HLSL How can one pass data between shaders / read existing colour value?

Question

Hello all,
I have 2 HLSL ps2.0 shaders. Simplified, they are:

Shader 1

• Reads texture
• Outputs colour value based on this texture

Shader 2

• Problem: Need to read in the colour from Shader 1
• Outputs the final colour which is a function of the input colour

(They need to be different shaders as I've reached the maximum vertex-shader outputs for 1 shader)

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My problem is I cannot work out how Shader 2 can access the existing fragment/pixel colour.

Knowing how to do any of these things with HLSL would solve my problem;

• Read existing pixel colour (I don't think this is possible)
• Pass result of Shader 1 to Shader 2 as a float4
• Render result of Shader 1 as a texture in memory, and have Shader 2 read that in

Answer 1

To do what you want, you can switch render targets between execution of shader1 and shader2. You output your first shader to a texture and then you pass this texture to your second shader.

This is done in Ogre with Compositor scripts and target.