Which memory barrier does glGenerateMipmap require?

glsl, memory-barriers, mipmaps, opengl

Solution

The OpenGL 4.6 specification clarifies this:

Any synchronization required before performing this reduction will be done within the `Generate*Mipmap` commands themselves.

So you don't need any kind of synchronization. If you have caused data to be written to the base mipmap level in any way, `glGenerateMipmap` will perform sufficient synchronization to make reads work.

Given that, it's probably a really good idea not to call this in the middle of rendering a frame.

Prior specifications had no answer, though information from this bug report suggests that prior implementations did exactly the above.

Problem

I've written to the first mipmap level of a texture using GL_ARB_shader_image_load_store. The documentation states that I need to call glMemoryBarrier before I use the contents of this image in other operations, in order to flush the caches appropriately. For instance, before I do a glTexSubImage2D operation, I need to issue GL_TEXTURE_UPDATE_BARRIER_BIT​, and before I issue a draw call using a shader that samples that texture, I need to issue GL_TEXTURE_FETCH_BARRIER_BIT​. However, which barrier do I need to issue before I am ensured that glGenerateMipmap will use the most recently written data?

Original source