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Cg Samplers

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I want to synthesize a texture in the vertex shader, and then pass the resulting sampler to the fragment shader. Is it possible to modify a sampler it some fashion like a per-pixel modification; in a vertex shader? Or are samplers pretty much read-only?

Essentially, I want to stencil an outline onto a blank sampler in a loop; and then in a second loop, fill in the shape.

Also, I'm working in Cg, if that is important; but really the same question holds for GLSL, too.

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Unless I miss a feature, you can't. You can read pixels, but you can't write them back. Neither can you pass a sampler from a vertex shader towards a fragment shader. Textures are just set active in one of the (16?) working "slots" by OpenGL, Direct3D or another API.

The way to generate textures via shaders is usually by:
- create a "Render Target" / FBO. Like you can draw towards your screen, you can also draw onto textures.
- Activate the Render Target (instead of giving output to the screen buffer). Now everything you render will be "drawed" onto that texture.
- Example, render a screen filling quad. Let the fragment shader do the pixel math. Or just render a bunch of dots and let the CPU decide the colors. Colors can be interpretated as positions, normals, depth data, and so on.
- When you are done, disable the render target. The texture you just produced can be used like any other texture, and eventually be processed further in other fragment shaders.

Should be plenty of tutorials covering this stuff, as many effects are using render targets.


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