| Martin Brown 2005-02-12, 3:55 pm |
| Christian Gollwitzer wrote:
> Uni wrote:
>
> Hmm? A jpeg file (more accurately JFIF) contains the quantization tables
> used for the compression, because they are needed to decompress the
> image. These tables are the only exhausting source for the "compression
> quality" of the jpeg image. The construction of the qtables from the
> "quality value" shown to the user is arbitrary and depends on the
> compressor. You are right, that this "quality setting" is normally not
> saved, but even if it would be, it could not compared between applications.
Quite a few applications do use unimaginatively scaled versions of the
original "sample" table given in the appendix of the JPEG standard. So
with a bit of cunning you can compute a very good guess at the original
quality setting from the quantisation tables in a JPEG image.
It only really breaks down if the quantisation tables are customised.
PhotoShop is a notable exception using its own symmetrical custom
quantisation tables. I recently saw a Nikon scanner JPEG that had all
values in the quantisation table idenically equak to 1.
Regards,
Martin Brown
|