OpenGL: Using shaders to create vertex lighting by using pre-calculated colormap?

First of all, I have very little knowledge of what shaders can do, and i am very interested in making vertex lighting. I am attempting to use a 3d colormap which would be used to calculate the vertex color at that position of the world, and also interpo开发者_高级运维late the color by using the nearby colors from the colormap.

I cant use typical OpenGL lighting because its probably too slow and theres a lot of lights i need to render. I am going to "render" the lights at the colormap first, and then i could either manually map every vertex drawn with the corresponding color from the colormap.

...Or i could somehow automate this process, so i wouldnt have to change the color values of vertexes myself, but a shader could perhaps do this for me?

Questions is... is this possible, and if it is: what i need to know to make it possible?

Edit: Note that i also need to update the lightmap efficiently, without caring about the size of the lightmap, so the update should be done only at that specific part of the lightmap i want to update.

---

It almost sounds like what you want to do is render the lights to your color map, then use your color map as a texture, but instead of decal mode set it to modulate mode, so it's multiplied with the existing color instead of just replacing it.

That is different in one way though: instead of just affecting the vertexes, it'll map to the individual fragments (pixels, in essence).

Edit: What I had in mind wasn't a 3D texture -- it was a cube map. Basically, create a virtual cube surrounding everything in your "world". Create a 2D texture for each face of that cube. Render your coloring to the cube map. Then, to color a vertex you (virtually) extend a ray outward from the center, through the vertex, to the cube. The pixel you hit on the cube map gives you the color of lighting for that vertex.

Updating should be relatively efficient -- you have normal 2D textures for the top, bottom, front, etc., and you update them as needed.

---

If you cant use the fixed function pipeline functionality the best way to do per vertex lighting should be to do all the lighting calculations per vertex in the vertex-shader, when you then pass it on the the fragment shader it will be correctly interpolated across the face.

Another way to deal with performances issues when using a lot of light sources is to use deferred rendering as it will only do lighting calculation on the geometry that is actually visible.

---

That is possible, but will not be effective on the current hardware.

You want to render light volumes into 3d texture. The rasterizer works on a 2D surface, so your volumes have to be split along one of the axises. The split can be done in one of the following ways:

• Different draw calls for each split
• Instanced draw, with layer selection based on glInstanceID (will require geometry shader)
• Branch in geometry shader directly from a single draw call

In order to implement it, I would suggest reading GL-3 specification and examples. It's not going to be easy, nor it will be fast enough in the result for complex scenes.