Add ability to output color information in obj files.

[brugger1]

Is your feature request related to a problem?

Here is the e-mail request from Daniel Ross motivating the request.

From: Ross, Daniel rossdb@ornl.gov
Sent: Wednesday, August 14, 2019 6:17 AM
To: Brugger, Eric brugger1@llnl.gov
Subject: Visit image saving

Eric,
It has been a while since I contacted you I was hoping you could point me to some documentation on how to export the image produced from data into a obj file with the coloring still applied.
We have received the funding to upgrade our Virtual reality cave and the next generation will be using Unity or Unreal engine to push the images into Virtual reality.
Of course both packages mainly use mainstream CAD/CAM software and don’t have a plugin for visit. However, it would be handy to be able to output the results into something I can then render in either Unity or Unreal engine.
I have been able to save the image produced int a obj file but it does not contain any of the coloring which is actually needed to show heat distribution. However the png and jpeg are literal photos that lack any dimensional aspect and are useless in a virtual environment.

Thanks for your help

Daniel

Impact

• High - User productivity significantly improved
• Medium - User productivity partially improved
• Low - User productivity trivially improved

[biagas]

Our current writer (using vtkOBJWriter) cannot do this.
vtkOBJExporter may do this, but it requires a different mechanism than the writers. The input is taken from a render window, not a dataset.
I would have to see specification of OBJ files to even understand how color information should be stored.

[markcmiller86]

This update added color support in the reader. I suspect we can do something equivalent to the writer though I don't know where the color information would get associated with the VTK dataset objects passed to the writer.

[biagas]

Color information isn't associated with the dataset. Scalars are mapped to colors by the Mapper before rendering. Or the Actor has a single color stored in its Property if coloring isn't from scalars.

[markcmiller86]

Color information isn't associated with the dataset

What about material colors? We have ways for databases to specify colors to associate with materials (Silo and WavefrontOBJ already do this) upon read. Can't we pass material color information (which either comes from the database on read or which gets defined in VisIt during display of colors either by default material color or by user adjusting coloring in filled boundary plot) along to an export operation?

I guess I was trying to say that with the exception of how to associate colors with the dataset during export, I think the format details are known and already demonstrated with examples in our test suite.

[mclarsen]

Simple exapmple of a .mtl file:
https://people.sc.fsu.edu/~jburkardt/data/mtl/mtl.html

Another option would be to output texture coordinates (1D) associated with the vertices that correspond to the color map location (i.e., normalized scalar value). That would allow the end user to apply any color map in whatever tool they are working with.

[markcmiller86]

So, an mtl file is what the reader currently uses to read color information too.

I hadn't considered this approach, @mclarsen. Are you saying that the right answer is to allow the user to issue two sep. saves...one for the obj data to a .obj file and the other to export a VisIt color table to a .mtl file? Well, wait, that won't work either because the .obj file needs to be annotated with references to colors in the .mtl file.

So, as @biagas I think has described, the trick is getting the color values (or their indices into the color map) associated with each vertex (or facet) out to the file. That association happens so deep down in our rendering processes that it suggests this operation is more like a save than an export.

[mclarsen]

You would need both. I am going to change my suggestion to be in favor or texture coordinates, since it much more flexible. They can map whatever they want on the the mesh.