How to transform a 3d-Point in a 2d-Point??

[TheBlob]

Hi!
I have a 3d-Vector/Point with x,y,z and it´s rendered in my 3d-space. But what are the actual 2d-coordinates for this point, after it´s rendered (where I can find it on my screen)??
thnx,
Blob

[Bob]

You mean your 3D point projected in the viewport?

I'm not sure about this, but I think it's like this.

p = (x, y, z, 1) in other words, your 3D point/vector
mv = modelview matrix
pr = projection matrix
vp = projected point in untransformed viewport coordinates

vp = pr * mv * p.

To get real windows space coordinates, you have to scale and offset vp. This is dependand of the size of the viewport.

[TheBlob]

Well, Thanks.
But, how do I get the modelview matrix, how the viewport matrix and how do i multiply them with my vector??

[Bob]

You can get the modelview matrix with glGetFloatv(GL_MODELVIEW_MATRIX, &mv), where mv is a float mv[16];. The projection matrix is obtained in the same way, but with GL_PROJECTION_MATRIX.

As for multiplication, well, bah, use gluProject instead http://www.opengl.org/discussion_boards/ubb/biggrin.gif . You pass the modelview- and projectionmatrix, the viewport coordinates, the point, and it will return the projected point. Forgot about it in my first post, it will do the job for you.

[TheBlob]

Ahhh! Thank you very much! Works well!!!