What are the pros / cons of the three?
Answer
coderanger is right about HLSL targeting DirectX, GLSL targeting OpenGL and CG being available with both interfaces.
However there are other things to consider (learned on the OGRE forum) :
- CG will not allow you to use the latest features of GLSL (I'm not sure about HLSL). It's a middle ground so you'll not be able to fully exploit the GLSL features, only the common one until Shader Model 3.0 (AFAI understood).
- CG is made by NVidia that optimize it for it's card but don't seem to do lot of work for ATI cards... some people report that there might be problems with ATI cards using CG.
- OpenGL seem to be a bit slower than DirectX on Windows platforms. That's really relative to what you're doing and the reason for this find it's source in the drivers implementations but it's good to know before you choose the API and the associated shader language. However, OpenGL is the only one interface available on all (win/mac/unix/some consoles) platforms. So knowing that, using exclusively GLSL might be the better choice for a non-Microsft-centric-cross-platform game.
- NVidia drivers on Linux seem to be more stable than ATI drivers, making CG more interesting if you need to be sure it works nicely on linux (that's all reported informations, you'll have to check for details)
So, if you're not using the lastest shader features, CG seem a good choice. GLSL seem a beter one if you're going full OpenGL. HLSL if you're going exclusively on Microsoft platforms.
Now first developping in HLSL for windows to use DirectX and then convert to GLSL for linux and mac could be the better solution to be sure of performance and have the larger set of shader features available. It might however be a lot of work (didn't do it myself so I cannot tell). OGRE graphics engine (and other engines) allow to use any API (DirectX or OpenGL or others) so it helps, but there is still shader code to convert if you go this way.
That's all the information I gathered while choosing my shader language (I've not made my decision yet).
Update: Valve did a conversion of one of their game to OpenGL and found no way to make the DirectX version faster than the OGL one. So keep in mind that the state of driver implementation, API quality, etc., all that change too much each year for you to totally rely on raw performance as an argument to choose one or the other. With this in mind, choose OpenGL/GLSL to simplify your life when working (or having plans or hopes to work) with other platforms than Windows, use DirectX/HLSL if you really want to use only Microsoft platforms and focus and maybe have some good API faster than OpenGL (this is reversing now, so don't count on it); use CG if you want to provide both possibilities to the user, but if you have the work force (and tools) to do it, using both GLSL and HLSL might be a viable solution too.
Update (2012): It is important to note that CG has been discontinued, and is no longer supported or actively worked on by Nvidia. Nvidia recommends that all users switch to a combination of GLSL and HLSL, or a newer library such as nvFX (on github). This is because it was too difficult to maintain feature-compatibility between GLSL and HLSL.
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