Programming Web Graphics with Perl and GNU Softwar

Name

pixel — color of a pixel

Synopsis

$image->Set('pixel[x,y]'=>color);
$image->Get('pixel[x,y]')

The pixel attribute returns the red, green, blue, and opacity value (or colormap index for colormapped images) at the given coordinate as four decimal numbers separated by commas. For example, if an image has a white pixel at 20, 50:

@pixel = split /,/, $image->Get('pixel[20,50]');
print "Red: $pixel[0]\n",
      "Green: $pixel[1]\n",
      "Blue: $pixel[2]\n",
      "Opacity: $pixel[3]\n";

will print the following:

Red: 255
Green: 255
Blue: 255
Opacity: 0

If you want to change a group of pixels, use the Draw( ) method with the color primitive.

Color Hex Code and Name Conversion

When a color is returned by a GetAttribute( ) call, the returned value is a string of four integers separated by commas representing the red, green, blue, and opacity values of the image. Unfortunately, this string must be converted to a hex value string if it is to be used with other PerlMagick functions or within the HTML code of a web page. It is fairly simple to write a Perl subroutine that will do the decimal-to-hex conversion using Perl’s sprintf function:

sub HexColor {
    my ($number, $hex);
    # Take only the first 3 decimal values in a comma-delimited string
    # as the Red, Green, Blue input
    (shift @_) =~ /^(\d+),(\d+),(\d+)/;
    foreach $number ($1, $2, $3) {
        $hex .= sprintf("%02X", $number);
    }
    return "#".$hex;
}

You can easily compute the hex string that corresponds to a color name by calling the QueryColor( ) method before calling ...