Anti-alias is the expression used for when extra pixels are used to soften the
angular edges of a line, the result being a sharper looking edge. In many
software packages this process is done automatically by a filter within the
program (this is why text looks so sharp in word processing programs).
The process is actually quite simple. Create a color that is a blend between
the line color and the background color and use that color to soften some
or all of the steps of a line.
To give additional sharpness to the line you may choose to use a second
color to blend between the midtone and the background and use it for more
than one pixel length.
Just be careful not to overuse anti-alias, because too much will cause your
picture to look blurry. A little bit can go a long way and it is fairly easy to go
overboard. One thing that can help tremendously is to zoom out from the
picture occasionally to see how the image looks from far away.
Anti-aliasing shown on the right
shows how a bit of anti-alias added to the lines adds a good
deal of crispness to appearance of the lines. The image on the left is the original image of a skull drawn with two colors. After adding as few transition colors between the black and the other colors, the line has a much more natural and organic feel.
So, when working with lines there are a number of guidelines that help
to give desirable results. Avoid the lone wolf pixel, beware the circle tool, be mindful of pixel width, and get comfy with the cut and paste features and
you’ll do just fine. Use anti-alias when needed (but don’t overdo it) and try
to gradually use longer lines when you’re making curves.