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openglmatrixopengl-es3dwebgl
Everton
Everton

Reputation: 13845

Why do modeview and camera matrices use RUB orientation

I usually find matrix libraries building both modelview and cameras matrices from the RUB (right-up-back) vectors, as depicted in these pages:

http://3dengine.org/Right-up-back_from_modelview http://3dengine.org/Modelview_matrix

Is the RUB tuple just a common standard? Otherwise, is there a reason the RUB vectors are preferred over any other orientation (such as forward-up-right)?

Upvotes: 0

Views: 397

Answers (1)

Reto Koradi
Reto Koradi

Reputation: 54602

Particularly if you're using the programmable pipeline, you have almost complete freedom about the coordinate system you work in, and how you transform your geometry. But once all your transformations are applied in the vertex shader (resulting in the vector assigned to gl_Position), there is still a fixed function block in the pipeline between the vertex shader and fragment shader. That fixed function block relies on the transformed vertices being in a well defined coordinate system.

gl_Position is in a coordinate system called "clip coordinates", which then turns into "normalized device coordinates" (NDC) after dividing by the w coordinate of the vector.

Based on the vector in NDC, the fixed function rasterization block generates pixels. It will use the first coordinate to map to the horizontal window direction, and the second coordinate to map to the vertical window direction. The third coordinate will be used to calculate the depth, which can be used for depth testing.

This means that after all transformations are applied, the first coordinate has to be left-right, the second coordinate has to be bottom-up, and the third coordinate has to be front-back (well, it could be back-front if you change the depth test).

If you use a classic setup with modelview and projection matrix, it makes sense to use the modelview matrix to transform the original geometry into this orientation, and then use the projection matrix to apply e.g. a perspective.

I don't think there's anything stopping you from using a different orientation as the result of the modelview transformation, and then include a rotation in the projection matrix to transform the whole thing into the correct clip coordinate space. But I don't see a benefit, and it looks like it would just add unnecessary confusion.

Upvotes: 5

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