No, a jpeg pixel is not "the summation of 64 8 bit coefficients." I've written jpeg codecs (and many other image formats). This is not possible once you go to jpeg, for any implementation of jpeg. One example: given a so-so RAW, at 11 bits, you can pull out dark things or darken bright things smoothly. JPEG, besides being lossy, does not allow enough steps.
#Darktable export jpeg quality software
It's why photo processing software wants RAW, not JPEG. This makes gradients band no matter how you do them in JPEG. So the dynamic range is 11 stops, but the number of representable levels per channel is still 8 bit: 256 levels. It only allows you the 256 values that are squares. Think of it as squaring: taking as your input and squaring every value gives you a range of 0 to 255^2 = 65025, but that does not allow you to store any value in that range. But there are still only 256 possible levels per channel, too few for quality image editing. The 11-bit dynamic range part you claim is merely from a color profile, which takes the resulting 8 bits per channel (i.e., 256 possible values) and spreads them over a gamma curve to an 11-bit range. Jpeg compresses an 8x8 pixel block at a time by taking a DCT (which mathematically is lossless, but in practice is not due to rounding and quantization at this stage), which turns those original 8x8 values into another set of 8x8 values, then some of these are thrown away and/or quantized for lossy compression.ĭecompression is the reverse: take these 8x8 quantized DCT coefficients, perform an inverse 8x8 DCT top get pixel values. A jpeg pixel is not 64 eight-bit coefficients.
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