Fixed-Function Vs. Shaders - help understand the conceptual differences

Question

My background: I first started experimenting with OpenGL some months ago, for no particular purpose, just fun. I started reading the OpenGL redbook, and got as far as making a planetary system with a lot of different lighting. That lasted for a month, and my interest for openGL went away. It awoke again a week or so ago, and as I gathered from some SO posts, the redbook is outdated and the OpenGL Superbible is a better source for learning. So I started reading it. I like the concept of shaders but there's a real mess going on in my brain because of transition from my old memories of the fixed pipeline and the new concept of shaders.

Question: I would like to write some statements which I think are true and I am asking OpenGL experts to verify them (i.e. whether I am understanding correctly, not quite correctly or absolutely incorrectly). So...

1) If we don't use any shader program, nothing changes. We have current color, current normal, current transformation matrix, current everything, and as soon as we call glVertex**(...) these current values are taken and the vertex is fed to ... I don't know what. The fact is that it's transformed with the current matrix, the current color and normal are applied to it etc.

2) As soon as we use a shader program, all the above stops working. That is, glColor, glRotate etc. make no sense (Do they?). I mean, glColor still does set the current color, glRotate still multiplies the current matrix by the rotation matrix, but these aren't used at all. Instead, we feed vertex attributes by glVertexAttrib. Which attribute means what is totally dependent on our vertex shader and the in variable binding. We also find ans set the values of the uniforms and then call glVertex and the shader is executed ( I don't know immediately or after glEnd() is called). The actual vertex and fragment processing is done entirely manually in the shader program.

3) Shaders don't add anything to depth testing. That is, I don't need to take care of it in a shader. I just call glEnable(GL_DEPTH_TEST). Neither is face culling affected.

4) Alpha blending and antialiasing need not be taken care of in shaders. glEnable calls will suffice.

5) Is it a good idea to use gluPerspective, glRotate, glPushMatrix and other matrix functions, and then retrieve the current matrix and feed it as a uniform to a shader? Thus there won't be any need in using a 3rd party matrix library.

Answer 1

1. It depends on what version of OpenGL you're talking about. Up through OpenGL 3.0, all the fixed functionality is still present, so yes, if you decide to just use fixed functionality it continues to work like it always did. Starting from 3.0, quite a bit of the fixed pipeline was deprecated, and as of 3.1 it disappears completely. Using these, you no longer really have the option to just use the fixed pipeline.

2. Again, it depends. For example, up through OpenGL 3.0, glColor is still supported, even when you use a shader. The difference is that instead of automatically being applied to what gets drawn, it's supplied to your shader, which can use it unchanged, modify it as it sees fit, or ignore it completely. So, your fragment shader receives gl_FrontColor and gl_BackColor, and writes the actual fragment color to gl_FragColor. If you're using OpenGL 3.1 or newer, however, glColor (for example) just no longer exists -- a color will be just another value you supply to your shader like you could/would anything else.

3. That's correct, at least up to OpenGL 3.1. As of 4.0, there's a new compute shader that (I believe) can get involved in things like depth testing (but I haven't used it, so I'm a bit uncertain about that).

4. Yes, you can still use built-in alpha blending. Depending on your hardware, you may also want to consider using the gl_ARB_draw_buffers_blend extension (which is mandatory as of OpenGL 4, if I recall correctly).

5. Yet again, it depends on the version of OpenGL you're talking about. Current OpenGL completely eliminates all support for matrices so you have no choice but to use some other matrix library. Older versions supplied things like gl_ModelViewMatrix and gl_NormalMatrix to your shader as a uniform so you could go that route if you chose.

Answer 2

2) In modern OpenGL, there is no glColor, glBegin, glVertex, glRotate etc. so they don't make sense.

5) In modern OpenGL there are no built-in matrices, so you have to use a 3rd party library or write your own. So to answer your question, no, it's not a good idea.