How to save part of the screen and restore it later?

[espruino-discuss2]

Posted at 2025-01-30 by mbork

I'd like to be able to save a (small) part of the screen (that is, a bitmap) to a variable and restore it later, so that I can:

1. draw something
2. "erase" it later, restoring the screen contents to the previous state.
   How to do that?

Graphics.asImage() seems to store the whole screen. Using Graphics.getPixel() in a nested loop seems inefficient. Is there another way?

[espruino-discuss2]

Posted at 2025-01-30 by @gfwilliams

Graphics.asImage() seems to store the whole screen.

I'd have suggested you make a Graphics instance and then draw the Graphics onto it, but I just checked and it seems you can't do that at the moment (it only works for ArrayBuffer Graphics instances).

It does make sense to be able to supply options to asImage though - I've just added it... So if you install a 'Cutting Edge' firmware build, you can now do: im = g.asImage({x:16,y:16,w:64,h:64,type:"string"}) for example to get an image of exactly a certain area of the screen, which should work nicely for you.

If you don't want to rely on the latest builds, the other option is to actually do the drawing of the area you want to update a lot in an offscreen buffer created with Graphics.createArrayBuffer. You can even use https://www.espruino.com/Reference#l_Bangle_setLCDOverlay to actually overlay what you're drawing on top of the existing screen contents without changing them

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Posted at 2025-01-30 by mbork

Wow, thanks for the quick answer!
And no, I'll probably not install the "cutting edge", I'm a bit too afraid to do that. ;-) When is it going to be available in a "normal" way?

And thanks for pointing me to Bangle.setLCDOverlay – this should work ideally for my use-case (a "sprite", so to speak). One drawback (as opposed to drawImage) is that I can't easily rotate it – it means that I'll need to draw it anew every time I need a rotation – but that should not be a big issue.

Altogether, it means that it's possible that I'll make a nice clockface soon (for some value of "soon", I'll probably not be able to start working on it earlier than in ~2 weeks).

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Posted at 2025-01-30 by @gfwilliams

I'll probably not be able to start working on it earlier than in ~2 weeks

Realistically I'll probably have made a 2v26 release by then so you might find you can use asImage anyway!