Resize an image and maintain quality? - php

Question

I have a problem with resizing images.
What happens is that if you upload a file larger than the stated parameters, the image is cropped, then saved at 100% quality.
So if I upload a large jpeg which is 272Kb. The image is cropped by 100 odd pixels. The file size then goes up to 1.2Mb.
We are saving images at a 100% quality. I assume that this is what is causing the problem. The image is exported from Photoshop at 30% quality which reduces the file size. Resaving the image at 100% quality creates the same image but I assume with a lot of redundant file data.
Has anyone encountered this before? Does anyone have a solution?
This is what we are using.
$source_im = imagecreatefromjpeg ($file);
$dest_im = imagecreatetruecolor ($newsize_x, $newsize_y);
imagecopyresampled (
$dest_im, $source_im,
0, 0,
$offset_x, $offset_y,
$newsize_x, $newsize_y,
$sourceWidth, $sourceHeight
);
imagedestroy ($source_im);
if ($greyscale) {
$dest_im = $this->imageconvertgreyscale ($dest_im);
}
imagejpeg($dest_im, $save_to_file, $quality);
break;

Answer 1

Saving at 30% then re-saving at 100% will, as you say, create redundant data, whether you crop, resize, whatever.
Unfortunately, JPEG compression accumulates data loss, so compressing at 30%, processing the image, then re-compressing will always look worse than the original compression. The rule of thumb is to avoid compression (especially heavy compression like 30%) until as late in the process as possible. So upload at 100% (or 80ish% if necessary), then compress.
Edit: apparently, jpegtran (google it) can do operations such as cropping without first decompressing the image, as long as the image size follows certain constraints (usually width and height a multiple of 16 pixels). Haven't tried it myself, but it might suite your purposes.

Answer 2

When saving a JPEG with 30% quality, a lot of pixel information is not saved. When opening it again using gd, a new image is created with crisp new pixels. Whether these look good or bad to you (depending on the quality the image was originally saved with) is irrelevant. When saving this new image, you're create a new JPEG file. Setting the quality to 100% will basically save every single pixel, which of course takes a lot of space. (I'm generalizing, but you get the idea.) Whatever quality setting the original was saved at is irrelevant, saving a big image at 100% quality takes a lot of space.
The only solution is to save the image using a lower quality setting; usually something around 70% is virtually indistinguishable from perfect, but saves a lot of bytes. You may also want to try PNG, which is lossless but may (or may not) provide a better compression ratio.

Answer 3

Your assumptions is correct, you are essentially saving it at loss-less. I ran into this with video encoding where I would downsize the resolution but increase it to loss-less and the final size was twice that the original. If it is a small enough image I would save it using a good palette based image type or a type that is native loss-less not jpeg. Either that save it at 50-70% quality and let jpeg do what it is good at.