Large stickers lose their transparency when sent

[Amzd]

In iOS 17+ you can select a subject in a photo and drag it out as a sticker. When copying or dragging these they are not resized so if the image was massive, the sticker will be massive.

When pasted into delta chat, we can natively detect that it has transparent corners and send it as sticker. Now when I send a sticker that is too big, core resizes it and it loses transparency and gains a black background. Preferably the sticker would be resized by core without losing transparency. If that is out of scope for core we can resize natively in iOS.

• Operating System: iOS 17
• Delta Chat Version: master
• Expected behavior: Stickers are resized without transparency loss
• Actual behavior: Stickers gain black background
• Steps to reproduce the problem: Drag (or copy) a subject from a large image to Delta Chat
• Screenshots:

• Logs:

[link2xt]

First thing I want to know is if this is still sent as a sticker viewtype or gets converted to image by heuristics.
For some history regarding sticker heuristics:

• Some screenshots are sent as stickers #4611 (screenshots are sent as stickers, original issue mostly happened on iOS IIRC)
• fix: W/a sending images sent as stickers on some platforms (#4611) #4619 (added heuristics to convert stickers to non-stickers)
• Add a way to force sending the image as a sticker #4739 (request to make it possible to force sticker viewtype, used by desktop sticker picker to force the sticker viewtype)
• api(json-rpc): force stickers to be sent as stickers #4819 (PR to add the ability to force stickers)
  If UI is absolutely sure that it is a sticker, it can set viewtype to sticker and force sticker viewtype, then heuristics is not used.
  Core heuristics already checks for transparent corners an I think it does not look at the image size or dimensions.

If it is sent as a sticker, then the only bug is that it converts to JPEG instead of resizing the image as PNG which can preserve transparency.

[Amzd]

It is sent as view type DC_MSG_STICKER on iOS

[r10s]

then the only bug is that it converts to JPEG instead of resizing the image as PNG which can preserve transparency.

if goal is to have small byte sizes, this is not a bug. JPG is often at least a magnitude smaller than (even optimized) PNG, see #3956 (comment) and #4037 (comment)

ftr, the arguably too large stickers resulting from iOS-auto-generated-drag-from-gallery are scaled down on iOS at deltachat/deltachat-ios#2535 - so, well, maybe we should just stop changing stickers in core.

so, tbh, not sure, what do here. the iOS issue seems to be fixed. and tweaking things again might result in new bugs, might totally be we did not consider all cornercases, at least i cannot say i have a good overview 🤷‍♂️