Улучшена функция хронокли.Убрано ненужное смещение бит для каждого пикселя, уменьшено количество ложных негативов, добавлено объяснение в описании функции.
/**
* Estimates, if image has pixels with transparency. It shrinks image to 64 times smaller
* size, if necessary, and searches for the first pixel with non-zero alpha byte.
* If image has 1% opacity, it will be detected. If any block of 8x8 pixels has at least
* one semi-opaque pixel, the block will trigger positive result. There are still cases,
* where image with hardly noticeable transparency will be reported as non-transparent,
* but it's almost always safe to fill such image with monotonic background.
*
* Icons with size <= 64x64 (or having square <= 4096 pixels) are fully scanned with
* absolutely reliable result.
*
* @param resource $image
* @return bool
*/
function hasTransparency ($image): bool {
if (!is_resource($image)) {
throw new \InvalidArgumentException("Image resource expected. Got: " . gettype($image));
}
$shrinkFactor = 64.0;
$minSquareToShrink = 64.0 * 64.0;
$width = imagesx($image);
$height = imagesy($image);
$square = $width * $height;
if ($square <= $minSquareToShrink) {
[$thumb, $thumbWidth, $thumbHeight] = [$image, $width, $height];
} else {
$thumbSquare = $square / $shrinkFactor;
$thumbWidth = (int) round($width / sqrt($shrinkFactor));
$thumbWidth < 1 and $thumbWidth = 1;
$thumbHeight = (int) round($thumbSquare / $thumbWidth);
$thumb = imagecreatetruecolor($thumbWidth, $thumbHeight);
imagealphablending($thumb, false);
imagecopyresized($thumb, $image, 0, 0, 0, 0, $thumbWidth, $thumbHeight, $width, $height);
}
for ($i = 0; $i < $thumbWidth; $i++) {
for ($j = 0; $j < $thumbHeight; $j++) {
if (imagecolorat($thumb, $i, $j) & 0x7F000000) {
return true;
}
}
}
return false;
}
Использование:
hasTransparency( imagecreatefrompng("myfile.png") ); //returns true if img has transparency