Support for Xiaomi Pad 8 "focus stylus pro" pen

[mateosss]

The writing with this new xiaomi pen is really off on apps that do not support it directly (for now I found that only these apps have reasonable writing strokes: hyperos notes, mi canvas, kilonotes, and chinese versions of notein, jnotes, and touchnotes. Everything else feels awful to write, including Write.

Could you give me a pointer of where I could start looking in the code if I wanted to implement this?

For now, I found some documentation in xiaomi's website, translated in the image below:

[pbsurf]

Could you upload a video showing the problem?

Android touch and style events are handled in MainActivity::onTouch() implementing View.OnTouchListener. I think in general input prediction should not be needed for any active stylus, does not work well in any case, and results in disconcerting changes in the stroke when writing or when the stylus is lifted. The xiaomi documentation makes reference to authentication, so perhaps they have intentionally crippled the behavior for apps that don't specifically use the xiaomi APIs?

[mateosss]

My intuition is that their pen technology is not very precise and thus need to do a lot of heavy lifting with their custom prediction API. The prediction makes writing usable but still not as good as a wacom/s-pen experience. The authentication part I cannot really say what's that about.

You then think that it would be better to override onTouch rather than try to go to SDL and do something like what they did on libsdl-org/SDL#11753. I'm asking this because so far I've only found the pen input coming from SDL in Write, but maybe I didn't read well enough.

I am sending two videos below, one writing a full sentence on both Write and the hyperos Notes app. And the second video is of some specifics that I noticed are particularly bad without the prediction: the "nd" transition in the word finding (n ends going down, d starts high above), writing dots, and writing a k (three differently oriented lines)

Note that on Write this is using a smoothing of 4 and 0 simplification. I tried different smoothing settings and no smoothing but the result is similar.

1. Full sentence:

   fullsentence-compressed.mp4

2. Specifics:

   specifics-compressed.mp4

[pbsurf]

Write bypasses SDL for input because even the latest SDL3 doesn't support everything needed for the best pen input quality, for example, historical input points. In the future, SDL will be replaced with the platform interface code from https://github.com/styluslabs/maps

From the videos, it looks like there is a problem with rendering, which disabling GPU rendering in advanced preferences should fix. This might improve the writing a little. It also looks like disabling pressure sensitivity could help some.