PHP gd png image opacity filter function

php_gd_png_opacity.php

function filter_opacity( &$img, $opacity ) //params: image resource id, opacity in percentage (eg. 80)
{
    if( !isset( $opacity ) )
        { return false; }
    $opacity /= 100;

    //get image width and height
    $w = imagesx( $img );
    $h = imagesy( $img );

    //turn alpha blending off
    imagealphablending( $img, false );

    //find the most opaque pixel in the image (the one with the smallest alpha value)
    $minalpha = 127;
    for( $x = 0; $x < $w; $x++ )
        for( $y = 0; $y < $h; $y++ )
            {
                $alpha = ( imagecolorat( $img, $x, $y ) >> 24 ) & 0xFF;
                if( $alpha < $minalpha )
                    { $minalpha = $alpha; }
            }

    //loop through image pixels and modify alpha for each
    for( $x = 0; $x < $w; $x++ )
        {
            for( $y = 0; $y < $h; $y++ )
                {
                    //get current alpha value (represents the TANSPARENCY!)
                    $colorxy = imagecolorat( $img, $x, $y );
                    $r = ($colorxy >> 16) & 0xFF;
                    $g = ($colorxy >> 8) & 0xFF;
                    $b = $colorxy & 0xFF;
                    if ($r + $g + $b == 0) 
                        {  
                        $opacity2 = 0;
                        $alpha = ( $colorxy >> 24 ) & 0xFF;
                            //calculate new alpha
                             if( $minalpha !== 127 )
                                  { $alpha = 127 + 127 * $opacity2 * ( $alpha - 127 ) / ( 127 - $minalpha ); }
                              else
                                   { $alpha += 127 * $opacity2; }
                        }
                        else if ($r + $g + $b == 3)
                        { continue; }
                        else
                        {
                        $alpha = ( $colorxy >> 24 ) & 0xFF;
                    //calculate new alpha
                    if( $minalpha !== 127 )
                        { $alpha = 127 + 127 * $opacity * ( $alpha - 127 ) / ( 127 - $minalpha ); }
                    else
                        { $alpha += 127 * $opacity; }
                        }
                    
                    //get the color index with new alpha
                    $alphacolorxy = imagecolorallocatealpha( $img, ( $colorxy >> 16 ) & 0xFF, ( $colorxy >> 8 ) & 0xFF, $colorxy & 0xFF, $alpha );
                    //set pixel with the new color + opacity
                    if( !imagesetpixel( $img, $x, $y, $alphacolorxy ) )
                        { return false; }
                }
        }
    return true;
}