GLSL subroutine is not changed

Question

I have begun learning GLSL and OpenGL. RIght now I am trying to see how GLSL subroutines work, so I have my fragment shader here:

#version 330 core

out vec4 color;

subroutine vec4 ColorFunc();

subroutine (ColorFunc) vec4 colorBlue() {
    return vec4(0.0, 0.0, 1.0, 1.0);
}

subroutine (ColorFunc) vec4 colorGreen() {
    return vec4(0.0, 1.0, 0.0, 1.0);
}

subroutine uniform ColorFunc ColorSelector;

void main() {
    color = ColorSelector();
}

In the application's source I load, compile link the shader and start the program at the beginning. A triangle is created and the vertex shader doesn't do anything special. The display function (the one sent to glutDisplayFunc) looks like this:

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT);

    glBindVertexArray(TriangleVAO);
    glDrawArrays(GL_TRIANGLES, 0, NumVertices);

    GLint ColorSelectorLoc;
    GLuint colorBlueIndex;
    GLuint colorGreenIndex;

    ColorSelectorLoc = glGetSubroutineUniformLocation(program, GL_FRAGMENT_SHADER, "ColorSelector");
    if (ColorSelectorLoc < 0)
    {
        fprintf(stderr, "Error: ColorSelector is not an active subroutine uniform in the shader\n");
        exit(EXIT_FAILURE);
    }

    colorBlueIndex = glGetSubroutineIndex(program, GL_FRAGMENT_SHADER, "colorBlue");
    colorGreenIndex = glGetSubroutineIndex(program, GL_FRAGMENT_SHADER, "colorGreen");

    GLsizei n;
    glGetProgramStageiv(program, GL_FRAGMENT_SHADER, GL_ACTIVE_SUBROUTINE_UNIFORM_LOCATIONS, &n);
    GLuint *indices = new GLuint[n];
    cout << "colorGreen: " << colorBlueIndex;
    indices[ColorSelectorLoc] = colorGreenIndex;
    glUniformSubroutinesuiv(GL_FRAGMENT_SHADER, n, indices);
    delete [] indices;

    glFlush();
}

What I expect is the triangles to be green yet they are always blue, no matter the value of ColorSelector, which is correctly retrieved (its value is 0). colorBlueIndex is 0 and colorGreenIndex is 1. I don't know what am I missing.

Answer 1

I found the problem. The subroutine's code must go before glDrawArrays:

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT);

    GLint ColorSelectorLoc;
    GLuint colorBlueIndex;
    GLuint colorGreenIndex;

    ColorSelectorLoc = glGetSubroutineUniformLocation(program, GL_FRAGMENT_SHADER, "ColorSelector");
    if (ColorSelectorLoc < 0)
    {
        fprintf(stderr, "Error: ColorSelector is not an active subroutine uniform in the shader\n");
        exit(EXIT_FAILURE);
    }

    colorBlueIndex = glGetSubroutineIndex(program, GL_FRAGMENT_SHADER, "colorBlue");
    colorGreenIndex = glGetSubroutineIndex(program, GL_FRAGMENT_SHADER, "colorGreen");

    GLsizei n;
    glGetProgramStageiv(program, GL_FRAGMENT_SHADER, GL_ACTIVE_SUBROUTINE_UNIFORM_LOCATIONS, &n);
    GLuint *indices = new GLuint[n];
    cout << "colorGreen: " << colorBlueIndex;
    indices[ColorSelectorLoc] = colorGreenIndex;
    glUniformSubroutinesuiv(GL_FRAGMENT_SHADER, n, indices);
    delete [] indices;

    glBindVertexArray(TriangleVAO);
    glDrawArrays(GL_TRIANGLES, 0, NumVertices);

    glFlush();
}