JPEG decoding inconsistent with other (non-Rust libraries)

[hanslovsky]

I am exploring Rust for image processing and am working on a small toy project. As a sanity check, I compare results with Python, which I am more familiar with. I noticed that decoding a JPEG with the image crate yields slightly different results than using Python's imageio. It looks like those are generally off by one. If this is to be expected with the JPEG algorithm (numerical instability, rounding), please disregard and close.

Expected

This is what the pixel values are in Python with imageio:

In [78]: import imageio as iio

In [79]: iio.v3.imread('tests-tiny.jpg').reshape(-1, 3)
Out[79]: 
array([[152, 146, 114],
       [149, 143, 111],
       [161, 155, 123],
       [159, 153, 121]], dtype=uint8)

Actual behaviour

This is what I get in Rust, iterating over and printing all pixels:

Rgba([151, 147, 113, 255]) 
Rgba([148, 144, 110, 255]) 
Rgba([160, 156, 122, 255]) 
Rgba([158, 154, 120, 255])

Reproduction steps

Example image

tests-tiny.jpg

Rust code

use image::GenericImageView;
use image::io::Reader;

fn main() {
    let reader = match Reader::open("tests-tiny.jpg") {
        Ok(r) => r,
        Err(error) => panic!("Unable to open image: {:?}", error)
    };
    let image = match reader.decode() {
        Ok(i) => i,
        Err(error) => panic!("unable to decode image: {:?}", error)
    };
    
    for y in 0 .. image.height() {
        for x in 0 .. image.width() {
            println!("{:?} ", image.get_pixel(x, y));
        }
    }
}

Python Code

In [78]: import imageio as iio

In [79]: iio.v3.imread('tests-tiny.jpg').reshape(-1, 3)
Out[79]: 
array([[152, 146, 114],
       [149, 143, 111],
       [161, 155, 123],
       [159, 153, 121]], dtype=uint8)

[etemesi254]

hi, author of jpeg decoder, yes this is to be expected. Jpeg is lossy and decoders are free to do what they want to a certain degreee so as long as thr deviation isn't big, this is normal