C4dtoa: How to get proper motion blur from alembic out of Houdini?

[lurklurkson2000]

Hello Community,

for some time now I am trying to get a centered motion blur out of houdini into c4dtoa.

My settings in houdini for the Alembic ROP Motion Blur is

Samples: 8

Shutter -0.25 / 0.25

In c4dtoa I enable motion blur and get the following results

Motion Blur turned off:

Motion Blur enabled, 8 Keys, Shutter centered on frame with -0.25 / +0.25:

However if I just set it to start or end of frame, I get proper motion blur:

Am I missing something to get proper centered motion blur?

Does it make any diffrence in the animation in the end, if I use center, start or end?

Thanks a lot in advance!

[peter.horvath6V6K3]

Looks like the shutter in the scene is wider. Do you have a camera in the Alembic file, or do you use the scene camera? Can you send me the Alembic file, so I can take a look?

[lurklurkson2000]

Hi Peter, thanks for your answer. Here is a demo file that recreates the issue:

https://www.dropbox.com/s/6svszo662bv7pqc/demo_abc.abc?dl=0

[peter.horvath6V6K3]

Your file works fine with the Arnold Procedural, but not with the native Alembic import. Can you confirm that you are using the native import? It seems that the keys in the Alembic file are not equally distributed and the C4D Alembic object does not do any interpolation. Therefore the object has the same position in multiple samples which messes up the motion blur. The Standard Renderer with Sub-Frame Motion Blur gives the same result.

[lurklurkson2000]

Thanks Peter, that did it. Quick question on Alembic Procedurals in general: Can I use the Arnold tag on Alembic like I would on geometry, concerning subdiv and displacement and so on?

[peter.horvath6V6K3]

You can not use an Arnold tag with the Arnold Procedural object. With the new operators in Arnold 5.1 you are able to override any objects within a procedural, however it's a bit too technical at the moment. For instance you have to know the object name within the procedural and the parameter names on the Arnold node.

Here's a simple tutorial for overriding shaders: https://support.solidangle.com/display/A5AFCUG/Procedural+Overrides

[lurklurkson2000]

I just tried it out and it seems to work if I just select "show custom parameters" and then set the subdiv and displacement. looks correct from the ipr...

[peter.horvath6V6K3]

That's magic than. : ) The Arnold tag is not exported when assigned to an Arnold Procedural object. (If you import the Alembic via the native importer, than you can use the tag on the geometries, but I suppose we're talking about the Arnold Procedural object.)

[lurklurkson2000]

yeah that was too early, the shading works but subdiv and displacement not... Any tips on how to override that parameters? Thank you so far!

[peter.horvath6V6K3]

This is an example to enable caltclark subdivision with 4 iterations, and overriding the displacement height. You have to know the path of the object within the Alembic file, you can check it when importing via the native importer for instance.

The available polymesh parameters:

Hope this helps.

[lurklurkson2000]

awww yes Peter, THANKS!

[lurklurkson2000]

Ok I think I got almost everxthing to work, the only thing I dont get is how to specify the disp_map in that syntax. It doesnt accept it however I write it...

[peter.horvath6V6K3]

See this thread: https://answers.arnoldrenderer.com/questions/5350/how-to-link-files-in-overrides.html

[lurklurkson2000]

@Peter Horvath Sorry to consult you again, but it seems the motion blur does not work on the alembic. It somehow reads always the same deformation, even if the mesh is not moving. There is always the same motion blur, no matter how the mesh is moving. After an intense research the last days I found out, that the alembic writer of houdini is writing the velocity as a point attribut on the points. Is this somehow interpretable by c4dtoa? And if not, any ideas on how to make it work?

[peter.horvath6V6K3]

I think the Alembic procedural reads the point attribute, so that should work fine. Do you have an example which shows the problem?

[lurklurkson2000]

The one I linked earlier in this thread also does it. The motion blur stays the same no matter where in the animation...

[lurklurkson2000]

I finally fixed it. Had to write our the .abc from houdini and change there the velocity attribute from points to vertexes. Also the native 'motion blur' option from the houdini rop_alembic had to be disabled. This made the distinct shapes in the motion blur dissapear.

[peter.horvath6V6K3]

Hmm interesting, I think we should support point attributes as well. Can you send me the Houdini scene (hip) showing the wrong and good settings, or the two abc files, so we can compare them?