2004.06.23 07:15 "[Tiff] Tiff JPEG compression quality factor", by Albert Adell

Hi Philip,

I have modified the method JPEGSetupDecode in tiff_jpeg.c file, in order to calculate the Q-factor.

I have acceded to the JPEG struct at this function and I am able to apply the Q-factor computation.

JPEGSetupDecode(TIFF* tif)
{
        ...

        /* Read JPEGTables if it is present */
        if (TIFFFieldSet(tif,FIELD_JPEGTABLES)) {
                TIFFjpeg_tables_src(sp, tif);
                if(TIFFjpeg_read_header(sp,FALSE) != JPEG_HEADER_TABLES_ONLY) {
                        TIFFError("JPEGSetupDecode", "Bogus JPEGTables field");
                        return (0);
                }

                //$$$ QualityFactor begin
                q = sp->cinfo.d.quant_tbl_ptrs[0]->quantval[6];
                if (q > 51)
                        q = (50 * 51 + q / 2) / q;
                else
                        q = (5126 - 50 * q) / 51;
                sp->jpegquality=q;
                //$$$ QualityFactor end
        }

        ...
}

This method is automaticaly called when reading a tile or a strip but I suppose that is more interesting if it could be calculated before. I put the JPEGSetupCode explicit call after an Open call and it seems that works!

Thanks for the qFactor computation its a nice aproximation.

Thanks a lot,

Albert Adell

________________________________________________________________________

Albert Adell Fern?ndez              Desenvolupament Fotogrametria Digital
Institut Cartografic de Catalunya   Passeig Santa Madrona 45-51
E-08004 Barcelona                   www http://www.icc.es
_________________________________________________________________________

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