Finding JPEG image quality

Hi. I am implementing Double Compression on a JPEG image which requires me to compute a quantization step. This step is related to image quality by some formulae. I want a method to find the quality of the image so that i can compute the step. If any other way of Double Compression is possible then please help PS: I will have only one image so finding PSNR and MSE I find not helpful as they require comparison between two images

Answer 1

[Image Analyst]

This may help: http://en.wikipedia.org/wiki/Image_quality

Also, ssim, in the Image Processing Toolbox, is used to assess image quality. See http://en.wikipedia.org/wiki/Structural_similarity

But you have two images. You have the original perfect image, and you have the one that's undergone double compression and then restored. So you can compare the two. Why do you think that is not helpful? It tells you how close your algorithm came to recovering the original perfect image.

[Pratik]

Hi Image Analyst. I am actually doing DCT Coefficient analysis which needs 1st step as DCT of image and then Quantizing it based on a step. This step is found from the standard table as well as quality. The relationship between the JPEG quality, Q, and the quantization steps s(ωx,ωy) takes the form: s(ωx,ωy) = max(((200−2Q)/100)*C(ωx,ωy) + 1 2,1) ,50≤ Q ≤ 100 (50/Q)*C(ωx,ωy) + 1 ,0 < Q < 50, where C is the predefined quantization matrix