[Digikam-devel] [Bug 217984] offer and use by default for jpeg files: use quality of the original picture

[Gilles Caulier]

https://bugs.kde.org/show_bug.cgi?id=217984

--- Comment #7 from Gilles Caulier <caulier.gilles at gmail.com> ---
Code taken from IM JPEG codec :

  {
    ssize_t
      j,
      qvalue,
      sum;

    register ssize_t
      i;

    /*
      Determine the JPEG compression quality from the quantization tables.
    */
    sum=0;
    for (i=0; i < NUM_QUANT_TBLS; i++)
    {
      if (jpeg_info->quant_tbl_ptrs[i] != NULL)
        for (j=0; j < DCTSIZE2; j++)
          sum+=jpeg_info->quant_tbl_ptrs[i]->quantval[j];
     }
     if ((jpeg_info->quant_tbl_ptrs[0] != NULL) &&
         (jpeg_info->quant_tbl_ptrs[1] != NULL))
       {
         ssize_t
           hash[101] =
           {
             1020, 1015,  932,  848,  780,  735,  702,  679,  660,  645,
              632,  623,  613,  607,  600,  594,  589,  585,  581,  571,
              555,  542,  529,  514,  494,  474,  457,  439,  424,  410,
              397,  386,  373,  364,  351,  341,  334,  324,  317,  309,
              299,  294,  287,  279,  274,  267,  262,  257,  251,  247,
              243,  237,  232,  227,  222,  217,  213,  207,  202,  198,
              192,  188,  183,  177,  173,  168,  163,  157,  153,  148,
              143,  139,  132,  128,  125,  119,  115,  108,  104,   99,
               94,   90,   84,   79,   74,   70,   64,   59,   55,   49,
               45,   40,   34,   30,   25,   20,   15,   11,    6,    4,
                0
           },
           sums[101] =
           {
             32640, 32635, 32266, 31495, 30665, 29804, 29146, 28599, 28104,
             27670, 27225, 26725, 26210, 25716, 25240, 24789, 24373, 23946,
             23572, 22846, 21801, 20842, 19949, 19121, 18386, 17651, 16998,
             16349, 15800, 15247, 14783, 14321, 13859, 13535, 13081, 12702,
             12423, 12056, 11779, 11513, 11135, 10955, 10676, 10392, 10208,
              9928,  9747,  9564,  9369,  9193,  9017,  8822,  8639,  8458,
              8270,  8084,  7896,  7710,  7527,  7347,  7156,  6977,  6788,
              6607,  6422,  6236,  6054,  5867,  5684,  5495,  5305,  5128,
              4945,  4751,  4638,  4442,  4248,  4065,  3888,  3698,  3509,
              3326,  3139,  2957,  2775,  2586,  2405,  2216,  2037,  1846,
              1666,  1483,  1297,  1109,   927,   735,   554,   375,   201,
               128,     0
           };

         qvalue=(ssize_t) (jpeg_info->quant_tbl_ptrs[0]->quantval[2]+
           jpeg_info->quant_tbl_ptrs[0]->quantval[53]+
           jpeg_info->quant_tbl_ptrs[1]->quantval[0]+
           jpeg_info->quant_tbl_ptrs[1]->quantval[DCTSIZE2-1]);
         for (i=0; i < 100; i++)
         {
           if ((qvalue < hash[i]) && (sum < sums[i]))
             continue;
           if (((qvalue <= hash[i]) && (sum <= sums[i])) || (i >= 50))
             image->quality=(size_t) i+1;
           if (image->debug != MagickFalse)
             (void) LogMagickEvent(CoderEvent,GetMagickModule(),
               "Quality: %.20g (%s)",(double) i+1,(qvalue <= hash[i]) &&
               (sum <= sums[i]) ? "exact" : "approximate");
           break;
         }
       }
     else
       if (jpeg_info->quant_tbl_ptrs[0] != NULL)
         {
           ssize_t
             hash[101] =
             {
               510,  505,  422,  380,  355,  338,  326,  318,  311,  305,
               300,  297,  293,  291,  288,  286,  284,  283,  281,  280,
               279,  278,  277,  273,  262,  251,  243,  233,  225,  218,
               211,  205,  198,  193,  186,  181,  177,  172,  168,  164,
               158,  156,  152,  148,  145,  142,  139,  136,  133,  131,
               129,  126,  123,  120,  118,  115,  113,  110,  107,  105,
               102,  100,   97,   94,   92,   89,   87,   83,   81,   79,
                76,   74,   70,   68,   66,   63,   61,   57,   55,   52,
                50,   48,   44,   42,   39,   37,   34,   31,   29,   26,
                24,   21,   18,   16,   13,   11,    8,    6,    3,    2,
                 0
             },
             sums[101] =
             {
               16320, 16315, 15946, 15277, 14655, 14073, 13623, 13230, 12859,
               12560, 12240, 11861, 11456, 11081, 10714, 10360, 10027,  9679,
                9368,  9056,  8680,  8331,  7995,  7668,  7376,  7084,  6823,
                6562,  6345,  6125,  5939,  5756,  5571,  5421,  5240,  5086,
                4976,  4829,  4719,  4616,  4463,  4393,  4280,  4166,  4092,
                3980,  3909,  3835,  3755,  3688,  3621,  3541,  3467,  3396,
                3323,  3247,  3170,  3096,  3021,  2952,  2874,  2804,  2727,
                2657,  2583,  2509,  2437,  2362,  2290,  2211,  2136,  2068,
                1996,  1915,  1858,  1773,  1692,  1620,  1552,  1477,  1398,
                1326,  1251,  1179,  1109,  1031,   961,   884,   814,   736,
                 667,   592,   518,   441,   369,   292,   221,   151,    86,
                  64,     0
             };

           qvalue=(ssize_t) (jpeg_info->quant_tbl_ptrs[0]->quantval[2]+
             jpeg_info->quant_tbl_ptrs[0]->quantval[53]);
           for (i=0; i < 100; i++)
           {
             if ((qvalue < hash[i]) && (sum < sums[i]))
               continue;
             if (((qvalue <= hash[i]) && (sum <= sums[i])) || (i >= 50))
               image->quality=(size_t) i+1;
             if (image->debug != MagickFalse)
               (void) LogMagickEvent(CoderEvent,GetMagickModule(),
                 "Quality: %.20g (%s)",(double) i+1,(qvalue <= hash[i]) &&
                 (sum <= sums[i]) ? "exact" : "approximate");
             break;
           }
         }
  }

See JPEGSetImageQuality() method from here for details :

https://www.imagemagick.org/subversion/ImageMagick/trunk/coders/jpeg.c

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