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No chroma upsampling in Photoshop?

Aug 3, 2012 6:24 PM

Say I have a subsampled jpeg (in this case 4:2:0) that looks like this:


If I open this in Photoshop it looks like this:


Is there any way to get Photoshop to perform this upsampling? It's quite annoying to have to check every jpeg I want to edit for chroma subsampling, and if it is subsampled to have to convert it to png using something other than Photoshop so I can finally work with it.

  • Currently Being Moderated
    Aug 3, 2012 6:36 PM   in reply to Chromiax

    Not sure what chroma upsampling is, but it looks like heavy jpeg compression to me.

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  • Currently Being Moderated
    Aug 3, 2012 7:37 PM   in reply to Chromiax

    Ok i looked it up, but your examples from the wikipedia page still resemble heavy jpeg compression artifacts.

    It looks like photoshop uses some form of chroma subsampling for saving jpegs depending on quality setting.

    I guess some kind of video software using that method?

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  • Currently Being Moderated
    Aug 3, 2012 7:45 PM   in reply to Chromiax

    Photoshop does chroma upsampling when reading JPEG files.

    But there are no standards on how the upsampling is supposed to be done, so different JPEG implementations will get different results.

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  • Currently Being Moderated
    Aug 3, 2012 9:11 PM   in reply to Chromiax

    You probably only tried a few applications that use the default OS decoder or the Independent JPEG Group decoder.


    There are many JPEG implementations, and they really don't all match on chroma sampling.


    Photoshop does it "right" in that it does upsample.  But that result may not match other implementations, and it seems to be a minor difference only visible on very contrasting colors.

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  • Currently Being Moderated
    Aug 3, 2012 10:36 PM   in reply to Chromiax

    You have my sympathy for having to work with JPEGs in the first place, a format which I avoid like the plague except as a final target output.


    That being said, may I respectfully suggest you run your JPEG through a JPEG repair utility such as JPEG Repair in Alien Skin's Image Doctor suite of plug-ins before editing?

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  • Noel Carboni
    23,514 posts
    Dec 23, 2006
    Currently Being Moderated
    Aug 4, 2012 6:45 AM   in reply to station_two

    Station_two, isn't that just what he's saying he has to do and would rather not?  It's not unreasonable to want to just open JPEGs directly into Photoshop in general and get the best possible result.


    That said, I'm not sure I agree that the fuzzier (top) version shown is "best" right out of the chute per se.  I prefer sharper imagery myself, and the Photoshop interpretation is crisper looking.  The stairsteps seem a small price to pay for me since one of my own tools fixes them nicely.


    In the general case Adobe ought to revamp its JPEG decoder code to offer a configurable option for reconstruction of color-subsampled images, and while at it increase the maximum pixel count to 64K by 64K.


    See also this thread:



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