Ho scoperto che il problema si verifica quando cerco di inserire immagini che sono gif trasparenti convertite in png.
La funzione che sto utilizzando è la seguente:
Codice PHP:
function imagecopymerge_alpha($dst_im, $src_im, $dst_x, $dst_y, $src_x, $src_y, $src_w, $src_h, $pct){
if(!isset($pct)){
return false;
}
$pct /= 100;
// Get image width and height
$w = imagesx( $src_im );
$h = imagesy( $src_im );
// Turn alpha blending off
imagealphablending( $src_im, 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( $src_im, $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( $src_im, $x, $y );
$alpha = ( $colorxy >> 24 ) & 0xFF;
//calculate new alpha
if( $minalpha !== 127 ){
$alpha = 127 + 127 * $pct * ( $alpha - 127 ) / ( 127 - $minalpha );
} else {
$alpha += 127 * $pct;
}
//get the color index with new alpha
$alphacolorxy = imagecolorallocatealpha( $src_im, ( $colorxy >> 16 ) & 0xFF, ( $colorxy >> 8 ) & 0xFF, $colorxy & 0xFF, $alpha );
//set pixel with the new color + opacity
if( !imagesetpixel( $src_im, $x, $y, $alphacolorxy ) ){
return false;
}
}
}
// The image copy
imagecopy($dst_im, $src_im, $dst_x, $dst_y, $src_x, $src_y, $src_w, $src_h);
}