SDL renderers render text extremely slowly

[niansa]

Hi!

I've noticed that the more text you render the slower the SDL renderers (SDL2, SDL3) become. According to my profiler, almost all of the frame time is spent rendering text!
This gets to the point where software rendering a page with an on-screen virtual keyboard can take up to 5 ms on a fast system, or up to 250 ms on a slow system. Maybe rendered text should be cached somehow?

[dan99199]

The provided SDL2/SDL3 renderers use SDL_ttf. TTF_Font caches bitmaps of glyphs. TTF_Text caches the vertices of its string (shaped, layout, etc).

The renderer will create and destroy a TTF_Text object for each piece of text that is rendered. The renderer also changes the font size, so that can invalidate and force a rebuild of cached bitmaps. This is most likely the performance issue you are observing. Also the renderer based text engine is pretty slow. The gpu engine may fare a bit better, even with the invalidations.

I believe the problem is that SDL_ttf works better in a retained mode UI, rather than an immediate mode api. I also don't think there is a generalized caching solution. If the app changes fonts alot during a render or there are alot of strings across the entire app, the caching solution will be different.

[linkdd]

In my SDL renderer, I have a cache of TTF_Text objects, which is basically a hashmap of Clay_ElementID to TTF_Text* so that I don't have to rebuild them (I use then TTF_SetTextString which is a no-op if the text did not change, and TTF_SetTextColor)

I also ignore the fontSize of the text element config, and expect the TTF_Font* associated to the fontId to have been loaded with the correct ptsize.

The renderers in the repository are examples, not definitive solutions. You should roll your own to answer your game's specific needs.