Dettagli

The picture is of the John Moulton Barn at the base of the Teton Range.

Raison d'être

This is a cleaner/more acurate version of en:Image:YUV_components.jpg. The improvements are:

• Use a lossless compression algorithm, PNG removing jpeg artifacts
• Don't scale U, V to extreme values - use the raw value, which more closely approximates the chroma component
• Fix the inverted polarity of U, V components

Generation

To generate this map Brian Szymanski used this perl script, and then ran optipng with a full search to shrink the filesize. If you want to understand the below code, you should probably use perltidy - the code is mostly here for my reference, but it is wrong. The first channel of the blue/yellow map has to be set to U as well and the third channel of the red/green map has to be set to 0.5. I will fix it soon and update the image.

perl -e 'use strict; use warnings; use GD; GD::Image->trueColor(1);
my %color_cache;
sub cached_allocate { my $img = shift; my @rgb = @_;
   my $rgb = join(",", @rgb);
   $color_cache{$rgb} = $img->colorAllocate(@rgb) unless($color_cache{$rgb});
   return $color_cache{$rgb};
}

my $img = GD::Image->new("barn.png");
my ($height, $width) = ($img->height, $img->width);
my $yuv_img = GD::Image->new($width, $height*4);
foreach my $y (0..$height-1) {
   foreach my $x (0..$width-1) {
      my ($r, $g, $b) = map { $_ / 256 } my @rgb = $img->rgb($img->getPixel($x, $y));
      my $Y = 0.299*$r+0.587*$g+0.114*$b;
      my $U = 0.436*($b-$Y)/(1-0.114);
      my $V = 0.615*($r-$Y)/(1-0.299);
      $Y *= 256;
      $U += .436; $U *= (256/.872);
      $V += .615; $V *= (256/1.23);
      ($Y, $U, $V) = map { int(0.5+$_) } ($Y, $U, $V);
      die "YUV: $Y,$U,$V" if (($Y > 255) or ($U > 255) or ($V > 255));
      $yuv_img->setPixel($x, $y, cached_allocate($yuv_img, @rgb));
      $yuv_img->setPixel($x, $height+$y, cached_allocate($yuv_img, $Y, $Y, $Y));
      $yuv_img->setPixel($x, 2*$height+$y, cached_allocate($yuv_img, 0, 255-$U, $U));
      $yuv_img->setPixel($x, 3*$height+$y, cached_allocate($yuv_img, $V, 255-$V, 0));
   }
};
print $yuv_img->png' >barn-yuv.png

Licenza

Io, detentore del copyright su quest'opera, la rilascio nel pubblico dominio. Questa norma si applica in tutto il mondo. In alcuni paesi questo potrebbe non essere legalmente possibile. In tal caso: Garantisco a chiunque il diritto di utilizzare quest'opera per qualsiasi scopo, senza alcuna condizione, a meno che tali condizioni siano richieste dalla legge.