RE: Lossy, smaller JPG files

Subject: RE: Lossy, smaller JPG files
From: "Combs, Richard" <richard -dot- combs -at- Polycom -dot- com>
To: Margaret Cekis <Margaret -dot- Cekis -at- comcast -dot- net>, 'Dan Goldstein' <DGoldstein -at- riveraintech -dot- com>, "techwr-l -at- lists -dot- techwr-l -dot- com" <techwr-l -at- lists -dot- techwr-l -dot- com>
Date: Thu, 15 Mar 2012 08:10:07 -0700

Margaret Cekis wrote:

> Dan Goldstein asked whether saving JPG files (a lossy format) received
> from
> someone else (without changes) in Paint Shop Pro, degrades the image
> quality, even if he can't detect any loss of quality.
> ----------------------------
> Dan:
> I think that the answer is "not necessarily". Paint Shop Pro and other
> modern high-quality graphics programs have much more sophisticated
> compression and down-sizing algorithms than earlier programs (or even
> earlier versions of the packages we are using now). They can compress
> JPGs
> and other common graphics format files with less loss than earlier
> compression methods.

You may be right about algorithm improvements, but resaving a JPG nevertheless results in a loss of information. It's the nature of the beast.

<snip> If you try to display the multi-megapixel image
> captured by
> a modern high-end digital camera on a laptop, or even on a high-
> resolution
> wide-screen monitor, you'll be able to see only a small portion of the
> image. These cameras now save images both in this super-megapixel "raw"
> format, and in JPGs that "normal" PCs and monitors can handle. Unless
> you
> have a billboard-size screen, downsizing JPG images to the resolution
> your
> screen or application can handle will not degrade the image enough for
> your
> eye to detect it.

RAW and JPEG are file formats. That has nothing to do with pixel dimensions. A 10-megapixel image is going to be (typically, if 4:3) 3648 x 2736 pixels regardless of whether it's a RAW file or a JPEG file. The difference is that the RAW file will be 10-15 MB (depending on bit depth) and the JPEG will be around 2 MB or less (depending on quality setting). That's because, using an algorithm based on how people see complex visual information, the JPEG format discards some of the information about the individual pixels and compresses what's left. (And it does this every time you save.)

Obviously, to display the entire 3648 x 2736 image on a 1920 x 1200 monitor, you have to zoom the image. Regardless of the file format.

But when working with the original image file, (1) you just want to zoom, not permanently discard half the pixels, and (2) you don't want to discard some of the information about those pixels (as JPEG does) every time you resave the file.

If someone sends you a JPEG (and there is no lossless source file available), save it in PSP's native file format (.psp) (it will get much bigger). Or as a PNG (also bigger, but not as much). Use PNG for your output (print, PDF, etc.). Or make a fresh JPEG from the PSP file and _never_ resave it. If any changes are required, edit the PSP file (or a copy) and then make another fresh JPEG.

Of course, if your deliverable is a web page or help file, there's no point in delivering a 3648 x 2736 image, so you _would_ want to discard a bunch of pixels. But don't do that in the original, just resize when creating the JPEG or PNG for the deliverable.

HTH!

Richard G. Combs
Senior Technical Writer
Polycom, Inc.
richardDOTcombs AT polycomDOTcom
303-223-5111
------
rgcombs AT gmailDOTcom
303-903-6372
------







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Follow-Ups:

References:
Lossy, smaller JPG files: From: Dan Goldstein
RE: Lossy, smaller JPG files: From: Margaret Cekis

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