Not sure what chroma upsampling is, but it looks like heavy jpeg compression to me.
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?
I saved that image using quality 6 purely for demonstration purposes, because I knew that quality setting uses 4:2:0. The jpeg artifacts are irrelevant. The issue is that such jpegs are very ugly when opened in Photoshop.
In case it wasn't clear: those 2 images above are the same, except the second image was opened in Photoshop and saved as png. Notice how much uglier it is (be sure not to look at the recompressed thumbnails).
Sometimes I don't notice when editing an image that I'm dealing with a subsampled jpeg (these are images from various sources) until I'm done editing, in which case I have to start all over again because Photoshop doesn't handle them properly.
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.
Unfortunately its results are consistently ugly and different from every other application I've tried which seem to all agree with each other (I suppose they might all use the same decoder). I've never seen Photoshop not do it wrong (where wrong = clearly uglier than the alternative). Is my only option really to inspect and possibly convert outside of Photoshop every single jpeg I wish to edit?
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.
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?
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: http://adobe.hosted.jivesoftware.com/message/3724972