glUniform and multiple textures

Can someone explain what precisely glUniform expects as a value when
using it to assign textures?

I have a shader that makes use of two textures. One is a cubemap used as
a heightmap and the other is a color map (2D Texture) to look up values
based on the height in the cubemap.

The heightmap is in texture unit 0, the colormap is in texture unit 1.

I've spent the whole morning trying to get this to work and it just
wouldn't which was really strange as in a previous test application I
got it to work just fine. Just when I took all the spaghetti code and
made a real engine out of it, things stopped working. Go figure...

Now in my old code, I had this:

glUniform1i(glGetUniformLocation(programId, "heightMap"), 0);
glUniform1i(glGetUniformLocation(programId, "colorMap"), 1);

And I used the same in my new code and it didn't work. It worked fine in
my old code....

So then I figure, let me look at the texture ID numbers that OpenGL is
giving my two textures, they turn out to be 1 and 2.

So I change the code to this:

glUniform1i(glGetUniformLocation(programId, "heightMap"), 1);
glUniform1i(glGetUniformLocation(programId, "colorMap"), 2);

It works...flawlessly.

And just for reference, in my shader the variables are defined as:

uniform samplerCube heightMap;
uniform sampler2D colorMap;

So what does glUniform expect here? The Texture unit or the texture ID?

And if it does expect the texture ID, why did it work in my old code
with a Texture ID of 0? And if it expects the texture unit, why is it
working in my new code with 1 and 2 even though the textures are in 0
and 1?

I don't like it when something works and I don't understand why it
works. That's just begging for an invitation to come back to haunt me
later and the OpenGL documentation for glUniform is very vague on this
as is trying to find any information about this on Google.

If someone could clear this up for me I would really appreciate it.

Thanks,

Stephan


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Author: Stephan Rose
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