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BlinksTale

Can shaders apply to specific polygons?

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

 

https://www.youtube.com/watch?v=pE9ZDNcg3kw

 

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?

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The typical use case for shaders might look something like this:

 

Start Frame

Setup some render states

Select shaders.

Draw Call

Draw Call

Change some render states

Draw call

Change Shaders

Draw Call

etc.

End Frame

 

All the triangles in any given draw call are going to use the same shader. However, you can change shaders between draw calls, so in your case you can simply use a different shader for your character.

 

Alternatively, if you want to avoid applying an effect (in this case changing colour according to position.y) to certain triangles but for some reason you don't want to change shaders or split up a draw call, then you can differentiate the triangles somehow and add some appropriate logic to your shader. For example, you could set the vertex colour alpha of your player character's verts to zero, then use that value within your shader to disable the effect, either with an if statement, or by including the vertex alpha in your effect's equation in such a way that when vertex alpha is zero, the effect becomes disabled.

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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

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

It's quite common for different objects to be drawn using different shaders.

Ideally, you want to draw as much as possible between switching shaders... but as long as you're not switching them in between every triangle (instead, drawing a few hundred triangles/pixels with each shader), then there isn't much of a performance impact at all.

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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.

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If you do use the alpha check to remove the shader effect, but still render - and are working in a group make sure you document this somewhere where they are likely to notice it. There are a few reasons to draw invisible triangles. One decent one I can think of is a window pane with what should be clear - with a light glare effect applied - being rendered black instead.
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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?

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