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RE: filmscanners: image samples of digital artifacts




--- Dan Honemann <danh@selectsa.com> wrote:
> One thing I've always been curious about is what
> causes the topographical
> map type of lines you see in the blue sky portion of
> this image:

The old JPEG (not JPEG 2000) does code three channels
Y, Cr, Cb. The channels Cr and Cb are downsampled.
Then each channel is divided in blocks of 8x8. For
each such block you do a Discret Cosinus Transform
(DCT), devide each of the 64 resulting values by one
of 64 numbers defined by the quantization table
(higher frequency values are divided by higher numbers
then low frequency values), and then Huffman
(arithmetic coding is also possible but is less
common) entropy encoded. This is true for lossy
compression. Now if you do a high compression you
divide the values after the DCT by higher factors so
you get more 0s. Because of that the transition of one
8x8 block becomes less smooth and you see 8x8 block in
the final image.

Robert

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