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Guess default chroma subsampling setting from the quality setting #243

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merged 1 commit into from Apr 14, 2017
Merged

Guess default chroma subsampling setting from the quality setting #243

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2 changes: 2 additions & 0 deletions cjpeg.c
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Expand Up @@ -608,6 +608,8 @@ parse_switches (j_compress_ptr cinfo, int argc, char **argv,
if (! set_quant_slots(cinfo, qslotsarg))
usage();

/* set_quality_ratings sets default subsampling, so the explicit
subsampling must be set after it */
if (samplearg != NULL) /* process -sample if it was present */
if (! set_sample_factors(cinfo, samplearg)) {
fprintf(stderr, "%s: can't set sample factors\n", progname);
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11 changes: 11 additions & 0 deletions rdswitch.c
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Expand Up @@ -553,6 +553,17 @@ set_quality_ratings (j_compress_ptr cinfo, char *arg, boolean force_baseline)
}
}
jpeg_default_qtables(cinfo, force_baseline);

/* For some images chroma subsampling significantly degrades color quality,
making it impossible to achieve high visual quality regardless of quality setting.
To make the quality setting more intuitive, disable subsampling when high-quality
color is desired. */
if (val >= 90) {
set_sample_factors(cinfo, "1x1");
} else if (val >= 80) {
set_sample_factors(cinfo, "2x1");
}

return TRUE;
}

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