static int enough_colors(MagickWand *mw, const struct imgmin_options *opt)
{
return
unique_colors(mw) >= opt->min_unique_colors ||
/*
* most color photos end up as the TrueColor type...
* for Grayscale we ignore color count...
* haven't run into many of the other types yet, be aggressive for now
*/
MagickGetType(mw) != TrueColorType ||
/*
* if a full-color JPEGs has exactly 256 colors it's likely
* been (poorly) converted from a GIF or PNG, these often look
* terrible anyway but can often be compressed quite a bit
*/
unique_colors(mw) == 256;
}
/*
* given a source image, a destination filepath and a set of image metadata thresholds,
* search for the lowest-quality version of the source image whose properties fall within our
* thresholds.
* this will produce an image file that looks the same to the casual observer, but which
* contains much less information and results in a smaller file.
* typical savings on unoptimized images vary widely from 10-80%, with 25-50% being most common.
*/
MagickWand * search_quality(MagickWand *mw, const char *dst,
const struct imgmin_options *opt)
{
MagickWand *tmp = NULL;
char tmpfile[MAX_PATH] = "/tmp/imgminXXXXXX";
if (0 == strcmp("-", dst))
{
if (-1 == mkstemp(tmpfile))
{
perror("mkstemp");
return CloneMagickWand(mw);
}
} else {
strcpy(tmpfile, dst);
}
/*
* The overwhelming majority of JPEGs are TrueColorType; it is those types, with a low
* unique color count, that we must avoid.
*/
if (unique_colors(mw) < opt->min_unique_colors && MagickGetType(mw) != GrayscaleType)
{
fprintf(stderr, " Color count is too low, skipping...\n");
return CloneMagickWand(mw);
}
if (quality(mw) < opt->quality_in_min)
{
fprintf(stderr, " Quality < %u, won't second-guess...\n", opt->quality_in_min);
return CloneMagickWand(mw);
}
{
ExceptionInfo *exception = AcquireExceptionInfo();
const double original_density = color_density(mw);
unsigned qmax = min(quality(mw), opt->quality_out_max);
unsigned qmin = opt->quality_out_min;
unsigned steps = 0;
/*
* binary search of quality space for optimally lowest quality that
* produces an acceptable level of distortion
*/
while (qmax > qmin + 1 && steps < opt->max_steps)
{
double distortion[CompositeChannels+1];
double error;
double density_ratio;
unsigned q;
steps++;
q = (qmax + qmin) / 2;
/* change quality */
tmp = CloneMagickWand(mw);
MagickSetImageCompressionQuality(tmp, q);
/* apply quality change */
MagickWriteImages(tmp, tmpfile, MagickTrue);
DestroyMagickWand(tmp);
tmp = NewMagickWand();
MagickReadImage(tmp, tmpfile);
/* quantify distortion produced by quality change
* NOTE: writes to distortion[], which we don't care about
* and to image(tmp)->error, which we do care about
*/
(void) GetImageDistortion(GetImageFromMagickWand(tmp),
GetImageFromMagickWand(mw),
MeanErrorPerPixelMetric,
distortion,
exception);
/* FIXME: in perlmagick i was getting back a number [0,255.0],
* here something else is happening... the hardcoded divisor
* is an imperfect attempt to scale the number back to the
* perlmagick results. I really need to look into this.
*/
error = GetImageFromMagickWand(tmp)->error.mean_error_per_pixel / 380.;
density_ratio = fabs(color_density(tmp) - original_density) / original_density;
/* eliminate half search space based on whether distortion within thresholds */
if (error > opt->error_threshold || density_ratio > opt->color_density_ratio)
{
qmin = q;
} else {
qmax = q;
}
if (opt->show_progress)
{
fprintf(stderr, "%.2f/%.2f@%u ", error, density_ratio, q);
}
}
if (opt->show_progress)
{
putc('\n', stderr);
}
MagickSetImageCompressionQuality(mw, qmax);
/* "Chroma sub-sampling works because human vision is relatively insensitive to
* small areas of colour. It gives a significant reduction in file sizes, with
* little loss of perceived quality." [3]
*/
(void) MagickSetImageProperty(mw, "jpeg:sampling-factor", "2x2");
/* strip an image of all profiles and comments */
(void) MagickStripImage(mw);
MagickWriteImages(mw, tmpfile, MagickTrue);
(void) DestroyMagickWand(tmp);
tmp = NewMagickWand();
MagickReadImage(tmp, tmpfile);
exception = DestroyExceptionInfo(exception);
}
return tmp;
}
