Detect JPEG image quality

Question

I allow users to upload images. However, I want to keep JPEG quality not more than 90%. What I plan to do is to detect the current quality: - If less than 90% do nothing - If mo

Answer 1

paddy is correct that this setting is not always stored in the JPEG file. If it is, then you can use identify from Imagemagick to read the quality. For example:

$ identify -format '%Q' tornado_ok.jpg

93%

Update: Based on the answer to this question https://superuser.com/questions/62730/how-to-find-the-jpg-quality I find out that apparently the identify command can still determine the quality by reverse engineering the quantization tables even if all the image EXIF / other meta data is lost. By the way, the title of your question as it stands now is a possible duplicate of that question I linked to.

But to me your question has merit on its own because in your question's text you explain what you are trying to do, which is more than simply detecting jpeg quality. Nevertheless, you should perhaps update the title if you want to reflect that you are trying to solve a more specific problem than just reading JPEG image quality.

Unless you are archiving original images, for web use even 90% is excessive. 75% used to be the default in the old days (degradation was visible only under close inspection between side-by-side images), and now in the days of high bandwidth 85% is a very high quality option. The 5% quality difference between 90% and 85% is virtually invisible, but will save you over 30% in file size typically. The JPEG algorithm is designed to begin by eliminating information that is invisible to human perception at its first compression stages (above 80% or so).

Update/note: The compression quality settings I am talking about are from tests with libjpeg, a very widely used JPEG library. Photoshop's compression percentages and other software's quality settings are all independent and do not necessarily mean the same thing as the settings of libjpeg.

paddy's idea of using image height and image width to calculate an acceptable file size is reasonable:

You can get the image height/width like this:

list($originalWidth, $originalHeight) = getimagesize($imageFile);

My own high-quality photos posted online, like this one: http://ksathletics.com/2013/wsumbb/nw.jpg are typically saved at a ratio of about 200 KB per megapixel.

So, for example, you can multiply width times height and divide by 1000000 to calculate the megapixels in the image. Divide the file size by 1024 to calculate the KB. Then divide the resulting KB by the megapixels. If the result is under 200 or whatever value you decide upon, then you don't need to re-compress it. Otherwise, you can re-compress it with a quality of 85% or whatever quality you decide on.