How to estimate the size of JPEG image which will be scaled down

Question

For example, I have an 1024*768 JPEG image. I want to estimate the size of the image which will be scaled down to 800*600 or 640*480. Is there any algorithm to calculate the size without generating the scaled image?

I took a look in the resize dialog in Photoshop. The size they show is basically (width pixels * height pixels * bit/pixel) which shows a huge gap between the actual file size.

I have mobile image browser application which allow user to send image through email with options to scale down the image. We provide check boxes for the user to choose down-scale resolution with the estimate size. For large image (> 10MB), we have 3 down scale size to choose from. If we generate a cached image for each option, it may hurt the memory. We are trying to find the best solution which avoid memory consumption.

Answer 1

I have successfully estimated the scaled size based on the DQT - the quality factor.

I conducted some experiments and find out if we use the same quality factor as in the original JPEG image, the scaled image will have size roughly equal to (scale factor * scale factor) proportion of the original image size. The quality factor can be estimate based on the DQT defined in the every JPEG image. Algorithm has be defined to estimate the quality factor based on the standard quantization table shown in Annex K in JPEG spec.

Although other factors like color subsampling, different compression algorithm and the image itself will contribute to error, the estimation is pretty accurate.

P.S. By examining JPEGSnoop and it source code, it helps me a lot :-)

Cheers!