Orthographic Projection in Modern OpenGL

Question

I'd like to set up an orthographic projection using only modern OpenGL techniques (i.e. no immediate-mode stuff). I'm seeing conflicting info on the web about how to approach this.

Some people are saying that it's still OK to call glMatrixMode(GL_PROJECTION) and then glOrtho. This has always worked for me in the past, but I'm wondering if this has been deprecated in modern OpenGL.

If so, are vertex shaders the standard way to do an orthographic projection nowadays? Does GLSL provide a handy built-in function to set up an orthographic projection, or do I need to write that math myself in the vertex shader?

Answer 1

If so, are vertex shaders the standard way to do an orthographic projection nowadays?

Not quite. Vertex shaders perform the calculations, but the transformation matrices are usually fed into the shader through a uniform. A shader should only evaluate things, that vary with each vertex. Implementing a "ortho" function, the returns a projection matrix in GLSL is counterproductive.

I'd like to set up an orthographic projection using only modern OpenGL techniques (i.e. no immediate-mode stuff). I'm seeing conflicting info on the web about how to approach this.

The matrix stack of OpenGL before version 3 has nothing to do with the immediate mode. Immediate mode was glBegin(…); for(…){ ...; glVertex(…);} glEnd(). And up to OpenGL version 3 is was rather common to use the matrices specified through the matrix stack in a vertex shader.

With OpenGL-3 it was aimed to do, what was originally planned for OpenGL-2: A unification of the API and removing old cruft. One of those things removed was the matrix stack. Many shaders already used more than the standard matrices already, anyway (like for example for skeletal animation), so matrices already had been passed as uniforms and the programs did already contain the whole matrix math code. Taking the step of remocing the matrix math stuff from OpenGL-3 was just the next logical step.

Answer 2

In general, you do not compute matrices in shaders. You compute them on the CPU and upload them as uniforms to the shaders. So your vertex shader would neither know nor care if it is for an orthographic projection or a perspective one. It just transforms by whatever projection matrix it is given.

Answer 3

When you target an OpenGL version > 3.0, glOrtho has been removed from the official specs (but still is present in the compatability profile), so you shouldn't use it anymore. Therefore, you will have to calculate the projection you want to use directly within the vertex shader (for OpenGL up to 3.0 glOrtho is perfectly ok ;).

And no there is no "handy" function to get the projection matrix in GLSL, so you have to specify it yourself. This is, however, no problem, as there is a) plenty of example code in the web and b) the equations are right in the OpenGL spec, so you can take it simply from there if you want to.