问题
I had troubles resizing a PNG and maintaining small file sizes. Solution found here.
When resizing the PNG, however, I ran into problems regarding image quality. As far as I could see, GD uses indexed 8-bit-color palette which distorts text and colors get lost, see:
- Original Image
- Resized Image with solution given above
- Resized Image with a tweak²
²The idea for the tweak I found here in stackoverflow: Create truecolor-image, resize it, and copy it to a new image, so the palette is determined based on the resampled result and the image quality is better as you can see in the image above.
// create new image
$newImageTmp = imagecreatetruecolor($newwidth,$newheight);
// we create a temporary truecolor image
// do the image resizing by copying from the original into $newImageTmp image
imagecopyresampled($newImageTmp,$src,0,0,0,0,$newwidth,$newheight,$width,$height);
// create output image
$newImage = imagecreate($newwidth,$newheight);
// copy resized truecolor image onto index-color image
imagecopy($newImage,$newImageTmp,0,0,0,0,$newwidth,$newheight);
// write image to buffer and save in variable
ob_start(); // stdout --> buffer
imagepng($newImage,NULL,6);
$newImageToSave = ob_get_contents(); // store stdout in $newImageToSave
ob_end_clean(); // clear buffer
// remove images from php buffer
imagedestroy($src);
imagedestroy($newImageTmp);
imagedestroy($newImage);
Problem: None of both results are satisfactory.
I am quite sure that there must be a way to 1. determine the color palette, and 2. maintain most of the image's colors, so that 3. the PNG looks similar to the original and has an acceptable file size.
Now, I only see going for JPG instead of PNG. But if you know a solution, it would greatly be appreciated if you let me/us know.
Thank you!
回答1:
All you need is to replace
$newImage = imagecreate($newwidth,$newheight);
With
$newImage = imagecreatetruecolor($newwidth, $newheight);
Output $maxImgWidth = 200;
回答2:
PHP's fork of GD doesn't have usable palette generation, so you only get PNG32 with vanialla libpng compression.
For small PNG8 with palette use pngquant, e.g. http://pngquant.org/php.html
And then compress it further with advpng or zopfli-png.
来源：https://stackoverflow.com/questions/13642018/optimize-png-when-resizing-with-gd-php-how-to-determine-color-palette