OpenGL Compute Shader rendering to 3D texture does not seem to do anything

Question

I have written a small shader to set the color values of a 3D volume.

#version 430

layout (local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
layout (r8, binding = 0) uniform image3D destTex;

void main() 
{
    imageStore(destTex, ivec3(gl_GlobalInvocationID.xyz), vec4(0.33, 0, 1.0, 0.5));
}

It compiles and links without errors.

The volume is created and the compute shader is dispatched like this

    generateShader->use();

    glEnable(GL_TEXTURE_3D);
    glGenTextures(1, &densityTex);

    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_3D, densityTex);

    glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

    glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_WRAP_R, GL_CLAMP_TO_EDGE);

    glTexImage3D(GL_TEXTURE_3D, 0, GL_R8, chunksize + 1, chunksize + 1, chunksize + 1, 0, GL_RED, GL_UNSIGNED_BYTE, NULL);
    glBindImageTexture(0, densityTex, 0, GL_TRUE, 0, GL_READ_WRITE, GL_R8);

    glDispatchCompute(chunksize + 1, chunksize + 1, chunksize + 1);
    // make sure writing to image has finished before read
    glMemoryBarrier(GL_ALL_BARRIER_BITS);

asdf(0); //Here I print the content of my texture. 
         //Strangely it contains the values 205 everywhere and independent of what I set in the shader: vec4(0.33, 0, 1.0, 0.5)

    glBindTexture(GL_TEXTURE_3D, 0);

The method for printing the image data looks like this:

void Chunk::asdf(int slice) {

    GLubyte* image;
    image = new GLubyte[(chunksize + 1) * (chunksize + 1) * (chunksize + 1)];

    glGetTexImage(GL_TEXTURE_3D, 0, GL_R8, GL_UNSIGNED_BYTE, image);
    std::cout << "Reading texels" << std::endl;
    for (int i = 0; i < (chunksize + 1); i++) {
        for (int j = 0; j < (chunksize + 1); j++) {

            int start = (((chunksize + 1) * (chunksize + 1) * slice) + (i * (chunksize + 1)) + j);
            std::cout << "Texel at " << i << " " << j << " " << slice << " has color " << (float)image[start] << std::endl;
        }
    }

    delete[] image;
}

What is wrong in this code? Do I need some special method to retrieve the data after setting it on the GPU? Also, I dont quite understand how the value is set in the GPU because when using imageStore one has to set a vec4 value, but the texture only contains one channel for red (it does not make a difference if I use a RGBA format)

edit: It turns out I didn't get the problem right. I wrote myself a shader to render the content of my 3D texture to the screen. It seems my print-function (asdf) does not get the right values of the texture because if rendered to the screen the color values are different

as you can see it still doesn't look quite right, but I assume this is a topic fo another issue?

Answer 1

glGetTexImage(GL_TEXTURE_3D, 0, GL_R8, GL_UNSIGNED_BYTE, image);

That will not copy any data to image , but will only generate a GL_INVALID_ENUM error. GL_R8 is just not a vaild format parameter. You most likely mean GL_RED here (note this is the same you used for glTexImage3D 's format parameter. GL_R8 is only an enum for internalFormat , and it is never relating to anything you see on OpenGL client side, see also this question ).

As a more general recommendation, I really advice everyone to use OpenGL's Debug Output feature during development, so that no OpenGL error will go unnoticed.