Friday, July 1, 2005

July/August 2005 HelpLine

JPEG Reloaded

    * Saved By JPEG
    * Timing Of File Saving
    * It's All In The Name

DPMag Published in HelpLine

Here's the important part. I closed the original file in Photoshop and opened c01.jpg. This has now reconstructed the compressed file.

Then, in order to simulate some editing, I cropped the file by a pixel on the top and left. After that, I did a Save As, again selecting JPEG for the copy, using maximum quality settings, and called it c02.jpg. I closed c01.jpg, opened c02.jpg (reconstructing the file again) and continued this process (Open, Crop, Save As, Close) until I got to c10.jpg.

Finally, I opened the original, together with the c10.jpg image and took a look at them to see what kind of compression artifacts I'd get. You can see them, although you have to enlarge the image a bit to do so. Around the child's head, you can see obvious compression artifacts. You also can see them in the skin tones next to face details. While they might not show up in certain uses like small Web images, they will appear when printed.

For the record, I did this same test without adjusting the image at all—no cropping—just Open, Save As and Close 10 times. The artifacts still were there, but they were just a little less obvious.

So if you want to keep as much honest data in your images by keeping them artifact-free for as long as possible, only compress when you have to and stay away from using JPEG as an archival or storage format. Why wouldn't you want to use JPEG for archives? First of all, when you save the image for the last time, you're compressing it again, so what you see on the screen isn't what you saved. Also, can you honestly say that you'll never reprocess that image? Might you use it in a montage with another image, or would you edit differently for a different type of output? If you can avoid compression, you'll have more imaging options in the future.

Timing Of File Saving

Q)  In the May 2005 PCPhoto, you suggest the following: "[I resave] the edited images in the native format of the image-editing application that I'm using. When I'm satisfied with my adjustments, I also save a copy of the file in TIFF format." I believe this leads you to a wrong sequence of events.

Tom McGrath
Via e-mail

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