Optional samplers in shaders

Question

I have a scene. Sometimes I draw untextured, coloured, semi-transparent triangles, sometimes I texture them, and I draw a lot of both in a fairly interleaved manner.

In a fixed function pipeline the difference would be if I had set the GL_TEXTURE_2D to be 0 or a texture handle. My codebase is being migrated from FFP to OpenGLES 2, so these bindings to 0 already exist everywhere they are needed. My ambition is for my generic OpenGL code to run on both OpenGL 1.x and OpenGLES 2. So I'm looking for shaders that are a cut-down FFP imitation.

How can an OpenGLES 2 shader know if the Sampler2D is set to 0 or has a valid texture?

My reasoning is that an if-statement that all fragments take the same branch is much cheaper than switching program pretty much every other statement as I alternate between textured ops and fixed-colour ops.

If I add a uniform then I have to go visit my code and each time I bind a texture I'll have to add a call to set the uniform flag. I have a large codebase that I have to ensure I am strictly setting it appropriately.

If I introduce a opaque white 1x1 texture this is like the uniform - I have to go everywhere in the code base ensuring I am binding it instead of binding to 0 to get colour-only primatives.

So I'd much prefer an approach where the sampler is interrogated in a shader. Is this possible?

Answer 1

Testing for the sampler in the fragment shader is going to make either a useless test or a useless texture lookup for each fragment being rendered. There is really no reason to do that.

A really nicer method is to bind a program using a specific fragment shader when you want to render flat triangles:

precision highp float;
uniform vec4 u_FixedColor;
void main()
{
    gl_FragColor = u_FixedColor;
}