Saving RgbaImage as JPEG is incorrect

[kmdreko]

This happens in the following code:

[dependencies]
image = "0.25.0"

fn main() {
    image::open("input.jpg")
        .unwrap()
        .into_rgba8()
        .save("output.jpg")
        .unwrap();
}

Expected

I would've expected this to create roughly the output image as the input.

Actual behaviour

The output is a jumbled mess.

Reproduction steps

Here is the input.jpg file tested with:

And here is the output.jpg result:

Just now seeing it also looks different between image viewers: Windows Photos App vs Firefox.

Notes

This can be worked around by either:

• downgrading to 0.24.9, or
• using .into_rgb8() (no alpha channel), or
• saving as .png

[vallentin]

I was just writing an issue as well :)

For context, we're coming from this Stack Overflow question.

Here's a simpler example (not requiring any input). It breaks even when just creating a RgbaImage using from_pixel():

use image::{Rgb, RgbImage, Rgba, RgbaImage};

fn main() {
    let (w, h) = (64, 64);

    let img = RgbImage::from_pixel(w, h, Rgb([255, 0, 0]));
    img.save("rgb.jpg").unwrap();

    let img = RgbaImage::from_pixel(w, h, Rgba([255, 0, 0, 255]));
    img.save("rgba.jpg").unwrap();
}

version	rgb.jpg	rgba.jpg
0.24.9
0.25.0

[fintelia]

JPEG doesn't support storing an alpha channel, so attempting to encode an RGBA image as JPEG should return an error. It is a bug that we silently produce a garbled image instead. (Silently dropping the alpha channel like in 0.24.9 is also a bug IMO).

I'd suggest using one of the workarounds suggested in the linked stack overflow answer: switch to a different format like PNG which actually supports alpha or manually remove the alpha channel via to_rgb8.

[yuanyan3060]

#2169

fix rgba8 image jpeg encode bug

[mqudsi]

Is there no way to statically prevent such code from compiling?