I just can't get vector motion blur to work right (RSMB, Blackmagic Fusion)

[Paul-Simon]

I've been struggling for many months to get motion vectors working, and I just never seem to be able to.
No matter what settings I choose, no matter which tutorials I watch, and no matter how many forum posts I read - vector blur remains completely impossible to use.
To be more precise, the blur always goes in the wrong direction. Usually it's diagonal, regardless of whether or not the subject is moving.
Yes, there's a lot of writeups and videos on people having similar problems, but NONE of the supposed fixes have worked.
Even Coronas official YouTube channel has a video on how to use it with Blackmagic Fusion, aaaand surprise surprise, it doesn't work. I've tried everything I've come across.

I really want to be able to use vector motion blur via RSMB in After Effects with some of my projects. If it's just straight up impossible in RSMB for whatever reason, I can also resort to using Blackmagic Fusion.
Can someone explain what the workflow is to achieve... any sort of working result at all?

[pokoy]

If it's really broken, please submit a repro case together with a bug report, now is the time to make sure it's working correctly in Corona 2.

Also, check with the same scene and setup against another renderer like vray/mray/arnold. If Corona's output differs than it's clear it needs to be improved.

[Paul-Simon]

I get the feeling that I'm just not competent enough to figure it out, rather than it being flat out broken and worthy of a bug report.

[maru]

Can you write here a step-by-step explanation what exactly you are doing, what is the result, and what is the expected result? That would greatly help our team understand the issue, and fix it if it's broken. Maybe recording a video, or taking a series of screenshots would be the easiest solution? Whatever works best for you! :)

[TomG]

Another key question would be which version of Corona you are using - a long time ago, the Vector pass did not work properly (maybe back in 1.4 or 1.5) but should have been working since then - as you mention, the YouTube tutorial shows it working as expected.

[Paul-Simon]

Alright, here's some more details to explain what I'm trying to do here
I'm using Corona 1.7 (oct 2017 build) with 3DS Max 2018.

1 - I made a quick and simple animation with sporadic movement by making some teapots fall down on eachother


2 - I enable the vector blur pass, and run with default settings (apart from turning off filtering)


3 - I render the vector blur pass


4 - I export as 16bit .tiff w/ transparacy, and open in After Effects where I apply the RSMB Pro Vectors effect, and the resulting blur looks like this:


I expect the blur to somewhat match what the 3D blur would output, but this is not working correctly for me.
It would be nice if someone would share some insight in how to get it to work with RSMB, if possible. I don't really understand exactly how this works.

[TomG]

Could be the same issue as this thread https://forum.corona-renderer.com/index.php?topic=16585.0 ?

To quote from that thread:
'
 "Corona outputs vector data with 0 velocity as black - so values go into positive and negative direction from there. Reelsmart expects data with a midpoint at grey (from the old days when nobody knew what floating point data is). "

You could inspect the "black" areas to see if there is data there (right click on the VFB to inspect the pixel), negative values in effect that won't show (and is not what RSMB is expecting). I can't test, but adding an offset of 0.5 in the Velocity pass settings should correct for that - anyone with RSMB able to test? Also, just to clarify as you have mentioned Fusion and RSMB, it is RSMB that you are using?
'

Let us know if the offset fixes it. Cheers!

(EDIT unfortunately not able to test as RSMB is not a free plugin, but it solved the issue in that previous thread for the user that was having problems)

[Njen]

FYI, it's not just an offset you may need, but also a 'fit range' as well. For value maps with a mid point of black, usually the range is -1 to 1, which is a total range of 2. And for value maps with a mid point of grey, usually the range is 0 to 1 with a total range of 1.

So in short, perform the fit range calculation first (-1 to 1 becomes -0.5 to 0.5), then offset by 0.5.

[maru]

Have you tried saving the vector pass to 32-bit exr?