Can't get correct velocity info from VDB file

[Seila]

hi,
I export the vdb volume file from houdini 15 and it contain density, temperature, color an velocity data data in it. I use VRayVolumeGrid to import this vdb file and in the field of "Cache Info" of VRayVolumeGrid node all these channels are recongnized. But i find only the output of velocity is not correct after rendering it out (I've tried to set the velocity attribute of vdb with floats vel.x, vel.y, vel.z or vector V but both of them not retrieve correct value). Is this a bug ?

Another question is: Can i append "fuel" attribute to vdb file with extra float info such that i can take use of it ? I've tried this but it not works and i wonder maybe the attribute name in vdb is not "fuel" but something else.

Thanks.

[ivaylo.ivanov]

Ok, will check out this velocity RE. The fuel channel is supported in the nightlies and for the next SP.

[ivaylo.ivanov]

You can try using the "Volumetric Geoemtry" rendering mode to get the velocity channels. The "Volumetric" mode seems to be unable to write in the standard Velocity channel.

[Seila]

I try to render with "Volumetric Geoemtry" mode but still can't get velocity channel info. All channels work well beside velocity channel.

[ivaylo.ivanov]

Are you using the official 3.1 release?

[Seila]

My VRay version is 3.10.01. Is this the latest version ?

[Paul Oblomov]

Velocity is of the same size as other channels ?

[Svetlozar Draganov]

My VRay version is 3.10.01. Is this the latest version ?

Yes it is the latest official version.
Is there a chance to send us that scene for investigation?

[Seila]

hi Svetlozar,

You can get the maya file, VDB file and the corresponding houdini file that generate VDB file here:
https://drive.google.com/open?id=0B7...C1yaVg4dWZicVE

There're three problems:
1. The velocity information has effect on motion blur but the direction seems opposite (the velocity value i set for volume is +100 in x direction but the result motion blur take on -x direction)
2. Since the velocity attribute is embedded in volume i want to use it to tint the color. I set the "Diffuse Input" parameter of volume grid to "Speed" but it seems not work at all, is this a bug ? or Speed not means velocity ?

And a extra question: If i use a 3d texture on volume grid, how to fix the pattern as the volume object move through the space ?

Thanks.

[ivaylo.ivanov]

1) It's working correctly. Your object moves in +x world coordinates, but -x in screen space which is used by the velocity render element. Test with a sphere for example, you should get the same thing.
2) This is a bug. The speed channel is handled a bit differently and it seems we never made it work with anuthing but Phoenix FD caches. Will make a note to fix this.
About the 3d mapping, it's a tough question. The best method (which also has flows) is to generate uvw coordinates in the simulator and use them for texture sampling. Maya fluids and the VolumeGrid support this option - "coordinate method" which can be changed from fixed to grid.