Reading and updating texture buffer in OpenGL/GLSL 4.3

Question

I'm going a bit nuts on this since I don't really get what is wrong and what not. There must either be something that I've vastly misunderstood or there is some kind of bug either in the code or in the driver. I'm running this on AMD Radeon 5850 with the latest catalyst beta drivers as of last week.

OK, I began doing a OIT-rendering implementation and wanted to use a struct-array saved in a shader storage buffer object. Well, the indices in that one were reflecting/moving forward in memory way wrong and I pretty much assumed that it was a driver bug - since they just recently started supporting such thing + yeah, it's a beta driver. Therefore I moved back a notch and used glsl-images from texture buffer objects instead, which I guess had been supported since at least a while back.

Still wasn't behaving correctly. So I created a simple test project and fumbled around a bit and now I think I've just pinched where the thing is.

OK! First I initialize the buffer and texture.

//Clearcolor and Cleardepth setup, disabling of depth test, compile and link shaderprogram etc.
...
//
GLint tbo, tex;
datasize = resolution.x * resolution.y * 4 * sizeof(GLfloat);
glGenBuffers(1, &tbo);
glBindBuffer(GL_TEXTURE_BUFFER, tbo);
glBufferData(GL_TEXTURE_BUFFER, datasize, NULL, GL_DYNAMIC_COPY);
glBindBuffer(GL_TEXTURE_BUFFER, 0);

glGenTextures(1, &tex);
glBindTexture(GL_TEXTURE_BUFFER, tex);
glTexBuffer(GL_TEXTURE_BUFFER, GL_RGBA32F, tex);
glBindTexture(GL_TEXTURE_BUFFER, 0);
glBindImageTexture(2, tex, 0, GL_TRUE, 0, GL_READ_WRITE, GL_RGBA32F);

Then the rendering loop is - update and draw, update and draw ... With a delay in between so that I have time to see what the update does.

The update is like this...

ivec2 resolution; //Using GLM
resolution.x = (GLuint)(iResolution.x + .5f);
resolution.y = (GLuint)(iResolution.y + .5f);

glBindBuffer(GL_TEXTURE_BUFFER, tbo);
void *ptr = glMapBuffer(GL_TEXTURE_BUFFER, GL_WRITE_ONLY);
color *c = (color*)ptr; //color is a simple struct containing 4 GLfloats.
for (int i = 0; i < resolution.x*resolution.y; ++i)
{
  c[i].r = c[i].g = c[i].b = c[i].a = 1.0f;
}
glUnmapBuffer(GL_TEXTURE_BUFFER); c = (color*)(ptr = NULL);
glBindBuffer(GL_TEXTURE_BUFFER, 0);

And the draw is like this...

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glMemoryBarrier(GL_ALL_BARRIER_BITS);
ShaderProgram->Use(); //Simple shader program class
quad->Draw(GL_TRIANGLES); //Simple mesh class containing triangles (vertices) and colors
glFinish();
glMemoryBarrier(GL_ALL_BARRIER_BITS);

I just put some memory barriers around to be extra sure, shouldn't harm more than performance right? Well, the outcome was the same with or without the barriers anyway, so ... :)

The Shader program is a simple pass-through vertex shader and the fragment shader that's doing the testing.

Vertex shader

#version 430

in vec3 in_vertex;

void main(void)
{
    gl_Position = vec4(in_vertex, 1.0);
}

Fragment shader (I guess coherent & memoryBarrier() isn't really needed here since I do them on CPU in between draw/fragment shader execution... but does it harm?)

#version 430

uniform vec2 iResolution;
layout(binding = 2, rgba32f) coherent uniform imageBuffer colorMap;

out vec4 FragColor;

void main(void)
{
    ivec2 res = ivec2(int(iResolution.x + 0.5), int(iResolution.y + 0.5));
    ivec2 pos = ivec2(int(gl_FragCoord.x + 0.5), int(gl_FragCoord.y + 0.5));
    int pixelpos = pos.y * res.x + pos.x;

    memoryBarrier();
    vec4 prevPixel = imageLoad(colorMap, pixelpos);

    vec4 green = vec4(0.0, 1.0, 0.0, 0.0);
    imageStore(colorMap, pixelpos, green);
    FragColor = prevPixel;
}

Expectation: A white screen! Since I'm writing "white" to the whole buffer between every draw even if I'm writing green to the image after load in the actual shader.

Result: The first frame is green, the rest is black. Some part of me thinks that there is a white frame thats too fast to be seen or some vsync-thing that tares it, but it this a place for logics? :P

Well, then I tried a new thing and moved the update block (where i'm writing "white" to the whole buffer) to the init instead.

Expectation: A white first frame, followed by a green screen.

Result: Oh yes its green allright! Even though the first frame is with some artifacts of white/green, sometimes only green. This might probably be due to (lack of) vsync of something, haven't checked that out. Still, I think I got the result I was looking for.

The conclusion I can draw out of this is that there is something wrong in my update. Does it unhook the buffer from the texture reference or something? In that case, isn't it weird that the first frame is OK? It's only after the first imageStore-command (well, the first frame) that the texture goes all black - the "bind()-map()-unmap()-bind(0)" works the first time, but not afterwards. My picture of glMapBuffer is that it copies the buffer data from GPU to CPU memory, let's you alter it and Unmap copies it back. Well, just now I thought that maybe it doesn't copy the buffer from GPU to CPU and then back, but only one way? Could it be the GL_WRITE_ONLY which should be changed to GL_READ_WRITE? Well, I've tried both. Supposedly one of them was correct, wouldn't my screen when using that one always be white in "test 1"?

ARGH, what am I doing wrong?

EDIT: Well, I still don't know... Obviously glTexBuffer(GL_TEXTURE_BUFFER, GL_RGBA32F, tex); should be glTexBuffer(GL_TEXTURE_BUFFER, GL_RGBA32F, tbo);, but I think tbo and tex had the same value since they were generated in the same order. Therefore it worked in this implementation. I have solved it though, in a manner I'm not very happy with since I really think that the above should work. On the other hand, the new solution is probably a bit better performance-wise. Instead of using glMapBuffer(), I switched to keeping a copy of the tbo-memory on CPU by using glBufferSubData() and glgetBufferSubData() for sending the data between CPU/GPU. This worked, so I'll just continue with that solution.

But, yeah, the question still stands - Why doesn't glMapBuffer() work with my texture buffer objects?

Answer 1

glTexBuffer(GL_TEXTURE_BUFFER, GL_RGBA32F, tex); should be glTexBuffer(GL_TEXTURE_BUFFER, GL_RGBA32F, tbo);

perhaps there is something else wrong, but this stands out. https://www.opengl.org/wiki/Buffer_Texture