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JPEG quality - from QT tables?
Posted: 2014-03-01T22:34:14-07:00
by blue-j
Is the JPEG quality reported by identify based on statistics of the quantization tables, or a metadata report? How reliable is it?
Thanks so much!
J
Re: JPEG quality - from QT tables?
Posted: 2014-03-02T07:11:41-07:00
by glennrp
blue-j wrote:Is the JPEG quality reported by identify based on statistics of the quantization tables, or a metadata report? How reliable is it?
It computes a signature of the Q tables (sum and a hash) and compares that with the numbers from the tables used in IJG code.
See coders/jpeg.c, around lines 760-890 in the current version (6.8.8-7).