This is certainly interesting and informative for somebody who already kinda knows how the code->gpu->screen pipeline works and wants to know how it's made fast, but I'm still missing some of the "pre"/"post" background for how/when the request to the shading system is made and what happens after the GPU is done.

I just started poking at doing computer graphics "from scratch" (working my way up from ray tracers now). I know that modern graphics systems are huge and ultimately your C++ library or whatever is just gonna flip some bits on graphics-card registers/busses or something, but I was really hoping for some background on "here's a bit of code that asks the shader to add depth to a triangle" and then how that gets mapped back to the screen.