Writing to a compressed texture using a compute shader, with no extra copies

Question

I'm trying to figure out what the best way is to generate an OpenGL texture using a compute shader. So far, I've read that pixel buffer objects are good for non-blocking CPU -> GPU transfers, and that compute shaders are capable of reading and writing buffers regardless of how they're bound. Ideally, I'd like to avoid as many copies as possible. In other words, I'd like to allocate a buffer on the GPU, write compressed texture data to it, and then use that buffer as a texture object in a shader.

Currently, my code looks something like this:

GLuint buffer;
glGenBuffers(1, &buffer);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, buffer);
glBufferStorage(GL_SHADER_STORAGE_BUFFER, tex_size_in_bytes, 0, 0);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0);

// Bind buffer to resource in compute shader
// execute compute shader

glBindBuffer(GL_PIXEL_UNPACK_BUFFER, buffer);
glCompressedTexImage2D(GL_TEXTURE_2D, 0, fmt, w, h, 0, tex_size_in_bytes, 0);

Is this correct? I read somewhere about guaranteeing synchronization, too. What do I need to add to make sure that my compute shader completes execution prior to copying from the buffer object?

Answer 1

After looking into this for a while, I found out a couple of things:

1. You cannot avoid a memcpy: You cannot write directly into texture storage allocated for a compressed texture using only OpenGL API calls. This means that you cannot avoid the call to glCompressedTexImage2D with a bound PBO. That being said, you may be able to use a 16-bit RGBA texture and a GLSL image type in your compute shader.

2. You do need to synchronize memory: In order to make sure that your compute shader finishes writing to your storage buffer, you must make sure that all reads and writes to it finish. This is done by calling glMemoryBarrier with GL_SHADER_STORAGE_BARRIER_BIT.

The full code for something that writes into a buffer to be used as a compressed texture looks like this:

GLuint buffer;
glGenBuffers(1, &buffer);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, buffer);
glBufferStorage(GL_SHADER_STORAGE_BUFFER, tex_size_in_bytes, 0, 0);

glUseProgram(compute_shader_prog);
glBindBufferBase(GL_SHADER_STORAGE_BUFFER, compute_shader_bind_point, buffer);
glDispatchCompute(wg_x, wg_y, wg_z);
glMemoryBarrier(GL_SHADER_STORAGE_BUFFER_BIT);

glBindBuffer(GL_PIXEL_UNPACK_BUFFER, buffer);
glCompressedTexImage2D(GL_TEXTURE_2D, 0, fmt, w, h, 0, tex_size_in_bytes, 0);