Rendering

MyGo draws on the GPU with Metal on macOS, with Direct3D 11 on Windows, or with WARP, Windows' own software renderer, where no GPU driver works, and with OpenGL on Linux, in the GtkGLArea GTK shows. A shader computes rounded rectangles, borders, gradients and shadows from the distance to their edges, so they stay sharp at any size and scale, and text comes from a glyph atlas that only uploads what changes.

On macOS, frames that change little, such as a clock ticking, typing or the pointer over a button, are drawn on the CPU, which redraws only what changed, and go to the screen without waking the GPU: they take less time than the GPU takes to start, and spare the memory Metal's driver holds for a couple of seconds after each frame it draws. Scrolling, resizing and animations of much of the window use the GPU, as do animations that change little but cost the CPU much to redraw, such as a dot pulsing over translucent layers, gradients and shadows at the display's rate: once drawing a burst of frames on the CPU takes more than a quarter of its time, the rest of the burst draws on the GPU, and the frame after a pause on the CPU again.

On Linux, a window draws on the CPU until that costs too much, as when scrolling or animating much of a large window on a fast display, and only then loads OpenGL: Mesa, the driver, takes some 50 MB, which stay, about as much as the rest of a small app. Where OpenGL would not run on a GPU, as in virtual machines or in WSL (where GALLIUM_DRIVER=d3d12 gives Mesa the GPU), Linux always draws on the CPU: about a millisecond for a whole large window on a high-density display, on several cores, and less than a tenth of one for what typically changes, such as a button under the pointer, since it redraws, and has the compositor take, only that. Set MYGO_GPU=0 to use the CPU renderer everywhere, for instance to compare, and on Linux MYGO_GPU=1 to draw with OpenGL from the first frame, even where it runs on the CPU.

Set MYGO_FRAME_STATS=1 to log each frame that takes longer than 8 ms, or MYGO_FRAME_STATS=4 for another threshold in milliseconds (all logs every frame). Each line tells how long building the frame (and in how many passes), laying it out, painting and presenting it took, whether it was drawn on the CPU, on the GPU or in memory (where a window has no GPU renderer), what the process allocated and how many texts it laid out meanwhile, and whether the garbage collector ran, with the time goroutines spent helping it mark:

mygo: frame 412 took 17.3 ms: build 3.1 (2 passes), layout 1.2, paint 0.8, present 11.5 (drawn on the GPU), other 0.7; meanwhile the process made about 2312 allocations, 171.0 KB, 12 text layouts; GC: 1 cycles ended, 4.1 ms of assists

Allocations and the collector are the whole process's, other goroutines' included, as runtime/metrics counts them: a span at a time, so that small counts read low. While the variable is unset, frames measure nothing.

MyGo draws a frame only when something changes: input, Invalidate, After, or an animation that moves. An idle window draws nothing, and two seconds after its last frame it frees the frame it drew on the CPU.