Find out if GL_TEXTURE_2D is active in shader

12
votes

I would like to know if GL_TEXTURE_2D is active in the shader.

I am binding a color to the shader as well as the active texture (if GL_TEXTURE_2D is set) and need to combine these two.

So if texture is bound, mix the color and the texture (sampler2D * color) and if no texture is bound, use color.

Or should I go another way about this?

1
openglglsl

1 Answers

19
votes

It is not quite clear what you mean by 'GL_TEXTURE_2D is active' or 'GL_TEXTURE_2D is set'.

Please note the following:

  • glEnable(GL_TEXTURE_2D) has no effect on your (fragment) shader. It parametrizes the fixed function part of your pipeline that you just replaced by using a fragment shader.
  • There is no 'direct'/'clean' way of telling from inside the GLSL shader whether there is a valid texture bound to the texture unit associated with your texture sampler (to my knowledge).
  • Starting with GLSL 1.3 you might have luck using textureSize(sampler, 0).x > 0 to detect the presence of a valid texture associated with sampler, but that might result in undefined behavior.
  • The ARB_texture_query_levels extension does indeed explicitly state that textureQueryLevels(gsampler2D sampler) returns 0 if there is no texture associated with sampler.

Should you go another way about this? I think so: Instead of making a decision inside the shader, simply bind a 1x1 pixel texture of 'white' and unconditionally sample that texture and multiply the result with color, which will obviously return 1.0 * color. That is going to be more portable and faster, too.