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understanding glMultmatrix

Started by sapdev May 23, 2009 at 12:54 AM 7 replies 2.1k views

Original Post

May 23, 2009 at 12:54 AM

The Case : Let there be a matrix m[16], It contains some data, some custom transformations. Now we multiply this matrix to the ModelView Matrix (which is set as the current matrix ) by glMultMatrix(m); Now The Question: In which order the matrix is multiplied ? M = modelview matrix m = custom matrix. M * m or m * M Please reply . . . . .

inferno82

May 23, 2009 at 1:39 AM

If I remember correctly it will be

modelview_matrix *= custom_matrix

- me

sapdev

May 23, 2009 at 1:46 AM

Thanks . . . . . . :)

sapdev

May 23, 2009 at 3:52 AM

I got one more question . . . .

consider the pseudo code below :-

glPushMatrix();glLoadIdentity();gluLookAt();glTranslate();gRotate();glScale();/////////////glBegin(GL_TRIANGLES)**********glEnd();/////////////glPopMatrix();

In this particular case in what order is the original modelview matrix is multiplied.

is it ?

m * s * r * t * g

m = old modelview matrix
s = scaling matrix
r = rotation matrix
t = translation matrix
g = gluLookAt Matrix or camera martix

Brother Bob

May 23, 2009 at 4:05 AM

As inferno82 said, new matrices are multiplied in from the right hand side. Think about your code and answer for a second.

What does the matrix look like after the call to glLoadIdentity? It completely resets the matrix so you have a new fresh identity matrix. Then you multiply g in from the right, what is the current matrix? Now, from that current matrix, if you multiply t in from the right, what is the current matrix? Keep going like that.

sapdev

May 23, 2009 at 4:25 AM

Ok got it . . .

m * g * t * r * s.

But there is a concept (I got over the internet) like the last transformation is applied first. So this two are opposite to each other, Can you please explain that.

Sorry if this has been answered already, but I am really new in this field and am really confused.

Brother Bob

May 23, 2009 at 4:38 AM

That is completely dependent on how you see transformations. They can be applied from last to first, or from first to last, depending on if you see transformations on the object locally or globally.

sapdev

May 23, 2009 at 5:27 AM

Hmmmm . . . .

Can you please explain it a bit more . . .

Do you mean that the global space / coordinate system and Object space / coordinate system . . . .

Thanks for the reply.

Brother Bob

May 23, 2009 at 8:30 AM

I mean transformations applied relative the global coordinate system or the local coordinate system. That is, should a rotation be about the global origin, or about the local origin? Should a translation be along the global axes, or the local axes? The global origin is in a fixed position and orientation, while the local origin moves and rotates along with the individual transformations applied. That difference will give for forward or backward interpretation of the transformation order.

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