Expert: LizaL - 5/17/2005

Question
Hello,
I'm trying to understanding how image re-sizing works with jpeg images. If I scale down the size of an image (under 'image size' in photoshop), thus reducing the file size, and then save, am I losing any of the original information for that image or not? Some websites seem to say that I'm not - if so, can I just re-size up again later? How on earth does this work!? I have read references to loss-less compression, is that what is happening here and again, how does it work? Many thanks in advance. Jo

Answer
Hi again Jo,

Based on your comments, I wanted to add something -- you should ALWAYS, ALWAYS save your original file. It might seem pointless and a waste of hard drive space, but it's not, considering that that original file is untouched and contains all the original pixel information, intact.

If you have to resize up, you should resize on that original image -- that is, make a copy of the original, and resize up from that, never from an image that you've already worked on.

And as I mentioned, resizing UP is never the greatest scenario. To avoid having to do this, scan your original at the proper resolution needed for the project, or take the digital picture at the proper resolution needed for the project.

But bottom line, yes, always save your original image and work from copies of it.

Hope this helps!

Lisa

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Hi Jo,

When you scale an image down, using the Image Size dialog box (and this is what you should always use, BTW), you're discarding information in the form of redundant and similar pixels -- and thus, you do lose some information. But that's not as bad as scaling up.

Basically, you need to understand that sizing UP is not a good thing. This brings into play what's called resampling up, and this involves interpolation. Simply put, when you make an image larger, Photoshop has to add in the pixels that aren't there (this is interpolation), and that can result in poor image quality. You might find you need to use Unsharp Mask after you scale up.

To avoid having to sample up or down, and to avoid problems in doing these things, make sure you scan your images in at the proper resolution. If they're to be printed commercially, find out from the printer the proper line screen at which they'll be printed. Rule of thumb is line screen x 2 (roughly), so if your printer will be printing at a 150 line screen (150 lpi), scan your images at 300dpi.

If your image is for the Web, always use Save for Web to give the proper compression. Of course you know to save photos as JPEG, and vector graphics and type as GIF.

Lossy refers to a form of compression by which some (but theoretically not a lot) of information is lost. Lossless is more or less the discarding of no information when a file is compressed.

Here's a good article about compression, in general. Maybe it will explain some things for you:
http://www.gslis.utexas.edu/~l38613dw/website_fall_02/readings/Compression.html


I hope this helps, but if you need any more information or explanation, please don't hesitate to email me as I'm not going to be available on AllExperts for a couple of days. I'll be checking my email on Thursday, BTW, and that address is lizal@little-works.com.

Thanks!

Lisa