使用GD生成图像缩略图仅对PNG失败

Question

I got following code for generate a image thumbnail, but it only works for JPEG for unknown reason, i cant figure out why it doesnt work for PNG ?

function generate_image_thumbnail($source_image_path, $thumbnail_image_path)
{
        list($source_image_width, $source_image_height, $source_image_type) = getimagesize($source_image_path);
        switch ($source_image_type) {
            case IMAGETYPE_GIF:
$source_gd_image = imagecreatefromgif($source_image_path);
                break;
            case IMAGETYPE_JPEG:
$source_gd_image = imagecreatefromjpeg($source_image_path);
                break;
            case IMAGETYPE_PNG:{
$source_gd_image = imagecreatefrompng($source_image_path);
                break;
}
        } //$source_image_type
        if ($source_gd_image === false) {
            return false;
        } //$source_gd_image === false
$source_aspect_ratio    = $source_image_width / $source_image_height;
$thumbnail_aspect_ratio = THUMBNAIL_IMAGE_MAX_WIDTH / THUMBNAIL_IMAGE_MAX_HEIGHT;
        if ($source_image_width <= THUMBNAIL_IMAGE_MAX_WIDTH && $source_image_height <= THUMBNAIL_IMAGE_MAX_HEIGHT) {
$thumbnail_image_width  = $source_image_width;
$thumbnail_image_height = $source_image_height;
        } //$source_image_width <= THUMBNAIL_IMAGE_MAX_WIDTH && $source_image_height <= THUMBNAIL_IMAGE_MAX_HEIGHT
        elseif ($thumbnail_aspect_ratio > $source_aspect_ratio) {
$thumbnail_image_width  = (int) (THUMBNAIL_IMAGE_MAX_HEIGHT * $source_aspect_ratio);
$thumbnail_image_height = THUMBNAIL_IMAGE_MAX_HEIGHT;
        } //$thumbnail_aspect_ratio > $source_aspect_ratio
        else {
$thumbnail_image_width  = THUMBNAIL_IMAGE_MAX_WIDTH;
$thumbnail_image_height = (int) (THUMBNAIL_IMAGE_MAX_WIDTH / $source_aspect_ratio);
}
$thumbnail_gd_image = imagecreatetruecolor($thumbnail_image_width, $thumbnail_image_height);
        imagecopyresampled($thumbnail_gd_image, $source_gd_image, 0, 0, 0, 0, $thumbnail_image_width, $thumbnail_image_height, $source_image_width, $source_image_height);
 
        switch ($source_image_type) {
            case IMAGETYPE_GIF:
                imagegif($thumbnail_gd_image, $thumbnail_image_path, 90);
                break;
            case IMAGETYPE_JPEG:
                imagejpeg($thumbnail_gd_image, $thumbnail_image_path, 90);
                break;
            case IMAGETYPE_PNG:{
                //imagealphablending($thumbnail_gd_image, false); 
                //imagesavealpha($thumbnail_gd_image,true);
                imagepng($thumbnail_gd_image, $thumbnail_image_path, 90);
                break;
}
}
 
 
 
imagedestroy($source_gd_image);
imagedestroy($thumbnail_gd_image);
        return true;
}

Answer 1

the problem is in the second png case, exactly in

imagepng($thumbnail_gd_image, $thumbnail_image_path, 90);

according to imagepng function

quality

Compression level: from 0 (no compression) to 9.

so, change 90 to 9 (for example)

imagepng($thumbnail_gd_image, $thumbnail_image_path, 9);