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openglmatrixglsl
Michael IV
Michael IV

Reputation: 11506

Normal model matrix calculation for normal mapping in GLSL

I need to calculate a normal model matrix for doing normal mapping in GLSL shader. I want to make sure I am right on this: When I multiply view (camera model) matrix with geometry model matrix, is the view matrix supposed to be already inverted? It is not clear from the online examples like those found here and here. Also, I see in some cases people also transpose the resulting matrix. Why? So what is the right way to build a normal model matrix in OpenGL?

Currently I do it this way:

glm::mat4 view = inverse(GetCameraModel());
glm::mat3 normalModelMatr= glm::mat3(view * mesh.GetModel());

Is this the way to go ?

Upvotes: 2

Views: 5342

Answers (1)

Stephan van den Heuvel
Stephan van den Heuvel

Reputation: 2138

The correct normal matrix is the inverse transpose of the model-view matrix. If you do not do any non-uniform scaling, that is scaling axises by different amounts, the inverse of the matrix is equal to its transpose because it is orthogonal. Therefore, the two operations cancel out and it is just the original matrix.

If you do do non uniform scale, the matrix is not orthogonal and you must do the inverse transpose.

You take the top 3x3 matrix, because you only need to rotate and scale normals, not translate.

So your normal matrix is correct as long as you do not employ non-uniform scaling.

Upvotes: 5

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