Cannot create/ sample 3D Texture (Qt/ OpenGL) - c++

Question

I am trying to display a layer in a 3D texture created from some 3D data, but all points sampled are always black (I guess my texture creation/ allocation is failing somehow). It is being rendered on a plane using window coords. I have checked my data, it is a vector with the correct values. glEnable(GL_TEXTURE_3D) has been called earlier. Any clues why this would fail?
Function that creates the texture:
bool VolumeRender::setVolumeData(QOpenGLShaderProgram *program, vector<unsigned short> v, int x, int y, int z){
voxels.resize(v.size(), 0);
cout << "Processing texture" << endl;
unsigned short sMax = 0;
unsigned short sMin = 32768;
for (unsigned int i =0; i< voxels.size(); i++){
sMax = max(sMax, v[i]);
sMin = min(sMin, v[i]);
} for (unsigned int i =0; i< voxels.size(); i++) voxels[i] = (v[i] - sMin)/(float)(sMax-sMin);
cout << "Loading 3D texture" << endl;
gl.glActiveTexture(GL_TEXTURE0);
gl.glGenTextures(1, &volumeTexture);
gl.glBindTexture(GL_TEXTURE_3D, volumeTexture);
gl.glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
gl.glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
gl.glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_BORDER);
gl.glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_BORDER);
gl.glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_WRAP_R, GL_CLAMP_TO_BORDER);
gl.glTexImage3D(GL_TEXTURE_3D, 0, GL_RED, x, y, z, 0, GL_RED, GL_FLOAT, &voxels[0]);
gl.glBindTexture(GL_TEXTURE_3D, 0);
program->bind();
program->setUniformValue("VOXELS", 0);
program->release();
voxelsLoaded = true;
return true;
}
Simple fragment shader:
#version 330 core
uniform sampler3D VOXELS;
uniform vec2 SIZE;
out vec4 color;
void main(){
vec2 coords = (gl_FragCoord.xy - 0.5) / SIZE;
vec3 texcoords = vec3(coords, 0.5);
color = texture(VOXELS, texcoords);
}

Answer 1

glEnable(GL_TEXTURE_…) has no effect when using shaders. It's a relic from the fixed function pipeline era. On the other hand the texture must be actually bound when drawing.
In your code you have
gl.glBindTexture(GL_TEXTURE_3D, 0);
program->bind();
program->setUniformValue("VOXELS", 0);
program->release();
Now since this is in initialization code, it's unclear if you actually understand the consequences of these lines. So let's break it down:
gl.glBindTexture(GL_TEXTURE_3D, 0);
This means that texture 0 (which with shaders is the nil texture, but in old and busted OpenGL-1.0 it actually could be sampled form) is bount to texture unit 0. From there on, when trying to sample from texture unit 0, it will not sample anything.
program->bind();
program->setUniformValue("VOXELS", 0);
program->release();
Set the sampler uniform with name "VOXELS" to sample from texture unit 0. Whatever texture is bound to that texture unit at the moment of calling the draw function, that texture will be sampled from.
Somewhere in your program you're making a draw call. You didn't show us where. But in order for your drawing to actually sample from your texture you have to bind your 3d texture to texture unit 0.
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_3D, volumeTexture);
draw_stuff();