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pengsenl

OpenGL Question about the matrix in Opengl

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Hi all! I am new comer in opengl. I have trouble with the glGetFloatv(GL_MODELVIEW_MATRIX, lastmatrix); I did a simple test program like this: glLoadIdentity(); glRotatef(45,0,0,1); glGetFloatv(GL_MODELVIEW_MATRIX, lastmatrix); // the value of the lastmatrix is { 0.70710677, 0.70710677 , 0 ,0 -0.70710677 , 0.70710677 , 0 ,0 0.0000000000, 0.00000000, 1 ,0 0.0000000000, 0.00000000, 0 ,1} But I thought it should be { 0.70710677, -0.70710677 , 0 ,0 0.70710677 , 0.70710677 , 0 , 0 0.0000000000, 0.00000000, 1 ,0 0.0000000000, 0.00000000, 0 ,1} What is wrong with this? I get the transpose of the matrix I want. Then I use another program to test, it is all right; // glLoadIdentity(); GLfloat lastmatrix[16] = {2,5,0,1, 0,1,2, 2, 5,4,4,4, 9,5,2,0}; glMultMatrixf(lastmatrix); glGetFloatv(GL_MODELVIEW_MATRIX, lastmatrix); then I get the right answer, the value of the lastmatrix is {2,5,0,1, 0,1,2, 2, 5,4,4,4, 9,5,2,0}; So can anybody tell me what is wrong with this? Thanks a lot!

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guess u are a directx guy, in OpenGL matrices are all in column major order.. that is when u put something like

float mat[16] = {2,3,4,5,
6,7,8,9,
......
};

it actually means that

2,3,4,5 is the first COLUMN of the matrix, i.e.

|2,6
|3,7
|4,8
|5,9

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pengsenl,
OpenGL uses column-major format for matrix. So 4 elements in a column are filled first then move on the next column.

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Thanks a lot! But i still want to know if I multiply the current matrix M by another matrix like this
glLoadIdentity();
GLfloat A[16] = {2,5,0,1,
0,1,2, 2,
5,4,4,4,
9,5,2,0};
glMultMatrixf(A);
Then how opengl do it? the result is M*A' or M*A?
If I use the glGetFloatv(GL_MODELVIEW_MATRIX, m_lastMatrix); It return the matrix in column order or in row order?

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It's M*A of course. Just silly to transpose the matrix before multiplication. Same with glGetFloat; it returns the matrix, not it's transpose.

But then again, you're probably still confused about what the linear layout of elements really are, compared to the transposed layout.

A matrix M with elements like this:

| a, b, c, d |
M = | e, f, g, h |
| i, j, k, l |
| m, n, o, p |

is defined as a one-dimensional array in C as

float M[16] = {a, e, i, m, b, f, j, n, c, g, k, l, d, h, l, p};

From this layout and the corresponding mapping, you can determing how multiplication behaves in OpenGL.

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