screenshots are sometimes sent in poor quality

[r10s]

to send images, we scale them down to ~ 1280px width and jpg-compress them under 500k. if the image is still larger, we try over with a smaller size. avatar work similar with different constraints.

in many situations, this is a good approach, even for screenshots, refined recently a lot https://github.com/chatmail/core/pulls?q=is%3Apr+image

however, if you have a larger screenshot on desktop, with a good part of photo and another good part of text, the outgoing image looks bad.

the conversion png -> jpg is of course part of the quality loss, however needed to get images in a compatible way small at all, there were some measurements in the past, that part is really needed. done also by apple, btw.

main reason for the poor quality is the scaling down, and that by that the quality.

so, maybe do the following:

• if an image to sent is not jpg, try first to send it with far larger dimensions, eg 4096 px. only if this gets a too large file, try over with a smaller size. this was suggested already at recode large PNG #3956, but not done that time for $reasons

• using really the original dimensions is risky, as there are png around with eg. tens of 10000 px sides. this is also not the issue at hand

• we should not do that approach for images that are already jpg, as this is not the issue at hand, and likely results in side effects. it will also increase overall data usage

example

eg. the following 1618 x 949 px, 1.1 mb png-image:

sending the image with Delta Chat results in a 1280 x 751 px 79 kb jpg-image:

if you convert the image in its original size to jpg, using the same 75% quality setting, which still seems to be inside the sweet spot, it is far better - while being 114 kb only:

sure, that specific image can be improved further. but goal is to have an overall, simple approach, that does not make all images larger that would not need it.

[iequidoo]

I found an interesting commit, even i've forgotten about it: 430a712:

Also don't reduce wide/high images if they aren't huge. Among other benefits, this way most of PNG
screenshots aren't touched.

That was already a try to preserve screenshots quality, however it doesn't work for screenshots with a big photo like in this issue.

to send images, we scale them down to ~ 1280px width and jpg-compress them under 500k. if the image is still larger, we try over with a smaller size. avatar work similar with different constraints.

So, this isn't quite true already from 2023-04. In a dev chat i suggested:

the issue is about screenshots, however, small text can appear also on photos, so i suggest not to add this additional check and just compare the encoded image weight with {BALANCED,WORSE}_IMAGE_BYTES -- if it fits, why reduce the image size to 1280 px, we already have MAX_RCVD_IMAGE_PIXELS: u32 = 50_000_000 which we can use as a limit when sending too[...]

and @r10s answered:

i'd like to fix the issue on point. this is that screenshots (so, non-jpg, or png) should start hight resolution, this was also the initial idea, 2y ago. it makes sense to not handle that all the same way.
allowing larger images sizes in general will result in larger overall data usage - MAX does not mean we want to read that value always. also, it will slow down small devices, that suddenly have to handle lots of far larger images. iirc, also telegram recodes to 1280 still, so let's stay with that.

No complains have been received about "too large images" AFAIR, so it's a question for me if we should revert that part of 430a712. So, let's start with adding a pure recode-to-JPEG step w/o linear size reduction and not touch logic for JPEG images for now.

[r10s]

No complains have been received about "too large images"

sure there have been, this is reason for recoding, compression, options etc. :)

it just seems fine meanwhile.

as said, i would really like to fix that on point without changing much elsewhere. it is easy enough