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[OM] Re: RAW Bit Depth

Subject: [OM] Re: RAW Bit Depth
From: Dan Mitchell <danmitchell@xxxxxxxx>
Date: Mon, 09 Apr 2007 14:23:09 -0600
Moose wrote:
> I have no idea how JPEG compression works inside.

  See http://en.wikipedia.org/wiki/Jpeg#Encoding for a (for wikipedia) 
surprisingly good explanation, especially if you ignore the giant 
matrices in there and just look at the pictures.


  Basically, it splits the image into luminance, chrominance-red, 
chrominance-blue (same encoding as PAL). (YCbCr)

  Then it (possibly) throws away some detail in the Cb/Cr areas because 
the eye can't percieve as much detail in changes of colour as in brightness.

  Then it splits the image into 8x8 tiles (which you can see if you save 
a jpeg a really low quality), and does a DCT on the resulting tiles to 
turn the info from pixels into frequency info. (the same sort of thing 
as generating the fourier series for the image, if that's any more 
meaningful).

  Then, depending on how much compression you ask for, it throws away 
high frequency info. (this is why jpeg loses sharp edges between bits of 
image, because to encode that, you need high frequency info -- but for 
photographic images, there are normally no such very sharp transitions).

  Then it compresses the resulting data using a lossless compression 
method; this doesn't affect the image any more.

  -- dan

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