Problems with colors and samples mixing in GLSL shaders

OpenGL Development
#1

Hello all!

I have a slight issue here. I have a shader in which I pass the color and sample into the fragment. The texture renders fine, but when I want to draw a plain colored cube, it will be tainted black. This is because I am using mix(), mixing the color and texture.

I can fix this by passing in a uniform u_sampled, which will act as a switch between mixing the texture and color, or just the color. This is pretty messy, however… So my question is: how can I figure out if a texture (sample) is bound from the shader?

Thanks,
-wes :slight_smile:

#2

Few articles on internet said that starting GLSL 1.3, but I’ve never used it :


if( textureSize( SamplerDiffuse, 0).x > 0)
   // sample texture
else
   // use color

Say us if that works !

#3

Create a 4x4 texture filled with white pixels and bind that when you want to use “no” texture.

#4

I though of that solution too, but that will not break the mix between the color and the white pixel, no ?

Perhaps you can play and force the mix_factor to 0 or 1…

#5

This is what I’m seeing at the moment:

  • You’re shader expects that you have a texture and mixes between an input color and a texture sample.
  • You currently don’t have a texture bound.
  • You expect your shader to magically adapt and ignore the texture, but instead GLSL does the only logical thing and feeds you black.
#6

Conditional statements in shaders are worse than messy. They are slow. Very slow. Either do as @theagentd says, as @Gef says or just use a different shader. There is no other way I’m afraid.

#7

You can also use the same shader with preprocessor like “#ifdef”, to enable/disable parts in your shader at compilation.
At the end, you have one shader to maintain but two or more different versions declared in your Java code.


#ifdef USE_TEXTURE
    // mix color with texture
#else
    // use color
#endif
#8

This is a common misconception. If-statements aren’t expensive per se, but they have some quirks which can make them useless for performance gains.

First of all, conditional assignments are extremely cheap. Example: [icode]x = myBoolean ? 10 : 5;[/icode]

Secondly, on OpenGL cards with dynamic branching, if-statements can actually increase performance. Many people want to use if-statements to skip work in shaders, realize that they give no performance gain (but also don’t performance cost!) and proclaim that if-statements are slow. GPUs runs shaders in groups, usually of 16x16 pixels, but the processors on your GPU aren’t very flexible. If you have an if-else-statement in your shader and 255 pixels pass the if-statement and just a single pixel does (= enters the else-block), all 256 shader executions will run both the if and else blocks. For example, I had code that skipped work for pixels with depth=1.0 (meaning the sky was visible so I can skip some computations) and it worked very well, since the sky pixels were generally a continuous area on the screen, so a significant number of 16x16 blocks could skip the computation. If you on the other hand were to look at the sky through the branches of a tree, this would break down since most 16x16 blocks would have at least a few pixels that the branches or leaves covered, so the whole block would have to run the computations.