Re-saving JPEG while keeping quality

conversionjpeg

Sometimes I use XnView's* "batch conversion" tool to downsize a bunch of JPEGs (to maximum of 1600×1200). But some of them are equal or smaller than that and no resizing is done.

Is any image quality lost during the process? (Even if I choose JPEG 100% quality?)

Edit: I was asking about those images which are already smaller than the limit, and are not being resized, just re-compressed.

* My Linux laptop is toast, can't do any imagemagick tricks.

Best Answer

Yes, there will be loss of quality, even at 100%. That said, is it likely to be all but unnoticeable immediately. If you have Photoshop you can easily confirm the loss of quality:

Load up the original and the resaved image as two layers in the same image.
Change the top one's display mode to Difference.
Select All and then Copy Merged (Ctrl+A and Ctrl+Shift+C by default).
Paste the result in as a new layer.
Using the Magic Wand tool, (Tolerance 0, Contiguous unchecked, Antialias unchecked), select an all-black pixel (RGB 0, 0, 0).

If all pixels are selected (which won't happen), the two images are identical. Otherwise the unselected areas have been modified.

If you resave the same JPEG a few times it will become much more noticeable.

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Updated: This should work (-flatten did the trick for me)

convert -geometry 110% your.eps -flatten your.jpg

Progressive JPEG quality compare to normal JPEG

There are a couple of nice posts here that seem to describe why Progressive compression may be better and it seems to come down to the fact that in Progressive JPEG the compressed data is ordered more efficiently and that blocky image data and noise get separated and compressed separately.

I've highlighted the important pieces below, they describe it better than I ever could.

As a result I would expect that progressive JPEG is better compression, but not any significant change in image quality.

Progressive JPEG has two different coding features:

Spectral Selection

Successive Approximation

What you describe is just the first feature (first DC, then AC bands). However, it is the second feature which is the major reason for different coding efficiency of Progressive JPEG. With Successive Approximation, you first store the higher bits of the coefficients (low precision, coarse image), then the lower bits (high precision, detailed image). In usual images, the noise is concentrated in the lower bits. Therefore, the Huffman encoder can be more efficient in symbol coding in the higher bit range where less noise is. The Successive Approximation coding thus separates the noise from the image, and that is the reason for better coding efficiency (noise is hardly compressible).

The more noise (detail) the image has, the better the coding efficiency of Progressive JPEG. The less noise (blur) the image has, the better the coding efficiency of Sequential (Non-progressive) JPEG.

Note that most Progressive JPEG coders use a mixture of Spectral Selection coding and Successive Approximation coding. For better coding efficiency, mainly Successive Approximation is relevant, and you can try to find optimal Successive Approximation paramters (the point where to separate the precision - Ah/Al in the JPEG standard) for your class of images. For example, I sometimes found an optimal point Al=4 for my images (4 lower bits separately coded).

Best Answer

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How to convert eps file to a large jpeg image

Progressive JPEG quality compare to normal JPEG

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