Quote Originally Posted by Carlos Lindado


(so it would be incorrect to state that chroma subsampling is the other type of JPEG compression besides DCT, since the two are used simultaneously).


Well, the point is that sRAW uses chroma subsampling compression.



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Quote Originally Posted by Carlos Lindado


OTOH, an sRAW file shouldn't be of the same size as a full RAW, since you're effectively discarding a lot of pixels and therefore a lot of bytes.



Not really. Canon starts with only a single color sample per pixel location, like every Bayer sensor, say, 21 MP. Uncompressed and without an embedded preview jpeg, that's 35 MB.Then Canon does a demosaic. This results in three color samples at every location -- so they just tripled the quantity of information -- now it's 105 MB (!) instead of 35 MB. (Almost as bit as 16-bit TIFF files from a full size raw conversion.)


So next they downsize to sRaw1, 10 MP. In doing so, they should (and do) increase bit depth to 15 bits to prevent quantization error. Uh oh, file size is 53 MB -- still more than the full-size raw. They *have* to apply chroma subsampling compression (throwing away color detail) just to get it down smaller than the original raw. 4:2:2 brings it down to 25 MB -- which is smaller than the 35 MB we started with. Then apply the standard huffman-coding lossless compression and add in a 3 MB preview JPEG and you're done.


It's truly sad that *this* is the best we can get from Canon. Nikon's so-called "lossy" NEF is far, far superior for the same file size.