[joeld42]

Most of the drawing work of a GUI is drawing text, shape paths, and images. A GPU-accelerated UI layer draws these using GPU commands. It's not exactly the same as making an ortho 2D scene, but conceptually that's pretty much it. Often it means using crude geometry (such as a quad per glyph in a text renderer, or a "ribbon" of quads to cover a curve) and using a shader to draw the glyph or curve itself.

For a simple example, think of a rounded rect. Typically this would draw as a quad (a pair of triangles), and a shader would calculate the rounded corners.

There's also a lot of compositing and clipping that happens in a UI (e.g. a large widget inside a scrollbox) which is challenging to do on GPU as these get nested.