Is compression lossless? Visual differences appear

  • Resolved Lyk

    (@lyk-1)


    9 years, 7 months ago

    Hello,

    While I strongly appreciate the effort to keep providing for free part of your API, I had some weird results.

    Smushing made the colors appear “washed-out” (as if brightness changes) in a very obvious way.

    I upload the photo 2 times. I smush the 1st and then compare with the 2nd. The smushed one appears brighter.

    I see the same result with both Firefox and Chrome. Safari shows no difference between them(!)
    Saving the images from Firefox results in files looking the same(!)

    It is like Firefox and Chrome render the smushed image in some way that makes it look different(?!)

    For the tests, I opened the online images directly (i.e. the .jpg files), so no weird css could interfere or something.

    Any clues? Never had issues with smushit or ewww optimizer before.

    The tests were made on a retina MBP.

    EDIT: Check on a non-retina device with firefox. Everything looks as expected (images looking exactly the same)

    I am really confused…

    https://www.ads-software.com/plugins/wp-smushit/

Viewing 4 replies - 1 through 4 (of 4 total)

  • Lossless is lossless. For jpg files we simply strip the meta and rearrange the compressed data (DCT coefficients), without ever fully decoding the image. Therefore, its transformations are lossless: there is no image degradation at all. We also render progressive if that improves size.

    It sounds like something specific to your system (and maybe retina display) that is rendering the image different for you. Maybe you could give us a link to your original file so we can try to recreate?

    Thread Starter Lyk

    (@lyk-1)

    Thanks for replying.

    Yeap, I agree with what lossless is, that’s why I wrote

    It is like Firefox and Chrome render the smushed image in some way that makes it look different(?!)

    Moreover, I mentioned that saving the images and checking them, proves that the images are indeed identical visually.

    It has to be an issue with the Firefox, Chrome jpg rendering on retinas or something. It may worth some further investigations and maybe a report to the firefox/chrome teams.

    I used the Medium 640 image from here https://foter.com/photo/abandoned-buildings/

    Here is a split view (via a screenshot) of the differences between smushed and non-smushed images, as they are presented by firefox

    I can see the same thing, but the diff is a lot more minor than it sounds like. Lossy does the same thing. I’d have to leave it at something to do with the original encoding.

    Thread Starter Lyk

    (@lyk-1)

    I guess you mean on a retina screen right?

    I initially noticed the issue with other, normal images of my blog. (images from dslr)

    It is not a huge difference, but I randomly noticed when I saw a picture of a person being noticeably washed out. Then I started testing.

    I guess the original encoding has nothing to do, since the original pic -when uploaded- looks as expected

Viewing 4 replies - 1 through 4 (of 4 total)
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