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JPEG estimator and quantization
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    Q: How does the JPEG format works?

    A: JPEG is a lossy compression, meaning that any JPEG compression involves some loss of information. The compression consists of several steps:

    •Color space transformation from RGB to YCBCR
    •Reducing the spatial resolution of the CB and CR components by a factor of 2 in the horizontal axis, or both in the horizontal and the vertical axis (this step is optional).
    •Dividing each color components to blocks of 8×8 pixels.
    •Transforming each 8×8 block to the frequency domain using a DCT transform.
    •Quantization (reduction of precision by representation with a lower number of bits) of each of the frequency components using a quantization matrix. This is the main stage that causes loss of information.
    •Entropy coding (a lossless compression, similar to ZIP) on the frequency components which have undergone quantization.

    Q: Is saving an image in Photoshop in minimum JPEG compression (i.e. 12) lossless?

    A: Even at quality 12, the Photoshop quantization matrix has values which are greater than 1, so the precision of some of the frequency components is reduced, meaning that some information is lost (you can view the quantization matrixes for Photoshop’s quality 12 here).

    Actually, even when using a quantization matrix that has all values set to 1, there is a loss of quality due to the DCT transform and the conversion from floating point values to integer values.

    Read the rest at:

    Other interesting read on JPEGSnoop author page:
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  • jpegsnoop is an excellent proggie for anyone wanting to measure how good the mjpeg mode & quantisation is on the gh2. Along with stills.