Questions about raw, tiff, jpeg, and DNG

Started Mar 27, 2013 | Questions thread
Hugowolf
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Re: Questions about raw, tiff, jpeg, and DNG
In reply to Tom Axford, Mar 28, 2013

Tom Axford wrote:

Hugowolf wrote:

It has nothing to do with quitting and restarting the program, it is only the editing that matters. Photoshop won't let you save an unchanged image in the same format, the 'save as' actually saves over the old image file. You could write a script that openned, resaved, and closed several times. It will not have an effect on the image quality.

(And, there are lossy forms of TIFF).

Brian A

I've done some quick tests and I have to admit that you are correct if the image is not edited (or edited in ways that do not affect most of the image).  I think the JPEG algorithms have been improved over the years, because that wasn't the case many years ago when I first encountered this problem.

The algorithms are pretty well defined and haven't changed.

However, in some cases, the image quality still degrades each time you save a JPEG.

To illustrate this, I created an image of the text "AB" on a white background and saved it (at the lowest JPEG quality). I then reloaded the image, edited it (by slightly cropping the image size) and saved the result.

Oh for sure, cropping will cause major recompression, much more so than most other forms of editing. The compression extent is generally based on 16 x 16 square pixel blocks, if you slice a pixel off the edge of one of those, then the whole image will be go through recompression. Edit pixels in the middle of a block, and any loss will generally be localized to that one block.

Which still doesn’t negate the fact that just repetitive saving does not cause data loss from a jpeg. If you are editing a jpeg image, then frequently saving the image while editing is not a good idea if you want to preserve quality.

Brian A

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