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Image Preparation and Enhancement
by Jennifer Alspach, Shari Nakano, and Steve Samson
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 This
book extract from Adobe
Creative Suite 2 Workflow is an Indezine exclusive with
permission from O'Reilly Media, Inc.
Jennifer Alspach, Shari Nakano, and Steve Samson--the
authors of the book have several decades of combined experience
in using graphic design technologies.
The excerpts chosen to be published on Indezine explain
image and pixel basics.
I wish to thank Betsy Waliszewski , Regina
Wilkinson and Craig Palmer
for facilitating the permission to extract.
ISBN: 0-596-10236-4 |
Pixels Up Close
Transparent Pixels
Continued on Page 2...
Exploring Image and Pixel Basics
Images are made of pixels. If you rolled your eyes when reading
that last sentence, skip ahead to the “Sizing Images”
section. If not, read on: the following section describes what pixels
are and how they form images.
Pixels Up Close
Pixels are individual squares of color—any color—on a
grid of thousands of squares that, together, form an image. Nowadays,
even the smallest files can contain one or two million pixels. Digital
camera manufacturers use the term “megapixels” (MP), where
one megapixel equals about a million pixels.
Although one or two million pixels may seem like a lot, the resulting
image isn’t all that large. A typical 2-MP image is 1600 x
1200 pixels, or about the size of a 21-inch monitor. An 8-MP image
is about 3500 x 2300 pixels, or about twice the width and height
of a 2-MP image.
The interesting thing about pixels and megapixels is that the relative
density of pixels is what creates more or less detail in an image.
Look at the image shown in Figure 3-1, where one screen pixel is
equal to one image pixel. If you look really, really closely, you
can see the pixels.
Figure 3-1
Now look at the image shown in Figure 3-2, where four screen pixels
are equal to one image pixel. No need to look closely here; the
pixels are quite easy to spot.
Figure 3-2
Back
Transparent Pixels
In Photoshop, in addition to being a specific color, pixels can
also be partially transparent. If a pixel is transparent, you can
see part or all of the pixel underneath it. If there’s no image
pixel behind the transparent pixel, Photoshop substitutes white.
Figure 3-3 shows a partially transparent image on top of another
image. This example is offset to the left, so that you can see the
original image for comparison purposes. All of the pixels in the
“top” image are set to 50% transparency (via the Layers
palette). You can easily see through the top image to the image
beneath, although the result is actually not all that pleasing.
Figure 3-3
In this case, a better result might be gained from using transparency
only on the background of the original poker chip image, as shown
in Figure 3-4. Now—depending on your point of view—you
have either giant poker chips or a grass-like tabletop. I like the
giant chips myself.
Figure 3-4
To clarify the size of the poker chips, I’ll add a dog to
the image, as shown in Figure 3-5.
Figure 3-5
Back
Continued on Page 2...
Copyright © 2006 O'Reilly Media, Inc. All Rights
Reserved.
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