Re: Jpeg imags: always YCbCr (even for CMYK?) and always 8 bit per channel?
- From: Thomas Richter <thor@xxxxxxxxxxxxxxxxx>
- Date: Wed, 20 May 2009 08:36:05 +0200
ChristianBarnett2@xxxxxxxxx schrieb:
Hi Thomas,
Late reply, but still wanna say thanks! Your post cleared up quite
some fog in my understanding of jpeg files (or JFIF files actually,
with JPEG compression).
Makes me wonder why the storage of a separate transparency channel,
for which most people use PNG, is not commonly used in .jpg images,
even though it is perfectly possible. I guess if just the JFIF
standard had defined a default color space tag for it, it would have
been common practice.
There are two problems with that: First of all, for storing transparency information, you usually want to have a lossless channel. Even though JPEG offers lossless compression (rarely known mode, though), you cannot compress one channel in lossless and all others in the lossy mode. Second is that there is no accepted wrapper (or container) that would indicate a transparency (alpha) channel in JPEG. JFIF is just one of the containers, though not the only one existing. TIFF (yes, really) is also able to contain JPEG codestreams.
So long,
Thomas
.
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- From: ChristianBarnett2
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