For anyone looking to pick up shaders for the first time, I recommend not beginning with the book of shaders. It’s an excellent introduction to a particularly heady slice of shader programming that, in my opinion, is a blind alley for folks new to shaders. I think focusing on fragment shaders rendering as full-screen quads makes the process feel more arcane and mind-shattering than it does when you consider the frag shader’s role in a 3D rendering pipeline. In the latter context, the simultaneous invocation of every frag doesn’t feel like a puzzle to solve but a reasonable part of getting pixels onscreen as part of a 3D render. For that reason I recommend newbies work on using fragments in the context of materials that include vertex and potentially compute shaders to manipulate geometry and to replicate things like phong shading or a shader-based particle system first.