images are divided into columns and rows, called a resolution.
This division defines a large number of cells, called pixels,
short for "picture element." Pixels are the smallest image-forming
unit of a video display.
To better understand what pixels are and
how they affect images, lets examine the digitalization
of the picture above.
In order to digitalize the picture above,
we must first divide it into a set of columns and rows (these
columns and rows define the number of pixels used to represent
the image, COLUMNS x ROWS = PIXELS). If, for instance, we
decided to divide the above picture into 4 columns and 2
rows we would end up with a 4 x 2 resolution containing
8 pixels, 4 columns x 2 rows = 8 pixels (pictured below).
that a great deal of information gets lost when we convert
our original picture into a 4 x 2 digital image. Indeed,
the two pictures hardly look the same. But because a pixel
can only contain one color, a lot of information gets lost
in the digitalization process if we only use 8 pixels. To
reduce this loss, we will need to increase the resolution
(the number of columns and rows dividing the image) which,
as you can see, increases the number of pixels.
But before we continue, I should first
point out something commonly misunderstood about pixels.
If you examine the picture above you will notice that the
pixels are NOT square, they are rectangular. A common misconception
is that pixels have to be square. But as we will see later,
many arcade games use rectangular pixels.
3 x 4 digital copy of our original is hardly better than
our 4 x 2 copy, but notice here that the pixels are wider
than they are tall, unlike the 4 x 2 copy where they are
taller than they are wide.
From the above image, it's plain to see
that a 3 x 4 resolution is still not big enough to effectively
capture our original image.
above is a 4 x 3 digital copy of our original. Notice that
it differs from the 3x4 image above it, even though both
resolutions contain the same number of pixels, 12. As we
will see later, the resolution gives us much more information
than just the total number of pixels used. For example,
a 288 x 224 resolution is not the same as a 224 x 288 resolution
even though both resolutions have the same number of pixels,
224 x 288 = 288 x 224 = 64512.
Also notice that the pixels are square
in the above image. Windows by default uses square pixels,
as does Linux. Square pixels house several important advantages
over rectangular ones, the most important being compatibility.
The fact that arcade game developers couldn't agree on a
pixel shape, will make perfect emulation very difficult
in MAME. Indeed, it's the source of all our woes.
above is an 8 x 6 digital copy of our original. Notice here
that the pixels are square as well. This is not an accident.
In fact it's very easy to break an image up into perfectly
square pixels, if you know the image's aspect ratio.
An images aspect ratio tells you how much
wider an image is than it is tall. If an image is square,
then it has an aspect ratio of 1:1. This means that if you
divide it into an equal number of columns and rows you will
get square pixels.
TVs, PC monitors, and arcade monitors
are all wider than they are tall, they do not have an aspect
ratio of 1:1. How much wider are they? I suggest you find
out for yourself. Measure your monitor, its width and its
height. With these two numbers you can easily calculate
your monitor's aspect ratio (note, you will need your monitor's
aspect ratio later, so I suggest you get these numbers now).
we have a perfect digital duplicate of our original. By
using a resolution of 16 x 12 we manage to capture our original
image with 192 square pixels. While normally 192 pixels
couldn't capture a business card, much less a sophisticated
photograph, our image is very simple and compresses well.
We took an image from the analog world that had an infinite
number of pixels, and digitalized it into 192 pixels --
that's not too shabby. But what's important to note here,
isn't how well the image compresses, but how images are
represented by pixels. You should now understand that pixels
(1) only display one color (2) can be square or rectangular
and (3) are the product of a resolution's columns and rows.
Additionally, you should know that you can determine the
shape of a pixel given an image's aspect ratio.
Now that we have a basic understanding
of how the digital world works with images, we need to further
explore how the analog world works with images.