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Choosing Better-Quality JPEG Images With Software?
kpoole55 writes "I've been googling for an answer to a question and I'm not making much progress. The problem is image collections, and finding the better of near-duplicate images. There are many programs, free and costly, CLI or GUI oriented, for finding visually similar images — but I'm looking for a next step in the process. It's known that saving the same source image in JPEG format at different quality levels produces different images, the one at the lower quality having more JPEG artifacts. I've been trying to find a method to compare two visually similar JPEG images and select the one with the fewest JPEG artifacts (or the one with the most JPEG artifacts, either will serve.) I also suspect that this is going to be one of those 'Well, of course, how else would you do it? It's so simple.' moments."
File size may not be accurate if it has been converted multiple times at different quality, or if the source is actually lower quality.
The only way to properly compare is if you have the original as the control.
If you compare between 2 different JPEG quality images, the program won't know which parts are the artifacts. You still have to decide yourself...
Run the DCT and check how much it's been quantized. The higher the greatest common factor, the more it has been compressed.
Alternatively, check the raw data file size.
Others have mentioned file size, but another good approach is to look at the quantization tables in the image as an overall quality factor. E.g., JPEG over RTP (RFC 2435) uses a quantization factor to represent the actual tables, and the value of 'Q' generally maps to quality of the image. Wikipedia's doc on JPEG has a less technical discussion of the topic, although the Q it uses is probably different from the example RFC.
File size doesn't tell you anything. If I take a picture with a bunch of noise (eg. poor lighting) in it then it will not compress as well. If I take the same picture with perfect lighting it might be higher quality but smaller file size.
Why this is modded up, I don't know. Too many morons out there.
Even faster is look at the DCT coefficients in the file itself. Doesn't even require decoding - JPEG compression works by quantizing the coefficients more heavily for higher compression rates, and particularly for the high frequency coefficients. If more high frequency coefficients are zero, it's been quantized more heavily, and is lower quality.
Now, it's not foolproof. If one copy went through some intermediate processing (color dithering or something) before the final JPEG version was saved, it may have lost quality in places not accounted for by this method. Comparing quality of two differently-sized images is also not as straightforward either.
This seems to me the best suggestion, and there's a simple visual way to accomplish it! The hardest hit part of the image is going to be the chroma information, which your eye normally has reduced resolution sensitivity for in a normal scene. To overcome this, load your JPEGs into your favorite image editor and crank the saturation to the max(this throws away the luminance data). Now the JPEG artifacts in the chroma information will HIT YOU IN THE FACE, even in images that seemed rather clean before. Pick the least blocky of the two, and there you go!
Ok, so you know how two images differ. Which one is closer to the original? You don't know, because you don't have the original to compare.
:x
Here's a simple but expensive formula:
1. Get the image
2. Compress it severely.
3. Compare the difference between original and the compressed.
The lower the difference, the lower the image quality.
4. Profit!
Or you could just measure the amount of data in the DCT space. Duh.
Unfortunately, that's a subjective term based on the 'codec' used to make the jpg. Not everyone's 100 is the same nor is everyone working off the same scale (i.e. 1-10 vs 1-100).
In addition, I bought a program (Windows only, sorry) that allows the user to pick the areas of the image that need the most bits. Basically, it allows you to pick the quality for any abitrary region (using standard selection tools like lasso) when saving the JPEG.
I mostly got it for the batch processing and its excellent image quality when you set it to minimum compression.
Try file size on the set of images of interest to you and see if it coincides with your intuition. If it does, you're done.
That works, but only if you have exact, pixel-to-pixel correspondence between the photos. It won't work if you just grab 2 photos from flicker that both show the Eiffel tower, and you wonder which one is "better".
Luckly, there is a simple way to do it: use jpegtran to extract the quantization table form each image. Pick the one with the smaller values. This can easily be scripted.
Caveat: this will not work if the images have been decoded and re-coded multiple times.