Hey GameDev, I'm finally learning shaders! I just read the NeHe tutorial on glsl, and then watched the following excellent introduction video:

So I'm just about ready to dive in... only, I'm realizing now that my reason for learning shaders might not even apply. I wanted to apply an effect to all points drawn on the level, but not the player (a gradient from bottom to top based on position.y % 10, see here: http://www.galigogames.com/images/cubior/cubior3.png ). The player would be on a different plane for this calculation, so when he/she moved, the gradient stayed relative to the player's cube - but all other gradients would be static.

When I read and watched the tutorials, it looked like shaders applied universally. I'm pretty sure still that it's possible to do y position based coloring/brightness, but can I have shaders apply only to certain polygons, and not others?

Oh wow, that's really sneaky putting it in the alpha value. And then that would work as long as either the player or the level always had a static value for alpha, so that the other one could be detected as using it for an actual alpha value? That's really cool.

I did not realize that shaders were designed to be switched that frequently! And it wouldn't have a huge performance impact?

Thanks! :D

Also, if there is no dependency between two consecutive rendering states ( like write to a render target then read it, depth test on then depth test off, ... ), modern GPU will not wait for the previous one to end before stating the new one, I do not know how many parralel context there is on earlier GPU, but the latest AMD one have 8 :)

Also, changing context is not yet free, so grouping by states the primitives is a good and mandatory practice.

Thankfully, I already draw characters and levels in separate parts of my draw function, so adding a shader switch inbetween the two should be relatively easy (no weird alpha dependencies!). Now, should I have both these shaders loaded in and ready to go then, but ultimately saved in separate files? Or can I put two shaders in one file somehow?