Re: Why JPGs for screen captures? test results

Subject: Re: Why JPGs for screen captures? test results
From: "Richard G. Combs" <richard -dot- combs -at- voyanttech -dot- com>
To: "TECHWR-L" <techwr-l -at- lists -dot- raycomm -dot- com>
Date: Wed, 26 Feb 2003 16:45:14 -0700

Great discussion! Regarding sizing and clarity, there's no reason at all for
a screen capture to be blurry or unreadable when it's sized 4-6 inches wide
and printed on a 600-dpi printer. I'm not a graphics guru, but I can take a
stab at explaining why I say this. Gurus can chime in if I stumble. ;-)

Depending on monitor size and personal preferences, your screen width is
probably between 800 and 1280 pixels. At 600 dpi, your printer can put 3600
dots across the width of a 6-inch image. That's 3 to 4+ times as much
information density as your screen, so why should it be less sharp or
readable? (If your screen is full of 8-point text, you may need a magnifying
glass to read it when you make it half as large -- but, through the
magnifying glass, it should be just as sharp and readable as it was on
screen, not blurry or distorted.)

The problem is that most "resizing" operations involve discarding
information and/or resampling. To keep the math simple, let's say you
capture a window that's 1000 pixels wide, and its actual physical width is
10 inches. Your bitmap is therefore 100 dpi (in the ballpark for most
current Windows machines). If you want to make the image half as big (5
inches wide) to fit your page, you have two options, each with a variant
worth mentioning:

1. Maintain the 100 dpi resolution, but throw away every other pixel. Now
your image is 500 pixels wide (5 inches at 100 dpi), but contains half as
much horizontal resolution. Some of those discarded pixels helped define the
shapes of those delicate fonts you love to fondle. ;-)

1a. The same option gets worse if you want to make the image, say, 57% as
big. Now, instead of throwing away every other pixel, your graphics/dtp app
has to do some sophisticated interpolating to maintain the 100 dpi
resolution. I know little about this except that some programs do it better
than others. I suspect that the result will usually be even worse than
option 1.

2. Keep all 1000 pixels, but make the resolution 200 dpi. You're still at 5
inches (1000/200), but you haven't discarded any information. Of course,
your monitor can't display 200 dpi (so, on screen, you won't see the
difference between this and option 1 unless you zoom in to 200%). But your
600 dpi printer can. It can use exactly 3 dots for each dot in the original
graphic. There's no loss of information and your printed image should be
sharp as a tack.

2a. What if you go with option 2, but size your image so that its resolution
is 259 dpi? Well, the (600-dpi) printer can't print fractional dots (I know,
technically it probably can, but that's to *get* to 600 dpi), so you're back
to the scenario in 1a. I think the technical term is "yuck."

Conclusion: If you want sharp screen shots, don't resample; keep every damn
pixel when you resize! And try to size your images so that their dpi divides
evenly into the printer dpi.

In FrameMaker, you should size imported bitmaps by setting the dpi
explicitly upon import (you can change it later from the Object Properties
dialog box).

I target a 600-dpi printer and use FrameMaker. Most of my imported screen
shots are set to 100, 120, or 150 dpi. I'm usually capturing windows or
portions of windows, not the full screen. If a nice, evenly-divisible dpi
setting doesn't work size-wise, then I try the capture again with the
application window resized a little.

Works for me. No blurring ever.


Richard G. Combs
Senior Technical Writer
Voyant Technologies, Inc.
richardDOTcombs AT voyanttechDOTcom
rgcombs AT freeDASHmarketDOTnet


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