GLSL preprocessor

Question

I'm trying to figure out why the following GLSL code doesn't work:

#ifndef VertexPositionType
#define VertexPositionType vec3
#endif

in StandardVertexShaderInputs {
    VertexPositionType ds_VertexPosition;
};

vec4 ProjectVertexPosition(in vec4 v);

vec4 ProjectVertexPosition(in vec3 v);

void main() {
    gl_Position = ProjectVertexPosition(ds_VertexPosition);
}

The shader refuse to compile. The info log state:

error C1008: undefined variable "ProjectVertexPosition"

Even if it doesn't warn about the preprocessor, I got that the preprocessor symbol VertexPositionType is not replaced. If I remove the preprocessor definitions, everything is fine.

Now, the specification says:

#define and #undef functionality are defined as is standard for C++ preprocessors for macro definitions both with and without macro parameters.

Perhaps the following line is not a valid preprocessor line?

#define VertexPositionType vec3

Answer 1

Your shader is illegal. NVIDIA's compiler may not be spitting out the right errors, but your shader is doing the wrong thing (well, besides the fact that you didn't provide a #version declaration. I assumed #version 330, but it's always good to be explicit about GLSL versions).

I can only assume this is a vertex shader, since you're writing to gl_Position. Input interface blocks are illegal in vertex shaders, just as output interface blocks are illegal in fragment shaders. AMD's compiler is rather more explicit about this:

ERROR: 0:5: error(#328) interface block should not apply in 'Vertex Shader in'.
ERROR: 0:14: error(#143) Undeclared identifier ds_VertexPosition
ERROR: 0:14: error(#202) No matching overloaded function found ProjectVertexPosition
ERROR: 0:14: error(#160) Cannot convert from 'const float' to 'Position 4-component vector of float'
ERROR: error(#273) 4 compilation errors.  No code generated

When I removed the interface block definition, leaving it as just in VertexPositionType ds_VertexPosition;, it compiled fine.

If I remove the preprocessor definitions, everything is fine.

Then congratulations: you have found an NVIDIA driver bug. You should report it to them, because input interface blocks are not allowed in vertex shaders.