Digital 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).

Notice 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.

This 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.

Pictured 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.

Pictured 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).

Alas, 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.