As mentioned previously, we've skipped anything that deals with opaque geometry. In general, one would probably want to render any opaque geometry first, with the depth buffer enabled, and store the rendered fragments in a texture. Then, when rendering the transparent geometry, one would disable writing to the depth buffer, and build the linked list as shown previously. Finally, you could use the value of the opaque texture as the background color when blending the linked lists.

This is the first example in this book that makes use of reading and writing from/to arbitrary (shared) storage from a shader. This capability, has given us much more flexibility, but that comes at a price. As indicated previously, we have to be very careful to avoid memory consistency and coherence issues. The tools to do so include atomic operations and memory barriers, and this example has just scratched the surface. There's much more to come in Chapter 11, Using Compute Shaders when we look at compute shaders, and I recommend you read through the memory chapter in the OpenGL Programming Guide for much more detail than is provided here.