Why does glGetUniformLocation report some of my uniform locations as -1?

Question

I have been writing GLSL shaders for over a year but this is the first time I have run into this problem. I am on the iOS platform - if that matters.

In a vertex shader I have this:

uniform mediump mat4 projectionViewModelMatrix;

In a fragment shader I have been using for months I have the following uniforms:

uniform int channels;
uniform sampler2D hero;

Nothing to write home about. I added three additional uniforms to the fragment shader:

uniform int heroChannels;
uniform int heroWidth;
uniform int heroHeight;

Using

glGetUniformLocation

I tried to grab the location of all my uniforms. This is when my own personal hell began. This is the result:

uniform heroChannels location -1
uniform channels location 4
uniform hero location 5
uniform heroHeight location -1
uniform projectionViewModelMatrix location 0
uniform heroWidth location -1

Huh? Why are the recently added uniforms not given a valid location by the GL Gods?

Answer 1

Couple things come to mind.

First, it's possible each of your int uniforms actually takes a whole vector in uniform space, and thus you may be running out of space - you can try to combine them into vectors to see if it helps.

Second, it's possible that the shader compiler happily discards the uniforms in case they're not actually in use. If this is the case, then they naturally won't get a location.