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RE: Lossy, smaller JPG files

Subject: RE: Lossy, smaller JPG files
From: jimmy -at- breck-mckye -dot- com
To: <techwr-l -at- lists -dot- techwr-l -dot- com>
Date: Fri, 16 Mar 2012 10:43:34 +0100

Another point to raise: for images with a small number of colours and large blocks of constant colour - like screenshots - PNGs will often end up smaller than JPEGs anyway (unless you use *particularly* appalling JPEG compression quality). Ideally, of course, we should be working in vector images or some other infinitely zoomable format, but it's difficult to get scalable output from many applications.

On 15.03.2012 16:10, Combs, Richard wrote:

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
RE: Lossy, smaller JPG files: From: Combs, Richard

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