Is it possible to detect unique colours inside an SVG? - svg

Question

I want to access all the colours that an SVG may be using. I've played with convert, but would like some direction on how to figure out the unique colours that an SVG may contain.
This is the PHP code I've written to talk to convert, to try and determine whether a bitmap contains colours, however it is very limited:
/**
* containsColor
*
* #see https://www.imagemagick.org/discourse-server/viewtopic.php?t=19580&start=15
* #see https://superuser.com/questions/508472/how-to-recognize-black-and-white-images
* #see https://www.imagemagick.org/discourse-server/viewtopic.php?t=19580
* #access public
* #static
* #param string $path
* #return bool
*/
public static function containsColor(string $path): bool
{
$commands = array(
'convert ' .
($path) .' ' .
'-format "%[colorspace]" info:'
);
$command = implode(' && ', $commands);
$response = exec($command);
$color = strtolower($response) !== 'gray';
return $color;
}

Answer 1

If we review Format and Print Image Properties document, we should be able to identify the unique color count with -format %k.
public static function containsColor(string $path): int
{
$commands = array(
'convert ' .
($path) .' ' .
'-format "%k" info:'
);
$command = implode(' && ', $commands);
$response = exec($command);
return (int)$response;
}
If you want to evaluate all colors used by the SVG post render, you can use -unique-colors operator.
convert input.svg -depth 8 -unique-colors txt:-
Which will output something that can easily be parsed by PHP.
# ImageMagick pixel enumeration: 403,1,65535,srgba
0,0: (0,0,0,65535) #000000FF black
1,0: (257,0,0,65535) #010000FF srgba(1,0,0,1)
2,0: (257,257,257,65535) #010101FF srgba(1,1,1,1)
3,0: (257,257,0,65535) #010100FF srgba(1,1,0,1)
4,0: (514,257,0,65535) #020100FF srgba(2,1,0,1)
5,0: (771,771,771,65535) #030303FF grey1
...
Remember that SVG is really just XML, so it may be possible to load it into a DOMDocument class, and use DOMXPath to extract color attributes. But as correctly mentioned in the comments, you would not be able to identify CSS3 filters, or advanced color rendering & mixing.
Update with working example
public static function containsColor(string $input) : bool
{
$pixelInfoList = [];
exec(sprintf('convert "%s" -depth 8 -alpha Off --colors 255 -unique-colors txt:-', $input), $pixelInfoList);
// ... Insert error handling here ...
for($index = 1; $index < count($pixelInfoList); $index++) {
preg_match('/\((\d+),(\d+),(\d+)\)/', $pixelInfoList[$index], $colorParts);
if ($colorParts[1] == $colorParts[2] && $colorParts[2] == $colorParts[3]) {
// Color is gray. Do nothing?
} else {
// Non-gray color. Stop search, and return.
return true;
}
}
return false;
}
Not perfect, but a start.
This works be evaluating the color channels outputted by txt:-. If the red, green, and blue channels are the same, we can state it's a gray color & continue to the next line, else we can state a non-gray color exists & stop the iterations. I'm also taking the library of using -alpha Off -colors 255 to through out additional data.
YMMV

Answer 2

This probably won't help you, because you seem to want to use this for a server application, but it might help others with the same question.
If you're using Inkscape, you can also just export as .gpl.
File > Save a Copy : GIMP Palette (*.gpl)
This will, however, not save all the colors that are displayed within a gradient, only those that are actually used in the file.