OpenVDB doesn't export temperature channel correctly. #55377
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Reference: blender/blender#55377
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System Information
Windows 10, GTX1080Ti
Blender Version
Broken: 2.79
Worked: Probably never
When exporting smoke simulation out of Blender using OpenVDB file, Blender saves a channel referred to as "Temperature" as "Heat" in VDB file. When imported into a 3rd party software, which can read VDB caches, a "Heat" channel, which is supposed to be "Temperature" channel renders drastically different result, and on top of that, lacks any resolution added by Wavelet High Res re-simulation.
Density channel, intended for smoke, not temperature, renders 1:1 in both Blender and 3rd party software, and the Wavelet resolution is present. Therefore, this is most definitely a bug in a way Temperature channel is written in VDB.
Furthermore, this channel MUST NOT be named heat, but Temperature. Not all of the 3D packages which can read VDB files allow you to select channel name. Some don't, and expect the channel name to be correctly named "Temperature".
EDIT: Update. So it appears that the "Heat" channel is not actually "Temperature" channel. Instead, "Temperature" channel, the one, which contains Wavelet resolution, does not even get exported in VDB at all. This is a big problem, because it's impossible to export VDB out of Blender correctly.
Added subscriber: @Rawalanche
Added subscribers: @kevindietrich, @lichtwerk
Not sure what the current status of interop with other software is, would kindly ask @kevindietrich to also have a look...
Looking at ptcache_smoke_openvdb_write we have:
density, (density_low), flame, (flame_low) fuel, (fuel_low), react, (react_low), color, (color_low), heat, heat_old, texture coordinates, shadow, velocity to be written.
[The XXX_low part obviously written when you have wavelet turbulence [high resolution] disabled.
Heat should be written if there are inflows with Temperature.
Not an expert here, so couldnt tell why heat is not subject to high resolution without further research...
That being said, I'll pass this on to @kevindietrich ...
Added subscriber: @MrJoMo
I have also recently noticed this. I'm working on a mixed platform pipeline to which I introduced blender. The acceptance has been good and people are excited about what blender can do. That said, not everyone is going to change overnight to blender. And for the time being I still have to export the final scenes (and simulations) to be rendered in Arnold. Not having a correct naming structure is causing several problems with the workflow and being able to sell the team on blender's capabilities.
Added subscriber: @dr.sybren
Changed status from 'Confirmed' to: 'Archived'
Although I agree that interoperability is a good thing, I don't think this is considered a bug in Blender, but rather a known limitation of its current design.
If it is decided that it's important to adhere to a standard naming scheme for OpenVDB channels, this naming scheme should first be documented, including a list of other software that adheres to it. That will give a better foundation for such changes.
Note that I'm intentionally ignoring part of the bug report here. If a report is about X, and later changed to also cover Y, that only causes confusion. If there is still a bug in Blender, please write a new report that only covers the bug. A guideline for making a good bug report can be found at https://wiki.blender.org/wiki/Process/Bug_Reports
In that case the bug will remain there until someone else decides to report it. I won't.
Being this strict in bug triage may help you get some weight off your shoulders, but at the cost of Blender continuing to fail as a part of pipelines in many use cases.