Matrix multiplication order

[Timo]

Not probably a bug, but I just wanted to confirm whether it's intentional that the matrix multiplication is done in the wrong order. For example, if we have a simple multiplication like this:

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matrix4 M1, M2, M3;
...
M3 = M1*M2;

The result is not M1*M2, but M2*M1. This is assuming that the matrices are in row major format like said in the docs.

Anyway I think it should be mentioned in the documentation.

[Halifax]

Hmm, you are correct. I guess that would explain why Irrlicht requires that you do:

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core::matrix4 worldViewProj;
worldViewProj = driver->getTransform(video::ETS_PROJECTION);
worldViewProj *= driver->getTransform(video::ETS_VIEW);
worldViewProj *= driver->getTransform(video::ETS_WORLD);

instead of the classical way. That explains a lot.

[hybrid]

Well, both orders are properly defined and just need to be used correctly.

[Halifax]

Well, both orders are properly defined and just need to be used correctly.

Well yes. Shouldn't the documentation be updated then?

[hybrid]

IIRC, I have added all the necessary explanations already for 1.7. If there are things still unclear, just point out the exact places and maybe give a suggestion on how to update the sections.

[brferreira]

IIRC, I have added all the necessary explanations already for 1.7. If there are things still unclear, just point out the exact places and maybe give a suggestion on how to update the sections.

The trunk doc says:

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//! Multiply by another matrix.

While it should be something like

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//! Multiply other matrix by this
/** This function returns a new matrix multiplying the second matrix by the first one */

[Steel Style]

I agree that's a really weird behaviour.
Anyone have a clue about what it is writed like this ?

[hybrid]

You mean why? Well, multiplication of matrices is defined both ways. It's just a writing convention if a*b means one or the other. Niko defined most things as in D3D, so it might be a reason there.

[Halifax]

I don't mean to beat a dead horse. I'm fine with the current convention as it is defined; I'm just glad I know now for future reference. But http://msdn.microsoft.com/en-us/library ... S.85).aspx (D3DXMatrixMultiply) is defined as "the transformation M1 followed by the transformation M2 (Out = M1 * M2)."

[Steel Style]

Yes that what it's supposed to be.
Because it's actually the multiplication is not M1*M2 but (M1*M2).Transpose.

So the only reason to do this may be because it's computed as column vector (OpenGL use them) so m[4] replace m[1], m[3] replace m[2]... but in this m[12] m[13] m[14] couldn't be the translation vector. :/