Graphics Programs Reference
In-Depth Information
per pixel. Image files with a bit depth of 8 are very common, 2
8
= 256,
so these files can have 256 shades of gray per pixel.
Research has shown the minimum number of values needed to fool
our eyes into seeing a
continuous tone
image is around 240. So 8 bits
per pixel provides us with a file that appears continuous tone. Black is
encoded as 0 and white is encoded as 255. Photoshop's histogram is one
way to view the entire spread of tonal values from zero to 255 within an
image. Photoshop's eyedropper tool and info palette is a way to view the
actual numeric values of an individual pixel. With 8 bits or 256 different
values, our computers can create a continuous tone Grayscale image. In
order to create a color image, we simply need a file that contains three
of these Grayscale channels or color planes: one for red, one for green,
and one for blue. An RGB file is nothing more than a three-channel
Grayscale file, where each channel contains 256 possible values for each
color as seen in Fig. 1-3. This encoding method is often referred to as
24-
bit color
. A 24-bit file contains three channels or planes with each of these
channel composed of 8 bits of data. 24-bit color files have the potential
to describe 16.7 million colors. If you look at the math involved to
produce this enormous number of colors, you will see that a file com-
posed of three channels (where each channel has 256 possible values)
works out to 256 ¥ 256 ¥ 256, or 16.7 million colors. As we will see, the
ability to mathematically produce 16.7 million colors is a problem
because it isn't possible to reproduce anything close to that number of
actual colors on any output device.
Note that 8-bits commonly is referred to as a
byte
(in case you were
curious, 4 bits is a
nibble
). Therefore, if you have a file that is 100 ¥ 100
pixels and has 8 bits or 1 byte per pixel, that file is 10,000 bytes in size
or about 10 KB (1 kilobyte = 1024 bytes). If this were a color file with 3
bytes per pixel it would be about 30 KB. We normally work with much
bigger images in Photoshop, but the concept is the same. If a digital
camera creates an image 3072 ¥ 2048, that's 6,291,456 pixels times 3
bytes per pixel (RGB) or 18,874,368 bytes. Divide that by 1024 to get
18,432 KB. Divide that by 1024 to get 18 MB (megabytes). Now you know
where all those numbers come from.
Enough math! The concept to understand here is how images are rep-
resented by pixels, and that each pixel has a numeric value based on its
bit depth. Eight-bit Grayscale images are simple; a pixel can be one of
256 possible values. Color files are more complex since the pixel in
question can be one of 256 values per color channel. In the example
illustrated in Fig. 1-4, the color seen is three values: one for red, one for
green, and one for blue. Here we have the ingredients for a single color
pixel. A digital image may be composed of millions of pixels, all with
various combinations of values. Knowing the value of our pixels is one
thing, but what about the color of these pixels? Before that can be dis-
cussed, it is necessary to understand how we define color.
Definition
Continuous Tone:
A
term used to describe an
unbroken, smooth, and
uninterrupted tone making
up an image. Rather than
seeing an actual series of
dots or a mosaic of dots or
pixels, continuous tone
describes how we see the
world before us. A
photographic print is
considered continuous
tone. Few output devices
are truly continuous tone
(some, like a Lightjet or my
Fuji Pictrography, are).
However today, many
devices simulate the effect
of continuous tone even
though they really produce
a solid dot.
