Major: Merge primitives by material not working at scale

[avan069]

"Do not join same materials" option (preferences) must be selected or many parts in a complex model will have a blank texture or not be exported. When selected, that option results in all parts and materials appearing as expected. The result is consistent across multiple repeated exports without changing the .blend file, but is otherwise inconsistent when changes are being made to the model between exports.

Forces user to select "do not join same materials" and export with very high draw call counts.

[avan069]

Changing parenting of exported parts seems to avoid parts missing from export (perhaps they were being exported a million miles away due to some transformation issue?) Does not appear to affect the missing material problem.

Example of a material exporting with a blank/white/missing(?) material... note it does not have the multi-colored "missing texture" present when exporting without a material assigned.

[avan069]

This is pretty deep. Exporting the part as an OBJ and reimporting it doesn't help. Repeating that process but exporting OBJ without UVs, retaining only the geometry, STILL has same result as above when exported.

Also, the white parts in an export, when testing with "do not delete export collection" checked, have no UV data. All verts at UV origin. They do still retain the correct material prior to export.

[avan069]

The material is not missing. It just happens that the 0,0 pixel of all my albedo textures is an off-white color. Looks like it's strictly an issue of UVs being destroyed during export.

[avan069]

Got it.

Cause:

• When joining meshes (object.join()), Blender was dropping UV data for meshes that did not have the same UV map name (obj.data.uv_layers[x].name). If all UV map names in the export collection are the same, all UVs export properly.
• Likely ending up with multiple UV map names (and multiple UV maps) due to various workflows/plugins that use multiple UV layers. Unlikely to be able to avoid users' choice of workflow.

Solution:

• Simplest: ensure the plugin renames all UV layers (probably to default: "UVMap") prior to object.join()
• Another option: After join, remove/collapse extra UV layers so only one real UV map remains.
• Maybe should: check to_mesh() call to ensure UV data isn't dropped prior to join?

NOTE: exporting with "join materials" still results in occasional missing objects, specifically: parented childen. If there are no parent links used, everything is visible. Some objects when parented are in the export collection but are not visible in BML. It almost certainly relates to unapplied transformations in Blender