How to remove exif from a JPG without losing image quality?

Question

I have a PHP photo sharing application in which user-uploaded images are resized into various thumb formats using ImageMagick.

As a seemingly "smart" way to save on file size, I am stripping exif info from these thumbs as follow:

$imagick = new Imagick($image);
$imagick->stripImage();
$imagick->writeImage($image);

This works. It does remove the EXIF info, where a thumbs of 30KB saves 12KB and becomes 18KB. A significant saving when showing many of such thumbs on a single page.

The problem however is that it works a little too well. The resulting images seem to lose a lot of color information and look "flat" compared to their non-stripped versions.

Based on my research so far, my theory is that one or both of the following is true:

• Imagick throws away essential color profile information as part of the stripping process
• Imagick recompresses the image upon saving it, losing quality

Regardless of the cause of the problem, I'm looking for a way to remove EXIF information in such a way that it does not affect the image quality or color itself.

Is this even possible?

Update:

Based on Gerald Schneider's answer, I tried enforcing the quality setting to 100% prior to "stripping" the image:

$imagick = new Imagick($image);
$imagick->setCompression(imagick::COMPRESSION_JPEG);
$imagick->setCompressionQuality(100);
$imagick->stripImage();
$imagick->writeImage($image);

Unfortunately, the problem remains. Below is example output where despite setting the quality to 100%, images are still flattened.

Answer 1

Consider keeping the ICC profile (which causes richer colors) while removing all other EXIF data:

1. Extract the ICC profile
2. Strip EXIF data and image profile
3. Add the ICC profile back

In PHP + imagick:

$profiles = $img->getImageProfiles("icc", true);

$img->stripImage();

if(!empty($profiles))
    $img->profileImage("icc", $profiles['icc']);

(Important note: using the ImageMagick 3.1.0 beta, the result I got from getImageProfiles() was slightly different from the documentation. I'd advise playing around with the parameters until you get an associative array with the actual profile(s).)

For command line ImageMagick:

convert image.jpg profile.icm
convert image.jpg -strip -profile profile.icm output.jpg

Images will get recompressed of course if you use ImageMagick, but at least colors stay intact.

Hope this helps.

Answer 2

Having made similar changes to MIME types in file headers that were incorrectly stored, I'd suggest you verify the length of the EXIF data via the standard tools, and then "Zero" the data manually using multibyte string functions.

EXIF can only be a maximum of 64KB in a JPEG file, however I'm not positive if it's exacly 64KB, so I would begin with this.