使用glib在php中将透明png转换为jpg时出现颜色错误

Question

I try to convert a partly transparent png to a jpg in php with gdlib. I found two snippets to help me with that, but both methods have the same problem: The half transparent colors are darker and do not look right. Here a enlarged sample from photoshop: left the png (with white in background instead of transparent), right the converted png to jpg with both snippets I used:

difference png (left) to jpg (right)

Here the original Png-File: golf.png

Any help would be really appreciated!

$input_file = "card/golf.png";
$output_file1 = "card/golf1.jpg";
$output_file2 = "card/golf2.jpg";

$image = imagecreatefrompng($input_file);

$bg = imagecreatetruecolor(imagesx($image), imagesy($image));
imagefill($bg, 0, 0, imagecolorallocate($bg, 255, 255, 255));
imagealphablending($bg, TRUE);
imagecopy($bg, $image, 0, 0, 0, 0, imagesx($image), imagesy($image));
imagejpeg($bg, $output_file1, 100);
imagedestroy($bg);
imagedestroy($image);

list($width, $height) = getimagesize($input_file);
$image = imagecreatefrompng($input_file);
$output = imagecreatetruecolor($width, $height);
$white = imagecolorallocate($output,  255, 255, 255);
imagefilledrectangle($output, 0, 0, $width, $height, $white);
imagecopy($output, $image, 0, 0, 0, 0, $width, $height);
imagejpeg($output, $output_file2, 100);
imagedestroy($output);

Answer 1

You're suffering from quantization. JPEG does not handle this type of image well at all. If you want to lessen the color changes you need to adjust your quantization tables. If you use all 1s for the quantization tables you don't get the color changes.