time.

My best wishes for your project