How does opengl project a 3d point to screenspace?

I am trying to emulate a subset of opengl with my own software rasterizer.

I'm taking a wild guess that the process looks like this:

Multiply the 3d point by the modelview开发者_StackOverflow中文版 matrix -> multiply that result by the projection matrix

Is this correct?

Also what size is the projection matrix and how does it work?

The point is multiplied by the modelview matrix and then with projection matrix. The resultant is normalized and then multiplied with viewport matrix to get the screen coordinates. All matrices are 4X4 matrix. You can view this link for further details.

http://www.songho.ca/opengl/gl_transform.html#example2

(shameless self-promotion, sorry) I wrote a tutorial on the subject :

http://www.opengl-tutorial.org/beginners-tutorials/tutorial-3-matrices/

There is a slight caveat that I don't explain, though. At the end of the tutorial, you're in Normalized Device Coordinates, i.e. -1 to +1. A simple linear mapping transorms this to [0-screensize].

You might also benefit from looking at the gluProject() code. This takes an x, y, z point in object coordinates as well as pointers to modelView, projection, and viewport matrices and tells you what the x, y, (z) coordinates are in screenspace (the z is a value between 0 and 1 that can be used in the depth buffer). All three matrix multiplications are shown there in the code, along with the divisions necessary for perspective.