change float in vertex shader (GLSL)

Question

I thought i had it working but I still can't figure out how to change a value in my vertex shader. What i do in opengl is the following, for each render:

//i bind my shader
shader.bind();

// Then trying to set the value in my vertex shader like so:
GLuint test = glGetUniformLocation(shader.id(), "Test");
glUniform1f(test, 6.0f);

// Do some other stuff like rendering an object
// ...

// then unbind shader
shader.unbind();

This is the value i want to change in my vertex shader:

uniform float Test = 0.66; // Doesn't work
//const float Test = 0.66; // Works

But my shader doesn't work when i try that, it renders my object black.

If i comment the glGetUniform(Location) part out and if use the float as a const instead of a uniform then my shader works fine. But i want to be able to change the value in my shader in each render. I though i had to use a uniform for that...?

Is there something i'm missing..??

EDIT:

Appearently i'm getting the following GLSL error:

non constant expression in initialization

So it must be constant in order to work... Does that mean there is no way to have this as a dynamic value..?

This is my Vertex Shader:

const float FresnelPower = 1.0;
uniform float Eta;         // <---- MUST BE CONSTANT...

const float F  = ((1.0-Eta) * (1.0-Eta)) / ((1.0+Eta) * (1.0+Eta));

varying vec3  Reflect;
varying vec3  Refract;
varying float Ratio;

void main()
{
    vec4 ecPosition  = gl_ModelViewMatrix * gl_Vertex;
    vec3 ecPosition3 = ecPosition.xyz / ecPosition.w;

    vec3 i = normalize(ecPosition3);
    vec3 n = normalize(gl_NormalMatrix * gl_Normal);

    Ratio   = F + (1.0 - F) * pow((1.0 - dot(-i, n)), FresnelPower);

    Refract = refract(i, n, Eta);
    Refract = vec3(gl_TextureMatrix[0] * vec4(Refract, 1.0));

    Reflect = reflect(i, n);
    Reflect = vec3(gl_TextureMatrix[0] * vec4(Reflect, 1.0));

    gl_Position = ftransform();
}

Nicol Bolas Nicol Bolas · Accepted Answer · 2012-01-28T16:57:47

uniform float Eta;         // <---- MUST BE CONSTANT...

When GLSL says "constant", what it means is a compile time constant. The value of Eta is certainly uniform within a particular draw call. But it is not constant, because it can change between draw calls. The compiler does not know what this value will be at compile time.

A const variable is a compile time constant. It therefore must be initialized with an expression consisting only of compile time constants. uniforms are not compile time constants, so you cannot initialize a const variable with them.

change float in vertex shader (GLSL)

2 Answers