PHP image color analysis with transparency

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:

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