PHP image color analysis with transparency - php

Question

I am currently working on an application that needs to analyse a number of images and figure out what color they're closest to.
Therefore I found a code snippet that does exactly that:
function analyzeImageColors($im, $xCount =3, $yCount =3)
{
//get dimensions for image
$imWidth =imagesx($im);
$imHeight =imagesy($im);
//find out the dimensions of the blocks we're going to make
$blockWidth =round($imWidth/$xCount);
$blockHeight =round($imHeight/$yCount);
//now get the image colors...
for($x =0; $x<$xCount; $x++) { //cycle through the x-axis
for ($y =0; $y<$yCount; $y++) { //cycle through the y-axis
//this is the start x and y points to make the block from
$blockStartX =($x*$blockWidth);
$blockStartY =($y*$blockHeight);
//create the image we'll use for the block
$block =imagecreatetruecolor(1, 1);
//We'll put the section of the image we want to get a color for into the block
imagecopyresampled($block, $im, 0, 0, $blockStartX, $blockStartY, 1, 1, $blockWidth, $blockHeight );
//the palette is where I'll get my color from for this block
imagetruecolortopalette($block, true, 1);
//I create a variable called eyeDropper to get the color information
$eyeDropper =imagecolorat($block, 0, 0);
$palette =imagecolorsforindex($block, $eyeDropper);
$colorArray[$x][$y]['r'] =$palette['red'];
$colorArray[$x][$y]['g'] =$palette['green'];
$colorArray[$x][$y]['b'] =$palette['blue'];
//get the rgb value too
$hex =sprintf("%02X%02X%02X", $colorArray[$x][$y]['r'], $colorArray[$x][$y]['g'], $colorArray[$x][$y]['b']);
$colorArray[$x][$y]['rgbHex'] =$hex;
//destroy the block
imagedestroy($block);
}
}
//destroy the source image
imagedestroy($im);
return $colorArray;
}
Problem is that whenever I provide an image with transparency, GDLib consinders the transparency to be black, thus producing a wrong (much darker) output than is really the case.
For example this icon where the white area around the arrow is actually transparent:
example http://img651.imageshack.us/img651/995/screenshot20100122at113.png
Can anyone tell me how to work around this?

Answer 1

You need imageColorTransparent(). http://www.php.net/imagecolortransparent
Transparency is a property of the image, not of a color. So use something like $transparent = imagecolortransparent($im) to see if there is any transparency on your image, then just ignore that color in your $colorArray or have some other way to identify the transparent color in the return from your function. That all depends on how you're using the returned data.
--M