Issue overview: recently I have been using the H18.5 rbdbulletsolver workflow, particularly for glass destruction using the rbdconnected/disconnected faces to hide the prefactured faces. It has worked very well for many shots when exporting the alembics separately for example: Show_shot_fx_glass.abc and Show_shot_fx_frames.abc. However on this show the lookdev guys are requiring it to be merged together into a single alembic before export so that it has the identical path and hierarchy in Maya. After alot of fanagling I got that all to work by deleting all groups and unneccessary attributes, enabling path att, and motionblur etc. The uvs are coming through correctly with inside faces transformed to the second tile as requested. Rbddisconnectedfaces is working as planned. Velocity is there as a point attribute. The problem is that the motionblur (mb) is not working. In one iteration mb worked for one layer, and not the other. Now it is not working for either layer. When I load the alembic and try to render (vray)I get warnings about changing vertex counts. I read somewhere online that I might have to promote the v to a vertex attr with attribute promote. I tried that but still no luck. When I loaded it as a vray proxy the motionblur worked, so the v data must be getting in there! Problem is that lookdev doesn't want to change their workflow. Hoping someone has had the same/similar issue and can point me in the right direction, or provide some food for thought. thanks in advance.


Houdini 18.5 workflow

Glass destruction prep

-Geo Input: alembic model with glass and frame layers separated with blasts by name
-layer1 glass: rbdmaterialfracture/glass (named glass)
-layer2 concrete: rbdmaterialfracture/concrete (named frame)
-unwrap and transform inside faces to second udim
-rbdconnected/disconnected faces

sim: rbdbulletsolver (SOP level)

ouput: rbdio>>transformpieces + rest

split output back into two layers via blast: @name=glass* @name=glass* not
for both layers process before export:
-attribute delete* ^N, ^v, ^path, ^uv etc
-group delete (all groups)
-transform x100
**edit** clean sop with "remove unused points"
-merge both layers into same stream then export as alembic:

Alembic ROP
-path attribute: path: build heirarchy from attribute
-Motionblur: check "use motionblur"

Load alembic in Maya
-path is correct
-heirarchy is correct
-uvs are correct
-enabled motionblur in overrides
-Maya is throwing a warning about changing vertex counts.
-Looked into promoting the v to vertex using attribute promote no luck
-mb works with vray proxy but not when loading as a standard cache/alembic import

hi
I have had this problem before

you can convert velocity to color attribute by attribute promote and in maya and in alembic propertis in Introduce the Cd attribute.

Thanks this works. I never had to use except for redshift with a flipmesh, but really good to know that it works for RBD with disconnectedfaces too.

Cheers

If you do not change the topology, motion blure works in all software and rendering engines.
But if you have a topology change (like flip) for the correct motion blure you have to have an output attribute taken from Houdini.
sajad alavi

sajadalavi
hi
I have had this problem before

you can convert velocity to color attribute by attribute promote and in Maya and in alembic properties in Introduce the Cd attribute.

I have the exact same problem and I've been all over the internet to find the answer, your answer looks promising but I'm a bit of noob here, so if you can be a little bit more specific about how to do it step by step.

I have a ROP particle system and I want to import it to V-ray Proxy using alembic, it works except the motion blur,
my problem with your answer is specifically :
- how to change the velocity channel to color in attribute promote node and is it different from doing it using attribute rename?
- what are the steps in maya-vray; how to introduce the Cd Attribute.

thank you very much
my current node graph for exporting the ABC


My Alembic MeshInfo in VrayProxy:

Number of frames: 240
Number of voxels: 2
Resolved frame: 78
Preview has 0 triangles, 0 hairs, 80914 particles
Bounding box: -
UV sets: 0
Color sets: 1 ('velocity')
Shader sets: 1 ('/geo/attribpromote1')

I have exactly the same problem but I couldn't get it to work by using the same process with this thread. I want to ask how you all and @pskaifa pulled it off?

I can make Arnold for Maya read the velocity data that converted into point float color data and render the motionvector AOV successfully. But not for V-Ray for Maya.

I tried to convert Houdini velocity into vertex float color and read it in V-Ray using VRayUserColor/VRayVertexColor, but couldn't make the velocity Render Element to translate it correctly.

EDIT: Nevermind, I think everything I did was in the right direction, except I totally missed the part that people are using VRayProxy to load the Alembic cache, not using Maya's native Alembic import. Thanks for the experience shared by the community!

I am hitting this wall today with Redshift.

Followed this tutorial but still no motion blur in Redshift.

https://vimeo.com/254225706 [vimeo.com]

I am wondering if I still need to specify something on the Alembic Rop node as well.

This is a little late to answer Intuition's question, but I dug into that tutorial myself, and it seemed to work (thanks for the link). The most important thing for going from Houdini to Maya via alembic is that your attribute has to be not just a vertex attribute, but also one that is represented as RGB. If you just promote a velocity value straight to a vertex attribute, it's still going to be an XYZ attribute. It has to be RGB for Maya to read it correctly. I'm not sure if there's a way to specify that a vector attribute is RGB, but just assigning @Cd the velocity values will automatically make this value an RGB. Then when you promote it to vertex, the new vertex attribute is automatically RGB, regardless of what you name it. This is great in if you want to pack in more than just one color set, you can just repeat the process.