image processing - Is it possible to tell the quality level of a JPEG?

Question

This is really a two part question, since I don't fully understand how these things work just yet:

My situation: I'm writing a web app which lets the user upload an image. My app then resizes to something displayable (eg: 640x480-ish) and saves the file for use later.

My questions:

1. Given an arbitrary JPEG file, is it possible to tell what the quality level is, so that I can use that same quality when saving the resized image?
2. Does this even matter?? Should I be saving all the images at a decent level (eg: 75-80), regardless of the original quality?

I'm not so sure about this because, as I figure it: (let's take an extreme example), if someone had a 5 megapixel image saved at quality 0, it would be blocky as anything. Reducing the image size to 640x480, the blockiness would be smoothed out and barely less noticeable... until I saved it with quality 0 again...

On the other end of the spectrum, if there was an image which was 800x600 with q=0, resizing to 640x480 isn't going to change the fact that it looks like utter crap, so saving with q=80 would be redundant.

Am I even close?

I'm using GD2 library on PHP if that is of any use

Answer 1

You can view compress level using the identify tool in ImageMagick. Download and installation instructions can be found at the official website.

After you install it, run the following command from the command line:

identify -format '%Q' yourimage.jpg

This will return a value from 0 (low quality, small filesize) to 100 (high quality, large filesize).

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