Problems with openvdb files

[simonbjork]

I'm having some trouble rendering a smoke simulation from Houdini. I want to fade off the "thin" parts of the smoke (low density) and assumed I could use the opacity gradient on the VrayVolumeGrid for this.

1. I don't understand the gradient. Is it based on smoke density?
2. The small values are really hard to work with. I guess the remap controls should be used, but see issue 3.
3. If I just nudge the left key above 0.0 the whole render becomes solid. Why is this? Becuse of the small values?
3. Show info button and guess input doesn't work with my vdb files. Show info reports 0x0 and guess input does nothing. Is this broken?
4. When I have tweaked the gradient to look good, and I try to render, the result is changing a lot from frame to frame. Is this expected? Or is this once again becuse of the low values that it's so sensitive?

I tried to use the OpenVDB command line tools to extract some info that I could use in the input offset/scale, but I don't know what to make of it?

* I tried to upload a sample vdb file, but the forum says it's to big. There's a link in the Skype group where a sample could be downloaded.
** I'm only rendering smoke, no fire.

[vladimir.nedev]

1. I don't understand the gradient. Is it based on smoke density?

Yes, the X axis is the value in the grid channel ( Smoke in your case, which comes from the "density" channel in the vdb file, this connection is specified in the Volume Grid Channels tab).
The Y axis is the opacity.
The offset and scale are used to remap the grid values.
Maybe the docs have more info on this.

3. If I just nudge the left key above 0.0 the whole render becomes solid. Why is this? Becuse of the small values?

Most of the grid has density=0, if you associate this value with non-zero opacity, you will get a solid cube, I think that's expected.

3. Show info button and guess input doesn't work with my vdb files. Show info reports 0x0 and guess input does nothing. Is this broken?

Yes, sorry, both buttons were broken. I fixed them for the next nightly build ( 2020.08.22 ).

For your file, the values are : offset=0, scale=6.9811, for the smoke/density channel.

4. When I have tweaked the gradient to look good, and I try to render, the result is changing a lot from frame to frame. Is this expected? Or is this once again becuse of the low values that it's so sensitive?

I don't know, I don't have much experience using the volume grid.
Maybe Boyan can help.

Greetings,
Vladimir Nedev

[vladimir.nedev]

2. The small values are really hard to work with. I guess the remap controls should be used, but see issue 3.

You are supposed to use values between 0 and 1 in both X and Y, as far as I remember.
I am not sure what you mean by small values.

The offset and scale, if setup properly (the "guess" button) will remap [min, max] values from the grid to [0, 1], so they fit inside the gradient control.

Greetings,
Vladimir Nedev

[vladimir.nedev]

There is some info in this post of mine from when the V-Ray Volume Grid was added in V-Ray for Modo : https://forums.chaosgroup.com/forum/...960#post752960

Greetings,
Vladimir Nedev