The font rasterizer is a massive hack in modern UIs. Subpixel rendering is a serious pain in the ass. When you render text using subpixel rendering, you render the actual vectors at 3x the spatial resolution. But, not simply as if the vectors were 3x wider, because that would look too sharp: it needs to render as if there was 3x as many pixels, which is different.

Then there’s compositing. Normal layers can be composited using alpha blending, assuming some sane format like premultiplied alpha RGBA. But not subpixel rendered text, because alpha blending the components will fuck up the subpixel rendering.

And it goes on, because if you want to handle text like everything else, you need special cases for it to look right. Rotation? Need to render the vectors rotated; can’t rotate in raster. If you need to render to a surface then transform that surface, you’re SOL; it can’t go to rasters until the end.

Normal surfaces can also be rendered at subpixel positions, and of course this does not work for surfaces containing text, because again, it will destroy the subpixel rendering.

OK. So you can get rid of the subpixel rendering and render slightly blurrier glyphs instead. (R.I.P. anyone trying to tell hanzi/kanji apart.) It’s still going to murder legibility if you move it over by a subpixel value because text is already on the edge of readability at 96 DPI.

I haven’t considered gamma correction, hinting, blending different colors, different blending modes, GPU acceleration, etc. because I simply don’t have the brain power to try to reconcile it all. It’s a nightmare.

We already did some of this for text. Which is a herculean effort. We use a freakin virtual machine to power font hinting, and ugly, complex, slow special casing at many layers of already ridiculously complex vector graphics stacks (I mean if you disagree with that assessment, you may just be smarter than I am, but I have serious trouble following the Skia codebase and I doubt Cairo is really that much better.) And speaking of which, there only really seems to be a handful of them out there: there’s Skia, used by most web browsers; Cairo, used by GTK; Direct2D, in Windows; Whatever modern macOS uses that isn’t QuickDraw anymore; and I guess there’s Mozilla’s pathfinder, a promising Rust-based vector graphics engine that was built as part of Servo and seemingly mostly abandoned, much to the world’s detriment. This work is hard. It can be done, but it’s not something I think a single engineer can do, if you want to build one that competes with the big boys even disregarding a few things like performance. I’d love to be wrong, but I have a sinking feeling I’m not.

Even text isn’t done being overcomplicated. As nyanpasu has mentioned above, some software have started implementing SDFs for font scaling. We do this because text legibility is really that important, whereas a line in the UI being slightly blurry for users on older screens is really just not that important. Some languages flat out can’t be read with crappy font rendering, and any of them will give you eyestrain if it’s ugly enough. As much as it sucks, a blurry border on a button doesn’t have an accessibility issue. And rendering at 1x and making the compositor upscale is not a great solution either because again, it’s already hard enough to read text in some languages; the added blurriness of scaling text and ruining subpixels is basically intolerable.

These hacks aren’t free, and with high DPI displays, they’re not needed. There’s a reason Apple did what they did.