Can JPEG be saved in RGB colour space instead of YCbCr ?

AK
Posted By
Alexander_Kriegisch
Nov 6, 2005
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1366
Replies
6
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Closed
Hi!

I wonder if Photoshop CS for Windows supports saving JPEG images in RGB colour space instead of YCbCr. This creates bigger files, but can greatly improve the quality of anaglyph stereo images where strict separation of the red versus the cyan (=green+blue) channels is desirable, because it eliminates ghosting effects introduced by image compression. Those ghosting effects result in "pollution" of the red channel by pixels from the cyan channel and vice versa.

I know that Photoshop can easily load and display images in RGB colour space, but it seems as if there is no option to actually save images in that format.

If that option is unavailable, please put it on your to-do list, dear developers.
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CN
Cybernetic Nomad
Nov 6, 2005
?

If your image is in RGB mode, then it is saved in that mode.
JS
Jeff_Schewe
Nov 6, 2005
JPG compression, by it’s nature of compressing color data without compressing luminance data (to preserve appearence) needs to separate color from the luminance data. I don’t think hardly any jpg compression schemes do compression without color space conversion-it’s built in to the encoding/decoding.
CN
Cybernetic Nomad
Nov 6, 2005
Ah, I understand now.
AK
Alexander_Kriegisch
Nov 6, 2005
Well, I think RGB as an optional alternative to YUV as a colour encoding scheme is part of the original JPEG standard. The JPEG subset JFIF, falsely (but commonly) called JPEG just contains YUV, but obviously all viewers can still display RGB-encoded JPEGs. A piece of software called StereoPhoto Maker can even create files in that format. Please cf. to <http://stereo.jpn.org/eng/stphmkr/anaglyph/anaglyph.htm> in order to get an explanation and quite a few sample pics.

I hope I have explained myself more clearly now. I would really love to see this feature as an option in Photoshop or – even better – discover it is already there. If so, just go ahead and tell me. You would make me happy. 🙂
JS
John_Slate
Nov 7, 2005
So is it safe to say that a Jpeg goes through an on-the-fly type of conversion to this other color space in order to apply compression, but in the end it still is RGB.
AK
Alexander_Kriegisch
Nov 7, 2005
In the end (on the hard disk) it is not RGB anymore, but YUV. When to be displayed again, though, then you are right. As a last step in decompression, YUV is transformed to RGB again.
<http://en.wikipedia.org/wiki/Jpeg>

By the way: The English Wikipedia article does not mention the fact that RGB encoding is also allowed, but the German one does:
< http://de.wikipedia.org/wiki/Joint_Photographic_Experts_Grou p#Dateiformat>

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