Matrix world from driver expression is different from object's matrix world? #112816

New Issue

Andrej · 2023-09-24T21:51:05+02:00

Andrej commented

2023-09-24 21:51:05 +02:00

Blender 3.6.3

Noticed that object.matrix_world we get from the driver's expression is different from the same matrix we get from just getting the same matrix using Python API. Since this is basically exactly the same object's property I thought it should be consistent.

Steps to reproduce the issue.

Default Blender scene.
Go to scene->gravity and add 3 drivers similar to this one (issue is unrelated to gravity, just to visualise) with paths listed below:

Paths (Z basis is 2nd column therefore we use [i][2]):

matrix_world[0][2]
matrix_world[1][2]
matrix_world[2][2]

Rotate cube by 90 degrees X axis.
Since I was trying to get Z basis vector then expected values in drivers expected to be (0, -1, 0).
We can confirm it by running code in python console: C.object.matrix_world[0][2], C.object.matrix_world[1][2], C.object.matrix_world[2][2] and get (0,-1,0).

But the issue is that it's returning (0,1,0) which looks like Z-component of the X,Y,Z basises - so it's kind of like matrix_world we get from the driver expression is transposed I guess.

Blender 3.6.3 Noticed that object.matrix_world we get from the driver's expression is different from the same matrix we get from just getting the same matrix using Python API. Since this is basically exactly the same object's property I thought it should be consistent. Steps to reproduce the issue. 1. Default Blender scene. 2. Go to scene->gravity and add 3 drivers similar to this one (issue is unrelated to gravity, just to visualise) with paths listed below: ![image](/attachments/cc345ae2-2806-4804-a7df-0ba128c90e9c) Paths (Z basis is 2nd column therefore we use [i][2]): - matrix_world[0][2] - matrix_world[1][2] - matrix_world[2][2] 3. Rotate cube by 90 degrees X axis. 4. Since I was trying to get Z basis vector then expected values in drivers expected to be (0, -1, 0). We can confirm it by running code in python console: `C.object.matrix_world[0][2], C.object.matrix_world[1][2], C.object.matrix_world[2][2]` and get (0,-1,0). But the issue is that it's returning (0,1,0) which looks like Z-component of the X,Y,Z basises - so it's kind of like matrix_world we get from the driver expression is transposed I guess.

image.png

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drivers.blend

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Andrej added the

labels 2023-09-24 21:51:06 +02:00

YimingWu commented

2023-09-25 06:53:15 +02:00

Can confirm that accessing ob.matrix_world seems to be the transpose of the result accessed from driver variable expression.

This is the result of ob.matrix_world, it seems that the python api here uses row-priority (The last column being the translation component), however when evaluated in driver expression, it seems to have became column priority.

Can confirm that accessing `ob.matrix_world` seems to be the transpose of the result accessed from driver variable expression. This is the result of `ob.matrix_world`, it seems that the python api here uses row-priority (The last column being the translation component), however when evaluated in driver expression, it seems to have became column priority. ![图片](/attachments/01a0ad53-387d-4e7a-a75d-9e4a25c69ec0)

图片.png

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YimingWu added

and removed

labels 2023-09-25 06:59:19 +02:00

YimingWu commented

2023-09-25 10:38:20 +02:00

Hi, due to the design of the property, for the moment, you could try following method:

Set var as matrix_world (the expression will show red at the moment)
Set expression as var[1][2], you will get ~~the correct result~~ a result that's consistent with what you get in script terminal.

Hi, due to the design of the property, for the moment, you could try following method: - Set `var` as `matrix_world` (the expression will show red at the moment) - Set expression as `var[1][2]`, you will get ~~the correct result~~ a result that's consistent with what you get in script terminal.

👍 1

Andrej commented

2023-09-25 10:54:58 +02:00

Hi, due to the design of the property, for the moment, you could try following method:

Set var as matrix_world (the expression will show red at the moment)

Set expression as var[1][2], you will get the correct result.

it works! I can even do var.col[2].y which could be a bit more readable in some cases.
Didn't know that I can set variables to more complex objects like matrices and not just scalar values.

> Hi, due to the design of the property, for the moment, you could try following method: > > - Set `var` as `matrix_world` (the expression will show red at the moment) > - Set expression as `var[1][2]`, you will get the correct result. it works! I can even do `var.col[2].y` which could be a bit more readable in some cases. Didn't know that I can set variables to more complex objects like matrices and not just scalar values.

YimingWu commented

2023-09-27 04:42:25 +02:00

That's kind of a quirk of current design, which it will return a python object when it's not evaluated to a simple value, then it can be used in the expression above.

I'll mark this as a known issue, since people rarely drive stuff directly from matrix entries (like in your case, it can be done with dot products between object axis and global, which is a little bit more complicated), and due to the fact that the wrapping of the matrix is different so they will change row/column priority.

That's kind of a quirk of current design, which it will return a python object when it's not evaluated to a simple value, then it can be used in the expression above. I'll mark this as a known issue, since people rarely drive stuff directly from matrix entries (like in your case, it can be done with dot products between object axis and global, which is a little bit more complicated), and due to the fact that the wrapping of the matrix is different so they will change row/column priority.

YimingWu added

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Needs Info from Developers

and removed

labels 2023-09-27 04:46:01 +02:00

Andrej commented

2023-09-27 08:14:02 +02:00

That's kind of a quirk of current design, which it will return a python object when it's not evaluated to a simple value, then it can be used in the expression above.

But why matrix is transposed though?

> That's kind of a quirk of current design, which it will return a python object when it's not evaluated to a simple value, then it can be used in the expression above. But why matrix is transposed though?

Thomas Barlow commented

2023-09-27 13:54:11 +02:00

But why matrix is transposed though?

Internally, Blender's matrices are column-major, but accessed through the Python API they are row-major. I think drivers run entirely in C code if possible for performance, but I guess those drivers running entirely in C forgot to account for the differences in matrix access.

> But why matrix is transposed though? Internally, Blender's matrices are column-major, but accessed through the Python API they are row-major. I think drivers run entirely in C code if possible for performance, but I guess those drivers running entirely in C forgot to account for the differences in matrix access.

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Matrix world from driver expression is different from object's matrix world? #112816