Error on assigning different amount of verts to MeshPolygon.vertices in Blender 3.5 #106384

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Andrej · 2023-03-31T14:33:46+02:00

Andrej commented

2023-03-31 14:33:46 +02:00

Blender 3.5

Short description of error
Found that in 3.5 the code below that's trying to assign different number of verts to polygon than there was before results in error. Haven't found anything related in https://wiki.blender.org/wiki/Reference/Release_Notes/3.5/Python_API.

It's not that big of a deal, I can rework original code with bmesh. Just wanted to put it out there - maybe it's part of some different issue.

Exact steps for others to reproduce the error

Select cube and run the code below in default scene. You can test the same code in 3.4 - it doesn't result in error.

obj = bpy.context.object
obj.data.polygons.add(1)
obj.data.polygons[-1].vertices = (0,1,2)

# ValueError: bpy_struct: item.attr = val: MeshPolygon.vertices: array length cannot be changed to 3 (expected 0)

Blender 3.5 **Short description of error** Found that in 3.5 the code below that's trying to assign different number of verts to polygon than there was before results in error. Haven't found anything related in https://wiki.blender.org/wiki/Reference/Release_Notes/3.5/Python_API. It's not that big of a deal, I can rework original code with bmesh. Just wanted to put it out there - maybe it's part of some different issue. **Exact steps for others to reproduce the error** Select cube and run the code below in default scene. You can test the same code in 3.4 - it doesn't result in error. ```python obj = bpy.context.object obj.data.polygons.add(1) obj.data.polygons[-1].vertices = (0,1,2) # ValueError: bpy_struct: item.attr = val: MeshPolygon.vertices: array length cannot be changed to 3 (expected 0) ```

Andrej added the

labels 2023-03-31 14:33:47 +02:00

Andrej changed title from ~~Cannot assign~~ to Cannot assign different amount of verts to MeshPolygon.vertices in Blender 3.5

2023-03-31 14:49:04 +02:00

Andrej changed title from ~~Cannot assign different amount of verts to MeshPolygon.vertices in Blender 3.5~~ to Error on assigning different amount of verts to MeshPolygon.vertices in Blender 3.5

2023-03-31 14:49:23 +02:00

Thomas Barlow commented

2023-04-01 02:01:57 +02:00

If you add

print(len(obj.data.polygons[-1].vertices))
print(obj.data.polygons[-1].vertices[:])

to the end of the above code in 3.4, you should be able to see that the assignment to vertices doesn't do anything; vertices remains zero length.

It looks like 51e5417bd3 in 3.5 also has the effect of raising an error when the array cannot be resized by assignment and the array is smaller than the sequence it's being set to.

To set vertices like this, the loop_total of the polygon can be set to the number of vertices beforehand so that vertices changes to the desired length.

This should work in both 3.4 and 3.5, though for practical use you may want to add new loops and set the loop_start of the polygon too so you're not overwriting the vertex_index of any existing loops or overwriting unallocated memory when the loops don't exist (there don't appear to be any safeguards).

obj = bpy.context.object
obj.data.polygons.add(1)
obj.data.polygons[-1].loop_total = 3
obj.data.polygons[-1].vertices = (1,2,3)
print(len(obj.data.polygons[-1].vertices))
print(obj.data.polygons[-1].vertices[:])

If you add ```py print(len(obj.data.polygons[-1].vertices)) print(obj.data.polygons[-1].vertices[:]) ``` to the end of the above code in 3.4, you should be able to see that the assignment to `vertices` doesn't do anything; `vertices` remains zero length. It looks like https://projects.blender.org/blender/blender/commit/51e5417bd37eecbf6db2f2f5e37e5155db039996 in 3.5 also has the effect of raising an error when the array cannot be resized by assignment and the array is smaller than the sequence it's being set to. ----- To set `vertices` like this, the `loop_total` of the polygon can be set to the number of vertices beforehand so that `vertices` changes to the desired length. This should work in both 3.4 and 3.5, though for practical use you may want to add new loops and set the `loop_start` of the polygon too so you're not overwriting the `vertex_index` of any existing loops or overwriting unallocated memory when the loops don't exist (there don't appear to be any safeguards). ```py obj = bpy.context.object obj.data.polygons.add(1) obj.data.polygons[-1].loop_total = 3 obj.data.polygons[-1].vertices = (1,2,3) print(len(obj.data.polygons[-1].vertices)) print(obj.data.polygons[-1].vertices[:]) ```

Andrej commented

2023-04-01 07:58:07 +02:00

That's true. In the code I was using orginally assigning vertices with obj.data.polygons[-1].vertices = [0,1,2] prior to declaring obj.data.polygons[-1].loop_total basically had no effect and it was just a happy accident that it produced correct geometry.
Checked that it worked exactly the same way even with obj.data.polygons[-1].vertices = [0,1,2] commented out.

That's true. In the code I was using orginally assigning vertices with `obj.data.polygons[-1].vertices = [0,1,2]` prior to declaring `obj.data.polygons[-1].loop_total` basically had no effect and it was just a happy accident that it produced correct geometry. Checked that it worked exactly the same way even with `obj.data.polygons[-1].vertices = [0,1,2]` commented out.

Richard Antalik commented

2023-04-12 06:32:50 +02:00

I am not very familiar with workflow in this case, does this mean that there is no issue after all?