Thanks Mike. I realize that to be true to the subject, one should learn something of the mathmatics of the compression method, I am really only interested in what JPEGing does to the opened file. IE: If a file has gone through a JPEG phase, will it always be an 8 bit per channel (I undersand this, it's like the Zone system with higher math) image when opened in an imaging program such as Photoshop? Your answer seems to confirm that.
William Robb ----- Original Message ----- From: "Mike Ignatiev" Subject: Re[2]: JPEG Question > The "reduced" algorithm, from the FAQ is fairly simple: > > 1. Transform the image into a suitable color space. > 2. (Optional -- omitted) > 3. Group the pixel values for each component into 8x8 blocks. Transform each > 8x8 block through a discrete cosine transform > 4. In each block, divide each of the 64 frequency components by a separate > "quantization coefficient", and round the results to integers. > 5. Encode the reduced coefficients using either Huffman or arithmetic coding. > 6. Tack on appropriate headers, etc, and output the result. > > Basically, the 3rd step is where the "pixel" come into play. In theory, > one can write a codec for an arbitrary color depth. In practice, I am yet > to see one. Since all "observable" codecs work on 24 bit color, there's > a good reason to think of JPEG as 24 bit format (if you want to be able > to read it back, anyway). > > BTW, zipped 24 bit tiff would be still a 24 bit format, although > <q>the concept of "bits per channel" just doesn't exist inside a</q> > zip file :) > > Mishka > > -----Original Message----- > From: Steve Jolly > Subject: Re: JPEG Question > > > > > Sorry to rain on your parade, but JPEG is far more complicated than > > "eight bits per channel". Internally, there is no concept even of > > pixels! Regular JPEGs are usually created from 8-bit-per-channel > > bitmaps, and there's not much point in converting them back into bitmaps > > of greater depth than that before viewing or printing them, but there > > the concept of "bits per channel" just doesn't exist inside a JPEG file. > > > > If you want to enter the scary world of lossy compression techniques, a > > good starting place is the comp.compression FAQ at > > http://www.faqs.org/faqs/compression-faq/. JPEG is described in section > > 75, which is in part 2 of the FAQ. > > > > S > > >
