Re: Resolution of graphics for printed docs

Subject: Re: Resolution of graphics for printed docs
From: Bill Swallow <techcommdood -at- gmail -dot- com>
To: "TECHWR-L" <techwr-l -at- lists -dot- techwr-l -dot- com>
Date: Wed, 5 Oct 2005 11:37:05 -0400

> I thought I'd answered the OP's question earlier in this thread, but some
> of the responses from other posters show that there's still confusion
> about printing screen captures.

Yes, I was trying to correct the info you provided. ;-) Here comes
round two. ;-)

> - Pixel dimensions: 504 pixels wide by 315 pixels tall
> - Print size: 7 inches wide by 4.375 inches high at a resolution of 72
> pixels per inch (ppi).

First fuzzy issue... a screen capture does not have a dimension in
inches. Not at all. A graphic does not either. Whatever application
you're using to work with the image is telling you an output size
based on the dpi value you set. That's it.

> Now, if you wanted to include the capture in a manual for printing, 7 x
> 4.3 inches would be too big and 72 ppi printing at 300 or 600 dpi would
> look too crude, so what to do?

Well, crude how? And, again, electronic images do not have canned
print size dimensions.

> Well, you could simply change the resolution to 150 ppi, which would give
> decent results when printed, but the file size would double and you would
> still have to reduce the image's size in Quark or InDesign so it would fit
> on a page.

Well, not really. You can change the ppi values all you want and the
image file size will not change. Why? Because this is a number that is
passed to an output device, telling it how to render the graphical
info in the image file. You're dealing with numeric info in the image
file's metadata.

> A better answer would be to reduce the image's print size while at the
> same time increasing its resolution. That way the file size remains the
> same while the resolution is increased to print quality. So we'd reduce
> the print size by 50% to 3.5 W x 2.1875 H and double the resolution to 150
> ppi (it should be 144, but 150's close enough and works better with
> printers).

This makes no sense. Most printers don't care about the ppi you set
unless it's outside their output range (in which case it'll do the
best it can and leave it at that). Again, print size and resolution
are joined at the hip, and only the resolution matters, as print size
is arbitrary based on resolution.

> Think about it, when you look at a 72-ppi image, there's no more
> information to be had, you're seeing the image at its maximum clarity.
> It's like the graphics in a comic book, once you've magnified the image to
> to where only the individual dots that make up the image are visible,
> there's nothing more to see.

Kinda sorta. A screen shot will always be the same quality unless you
physically resize the image to alter the total number of pixels in the
image. Increasing ppi doesn't add pixels to the image, so it's safe to
alter the ppi value. It's a scale thing. But if you take a 300x200
pixel image and make it bigger by scaling it to 600x400 pixels, well,
you just added 300% more pixels to the image file. As a result, you
will get distortion in the image, because now for every one original
pixel, you have either 4 pixels or 9 pixels (depending on the process
used to resize) representing each one original pixel's color info.

> But just as that comic book graphic becomes clearer as you reduce the
> magnification (shrinking the image, in effect) if you reduce the size of a
> screencap by 50% while doubling the resolution, you've also doubled the
> amount of information being displayed, but it's in a smaller space. The
> benefits in doing it this way are that all screen captures will be sized
> to the same proportions for consistency, and if you view the reduced
> images online, they'll still be very readable.

No, you almost had it. Your magnification analogy was dead-on, as that
is what setting a ppi value does. It sets a magnification level for
the file when prepped for output. That is, your 72 ppi image saved to
144ppi tells the output device to output it smaller it would at 72
ppi. But you can take the very same image and apply a bunch of ppi
values to it. The integrity of the image won't change, but the size of
the output image will. And DO NOT muck with size reduction, as this
is lossy no matter what process you use... By doing this you are
throwing image information away, and there is absolutely no lossless
way of doing that.

> You can get the same effect by importing the 72-ppi image into Quark or
> InDesign and sizing it smaller, but pre-flight checks will usually choke
> on such images because they show up as 72 ppi even though by reducing
> their size they'll probably be printing at 150 dpi or higher.

Well now you're talking about scaling within a DTP tool, which is in
fact altering the ppi of the output image. It has nothing to do with
the resizing you were speaking of above.

Bill Swallow
HATT List Owner
WWP-Users List Owner


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