On definitions of coordinate systems

[GCChen97]

Hi,

Thanks for the convenient tool. I notice that the definitions of coordinate systems in mitsuba and blender are different when render normal map with AOV. It seems that the y axis in blender is z axis in mitsuba. So, what happens? Is SE3 transformation required when import/export scene.xml?

[GCChen97]

I also noticed that the ply meshes imported by the addon mitsuba-blender are -90 degree rotated around x axis comparing to those imported by blender. But obj meshes are consistently imported by both mitsuba-blender and blender.

[ros-dorian]

Hello, thank you for the comment.

Mitsuba and Blender have different Forward and Up vectors defining their coordinate systems. This means that if you are exporting a mesh from Blender or importing it from a Mitsuba scene (without using the add-on), you need to specify the correct values in the export/import dialog which are -Z Forward and Y Up.
Note that the add-on is doing this automatically when exporting/importing so it is always recommended to use it whenever possible.

Let me know if that answers your question.

Cheers!

[GCChen97]

Hi, thanks for the help. I further did some search and finally realized that Y UP means the XOZ plane is the "floor" and Z UP means the XOY plane is the "floor", which are not the definitions of camera frame.

So I have another question: what's the definition of camera coordinate system in blender and mitsuba? I guess Y UP also means "screen top==Y up" in camera frame. If so, are "+Z-forward" and "Y-up" and "X-left" the definitions of camera frame in mitsuba, and "-Z-forward" and "Y-up" and "X-right" in blender?

[GCChen97]

I also noticed some \pi rotation around X/Y in the codes of mitsuba-blender. What are these codes doing?

[ros-dorian]

As you correctly suggested in the discussion over at mitsuba-renderer/mitsuba3, the camera's forward direction is rotated 180 degrees around the up direction compared to the world frame. As Blender does not have the same definition, this code corrects that.

[GCChen97]

Thanks for the help! I am not familiar with CG and was confused about the differences between some common definitions in CV and CG.

[GCChen97]

Here are the definitions:
In mitsuba, the camera frame is "screen left==x, screen top==y and forward==z", the world frame is "Y up and -Z forward" (the "floor" is the XOZ plane).
In blender, the camera frame is "screen right==x, screen top==y and forward==-z", the world frame is "Z up and Y forward" (the "floor" is the XOY plane).

Thank @ros-dorian again!