How to prevent content from overflowing the display area?

[IC0hO]

I am currently reading the code to understand and learn. I want to know how to display chapters on the timeline, and during mouse movement, the chapter content does not exceed the osc display area. Other oscs like [mpv-osc-tethys]do not have such good effects, if the chapter is too long it will overflow the display area.

[christoph-heinrich]

We have a fairly accurate estimation of how much space text takes up thanks to compute_bounds, and with that we can clamp the position of the chapter popup.

If you want to read the relevant code, it's in text.lua.

Beware that compute_bounds is fairly slow, but when the overlay is hidden it's much faster for some reason (I think when it's hidden mpv doesn't wait for a frame redraw, but I've never checked the code to confirm). Combine that with some caching and it's usable.
Furthermore by default we don't measure whole strings, but instead individual characters and then compute string length from that, which makes it a little bit less accurate, but it's much faster when a lot of new strings are to be displayed in a large menu.

[IC0hO]

Although I don’t fully understand the whole process yet, thank you for providing the overall idea.I will delve into text.lua to better understand the implementation.

[IC0hO]

So far, mpv does not provide a direct way to retrieve the coordinates of rendered ASS, right?

[christoph-heinrich]

I don't know what you mean, after all compute_bounds gives you a bounding box of whatever was rendered.

[IC0hO]

I tried to use compute_bounds to reproduce this process. The whole process was quite complicated for me, so I skipped some steps, which made the accuracy look not that good, and it did feel a bit slow without the cache. I’m just thinking about this: it would be nice if mpv directly provided the rendered ASS coordinates. I don’t know if this explanation is clear or not.

[christoph-heinrich]

The result of compute_bounds isn't pixel perfect, the size increases in steps of 16 (not sure) pixels. To get something more accurate requires some tricks (e.g. we try to render at the largest text size possible and then scale the result down).

Afaik mpv can't give more accurate results because the textures it gets from libass increment in those sizes.

Maybe libass will one day provide something better for this purpose. There is an issue about that, but no activity in a long time libass/libass#348