Python API allows setting negative material indices #105577

New Issue

Thomas Barlow · 2023-03-09T02:50:12+01:00

Thomas Barlow commented

2023-03-09 02:50:12 +01:00

System Information
Operating system: Windows-10-10.0.19045-SP0 64 Bits
Graphics card: NVIDIA GeForce GTX 1070 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 516.94

Blender Version
Broken: version: 3.4.1, branch: blender-v3.4-release, commit date: 2022-12-19 17:00, hash: rB55485cb379f7 and version: 3.6.0 Alpha, branch: main, commit date: 2023-03-06 22:38, hash: 175b1b449ec2
Worked: 3.3.4

Caused by f1c0249f34

Short description of error
The Python API now allows for negative material indices to be set, frequently crashing Blender if the affected mesh has more than one material slot (there appear to be optimizations in place that assume every polygon is using the first material slot when there's only one material slot). If Blender doesn't crash, the polygons with negative material indices become invisible with Solid Viewport Shading.

The material indices can be set negative with both MeshPolygon.material_index and the newer material_index attribute.

This seems to have come about from the change in 3.4.0 that moved material indices to a generic attribute.

There was a similar issue with Geometry Nodes that got fixed: #100952

Exact steps for others to reproduce the error
Select a default cube and run the following script:

import bpy
p = bpy.context.object.data.polygons[0]
p.material_index = -1
# Fails in 3.4
assert p.material_index >= 0

Alternative steps that are likely to crash Blender:

import bpy
me = bpy.context.object.data
# If a mesh only has one material slot, Blender appears to ignore the material indices, so an extra material slot is added
me.materials.append(None)
for p in me.polygons:
    p.material_index = -1

If Blender doesn't crash immediately from running the script, I find that going in and out of Edit mode a few times causes a crash pretty quickly.

A .blend containing a default cube with two material slots and all material indices already set to -1 is also attached.

If the material_index attribute exists, the material indices can also be set negative using it:

for mat_idx_attr in me.polygon_layers_int["material_index"].data:
    mat_idx_attr.value = -1

Interestingly, I noticed that the hard_min of the material_index property of MeshPolygon is still zero in 3.4, despite allowing negative values to be set:
assert bpy.types.MeshPolygon.bl_rna.properties["material_index"].hard_min == 0
I'm not sure if there's an equivalent hard_min for the material_index attribute, since it's a generic MeshPolygonIntProperty.

**System Information** Operating system: Windows-10-10.0.19045-SP0 64 Bits Graphics card: NVIDIA GeForce GTX 1070 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 516.94 **Blender Version** Broken: version: 3.4.1, branch: blender-v3.4-release, commit date: 2022-12-19 17:00, hash: `rB55485cb379f7` and version: 3.6.0 Alpha, branch: main, commit date: 2023-03-06 22:38, hash: `175b1b449ec2` Worked: 3.3.4 Caused by f1c0249f34c4 **Short description of error** The Python API now allows for negative material indices to be set, frequently crashing Blender if the affected mesh has more than one material slot (there appear to be optimizations in place that assume every polygon is using the first material slot when there's only one material slot). If Blender doesn't crash, the polygons with negative material indices become invisible with Solid Viewport Shading. The material indices can be set negative with both `MeshPolygon.material_index` and the newer `material_index` attribute. This seems to have come about from the change in 3.4.0 that moved material indices to a generic attribute. There was a similar issue with Geometry Nodes that got fixed: https://projects.blender.org/blender/blender/issues/100952 **Exact steps for others to reproduce the error** Select a default cube and run the following script: ```py import bpy p = bpy.context.object.data.polygons[0] p.material_index = -1 # Fails in 3.4 assert p.material_index >= 0 ``` Alternative steps that are likely to crash Blender: ```py import bpy me = bpy.context.object.data # If a mesh only has one material slot, Blender appears to ignore the material indices, so an extra material slot is added me.materials.append(None) for p in me.polygons: p.material_index = -1 ``` If Blender doesn't crash immediately from running the script, I find that going in and out of Edit mode a few times causes a crash pretty quickly. A .blend containing a default cube with two material slots and all material indices already set to -1 is also attached. If the `material_index` attribute exists, the material indices can also be set negative using it: ```py for mat_idx_attr in me.polygon_layers_int["material_index"].data: mat_idx_attr.value = -1 ``` Interestingly, I noticed that the `hard_min` of the `material_index` property of `MeshPolygon` is still zero in 3.4, despite allowing negative values to be set: `assert bpy.types.MeshPolygon.bl_rna.properties["material_index"].hard_min == 0` I'm not sure if there's an equivalent `hard_min` for the `material_index` attribute, since it's a generic `MeshPolygonIntProperty`.

NegativeMaterialIndicesCube.blend

102 KiB

Thomas Barlow added the

labels 2023-03-09 02:50:12 +01:00

Germano Cavalcante commented

2023-03-09 15:32:44 +01:00

In fact, given the way the viewport handles negative material indices (crashing), negative material indices should not be allowed to be set in Python. Or the viewport shouldn't crash.

Iliya Katushenock added the

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Python API allows setting negative material indices #105577