OpenGL shader not passing variable from vertex to fragment shader

Question

I'm encountering something really really strange.

I have a very simple program that renders a simple full-screen billboard using the following shader pipeline:

VERTEX SHADER:

#version 430

layout(location = 0) in vec2 pos;

out vec2 tex;

void main()
{
    gl_Position = vec4(pos, 0, 1);
    tex = (pos + 1) / 2;
}

FRAGMENT SHADER:

#version 430

in vec2 tex;

out vec3 frag_color;

void main()
{
    frag_color = vec3(tex.x, tex.y, 1);
}

The program always renders the quad in the correct positions (so I've ruled out the VAO as culprit), but for some reason ignores whatever values I set for tex and always set it to vec2(0,0), rendering a blue box.

Is there something I'm missing here? I've done many opengl apps before, and I've never encountered this. :/

yes. the box is bounded from (-1,-1) to (1,1). I can also do a direct assignment (eg: tex = vec2(0.5,0.5); ) and it still doesn't work. — Andrew S. Allen
– Andrew S. Allen, Commented Nov 7, 2014 at 16:56
I was thinking it was doing integer division, but if direct assignment still gives you the same issue, perhaps not. — Scraniel
– Scraniel, Commented Nov 7, 2014 at 17:00
Using a uniform, I can set color in the fragment shader. Uniforms don't work in the vertex shader (just like direct assignment)... — Andrew S. Allen
– Andrew S. Allen, Commented Nov 7, 2014 at 17:20
Do you have alpha channel enabled? If so, you should output vec4 from fragment shader. Check your formats. Also, there should be layout(location = 0) for frag_color, but that shouldn't be the issue. — Jaa-c
– Jaa-c, Commented Nov 7, 2014 at 17:24

Lewin · Accepted Answer · 2014-11-08 19:12:16Z

0

I've seen implementations of OpenGL 2.0 having problems at compile time when adding floats and not explicitly casted integers.
Even if it is not a problem I would think possible on a 4.3 implementation, maybe you should just add dots and suffixes to your "integer" constants.

The last line of you're vertex shader seems to be the only one that could be a problem (I believe values are always casted in constructors :p ) so you would have something like this instead:

tex = (pos + 1.0f) / 2.0f;

Remember that you're shaders can compile on thousands of different implementations so it's a good habit to always be explicit!!!

answered Nov 8, 2014 at 19:12

Lewin

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1 Comment

Reto Koradi Over a year ago

Note that 1.0 is of type float in GLSL, unlike in C/C++, where it is double. So the f suffix is not needed, and actually not supported in some OpenGL versions, like OpenGL 2.0 and ES 2.0.

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