color looks wrong when CMYK jpg changed to RGB after opened and saved by image-rs

[wellitecho]

This happens when I simply used image-rs to open and save a CMYK jpg file

Expected

I expect the CMYK jpg file stays the same(file size, colorspace, etc.)
I did not expect a CMYK jpg be changed to RGB jpg automatically. Because this CMYK jpg is used for printing.

Actual behaviour

the original CMYK jpg file is 12.5Mb, in CMYK; after opened and saved by image-rs, the size was changed to 2.2Mb, color spaced was changed to RGB(found out with Photoshop), and the image color looks different than the original file.

Reproduction steps

Download the original CMYK image file from the link below, and simply run:

use image;
/*
Cargo.toml:
[dependencies]
image = "0.23"
*/
fn main() {
    let img = image::open("cmyk_original.jpg").unwrap();
    img.save("cmyk_open_and_saved.jpg").unwrap();
}

Provide source code, a repository link, or steps
original cmyk image file uploaded to mega.io

open and saved by image-rs

edit: added links to images

[wellitecho]

closed; moved to jpeg-decoder module

[wellitecho]

Previously I moved this issue to image/jpeg-decoder(issue image-rs/jpeg-decoder#185), and there is no update after two months. Maybe the issue should be reopened here for people who's interested? Don't know what the right protocol is here.
Sorry if any inconvenience/trouble caused.

[fintelia]

From the discussion in the other thread there seems to be two issues at play:

1. The image data is converted from CMYK to RGB. This is intended behavior for the image crate which always converts to a supported ColorType, but it may make sense for the jpeg-decoder crate to also support keeping the image in CMYK format.

2. The embedded ICC profile isn't applied when doing the conversion. This is not the desired behavior, but fixing it will take a whole bunch of work which no one has yet expressed an interest in doing. See Color space information #786 and the linked issues for more discussion on color space handling.

[wellitecho]

Thanks for your prompt reply.
Indeed, it automatically converts CMYK to RGB as comment shown here in image/decoder.rs by image/decoder.rs fn cmyk_to_rgb. It's just that this RGB output image is not very usable for printing.
From the looks of it, it will take quite some time and effort to achieve the desired result.
Thanks for working on and improving this great crate. It works well in many cases.

[wellitecho]

Hi, is there any update on this issue? Is CMYK support, or other color space related support added? Thanks.

[fintelia]

Alas, we're still blocked on someone volunteering to design and implement the relevant changes