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Diffstat (limited to 'plug-ins/file-jpeg/jpeg-quality.c')
| -rw-r--r-- | plug-ins/file-jpeg/jpeg-quality.c | 148 |
1 files changed, 148 insertions, 0 deletions
diff --git a/plug-ins/file-jpeg/jpeg-quality.c b/plug-ins/file-jpeg/jpeg-quality.c new file mode 100644 index 0000000..52bd2c7 --- /dev/null +++ b/plug-ins/file-jpeg/jpeg-quality.c @@ -0,0 +1,148 @@ +/* GIMP - The GNU Image Manipulation Program + * Copyright (C) 1995 Spencer Kimball and Peter Mattis + * + * jpeg-quality.c + * Copyright (C) 2007 Raphaël Quinet <raphael@gimp.org> + * + * This program is free software: you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation; either version 3 of the License, or + * (at your option) any later version. + * + * This program is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + * + * You should have received a copy of the GNU General Public License + * along with this program. If not, see <https://www.gnu.org/licenses/>. + */ + +#include "config.h" + +#include <glib/gstdio.h> + +#include <jpeglib.h> + +#include "jpeg-quality.h" + +/* + * Note that although 0 is a valid quality for IJG's JPEG library, the + * baseline quantization tables for quality 0 and 1 are identical (all + * divisors are set to the maximum value 255). So 0 can be used here + * to flag unusual settings. + */ + +/* sum of the luminance divisors for quality = 0..100 (IJG standard tables) */ +static gint std_luminance_sum[101] = +{ + G_MAXINT, + 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 +}; + +/* sum of the chrominance divisors for quality = 0..100 (IJG standard tables) */ +static gint std_chrominance_sum[101] = +{ + G_MAXINT, + 16320, 16320, 16320, 16218, 16010, 15731, 15523, 15369, 15245, 15110, 14985, + 14864, 14754, 14635, 14526, 14429, 14346, 14267, 14204, 13790, 13121, 12511, + 11954, 11453, 11010, 10567, 10175, 9787, 9455, 9122, 8844, 8565, 8288, + 8114, 7841, 7616, 7447, 7227, 7060, 6897, 6672, 6562, 6396, 6226, + 6116, 5948, 5838, 5729, 5614, 5505, 5396, 5281, 5172, 5062, 4947, + 4837, 4726, 4614, 4506, 4395, 4282, 4173, 4061, 3950, 3839, 3727, + 3617, 3505, 3394, 3284, 3169, 3060, 2949, 2836, 2780, 2669, 2556, + 2445, 2336, 2221, 2111, 2000, 1888, 1778, 1666, 1555, 1444, 1332, + 1223, 1110, 999, 891, 779, 668, 558, 443, 333, 224, 115, + 64 +}; + +/** + * jpeg_detect_quality: + * @cinfo: a pointer to a JPEG decompressor info. + * + * Returns the exact or estimated quality value that was used to save + * the JPEG image by analyzing the quantization table divisors. + * + * If an exact match for the IJG quantization tables is found, then a + * quality setting in the range 1..100 is returned. If the quality + * can only be estimated, then a negative number in the range -1..-100 + * is returned; its absolute value represents the maximum IJG quality + * setting to use. If the quality cannot be reliably determined, then + * 0 is returned. + * + * This function must be called after jpeg_read_header() so that + * @cinfo contains the quantization tables read from the DQT markers + * in the file. + * + * Return Value: the JPEG quality setting in the range 1..100, -1..-100 or 0. + */ +gint +jpeg_detect_quality (struct jpeg_decompress_struct *cinfo) +{ + gint t; + gint i; + gint sum[3]; + gint q; + + /* files using CMYK or having 4 quantization tables are unusual */ + if (!cinfo || cinfo->output_components > 3 || cinfo->quant_tbl_ptrs[3]) + return 0; + + /* Most files use table 0 for luminance divisors (Y) and table 1 for + * chrominance divisors (Cb and Cr). Some files use separate tables + * for Cb and Cr, so table 2 may also be used. + */ + for (t = 0; t < 3; t++) + { + sum[t] = 0; + if (cinfo->quant_tbl_ptrs[t]) + for (i = 0; i < DCTSIZE2; i++) + sum[t] += cinfo->quant_tbl_ptrs[t]->quantval[i]; + } + + if (cinfo->output_components > 1) + { + gint sums; + + if (sum[0] < 64 || sum[1] < 64) + return 0; + + /* compare with the chrominance table having the lowest sum */ + if (sum[1] < sum[2] || sum[2] <= 0) + sums = sum[0] + sum[1]; + else + sums = sum[0] + sum[2]; + + q = 100; + while (sums > std_luminance_sum[q] + std_chrominance_sum[q]) + q--; + + if (sum[0] == std_luminance_sum[q] && sum[1] == std_chrominance_sum[q]) + return q; + else + return -q; + } + else + { + if (sum[0] < 64) + return 0; + + q = 100; + while (sum[0] > std_luminance_sum[q]) + q--; + + if (sum[0] == std_luminance_sum[q]) + return q; + else + return -q; + } +} |