Let's consider strictly technical aspects of icon design
1. Lines, slopes
One of the vital elements of the composition is the framework of the object, i.e. rough linear marking, which limits the object from the rest of the image. When working with normal-sized images, the artist occasionally thinks about defining objects with additional lines. This is unnecessary because of the size: even non-contrasted objects do not blend into a one. Pixel graphics is different. Two solutions are available: either foreground and background colors have to be from different sides of the color wheel, or the object must be divided from the background by contrast lines and shadows. I would like to discuss lines in more detail.
If we think again to the big-size graphics, we see that in order to highlight contours, we can use any (including the most complicated) angles, Bezier splines and edges. Anyway, the line will look perfect thanks to anti-aliasing. When decreasing the graphic dimensions to the small icon size, situation changes dramatically. When every particular pixel is vitally important and can ruin the whole appearance of the icon, anti-aliasing is simply not applicable. It comes that you have to consider the possible line angles.
The slope you select for the line, determines the step of this line. Because every line is made of primary elements, the combination of which determines its neatness and visual appeal.
For instance, a 18-degree slope contains many tiny 3-pixel lines connected using a 1-pixel downshift:
This is how nearly each line looks like. Basic element with joining and another primary element. Unfortunately, not each angle makes the line look neet and not annoying.
For example, here are 25 and 20-degree slopes:
It can be easily known which line is more appealing. The 25-degree angle produses a line containing even 2-pixel primary elements. The 20-degree angle has a basic element containing of three lines: 3-pixel, 3-pixel and 2-pixel. The angle is more complicated, thus, when not supported by other visual effects, it creates a feeling of messiness: the basic elements have contrast which is too strong to hide from the human eye.
Now we can see the first conclusion. The most "appropriate" angles have to make as plain basic elements as they can. therefore, the ideal slopes are 0 and 90 degrees. In those, 1-pixel basic elements are joined without downshift and produce smooth lines. Less ideal are angles which form primary elements of 1, 2, 3 pixel with 1-pixel downshift. Of course, the lines can not be smooth, but the human sight will process the image and provide to the viewer what you intended to show to him. Also, the mentioned slopes can be considered correct (but at a stretch): slopes with basic elements consisting of different lines shifted by more than 1 pixel.
It is the time to touch another problem. In the last paragraph, I intentionally defined slopes with primary elements of 1, 2 and 3 pixels as correct. Such small lines, especially when a great amount of them is combined, appear as a single whole. But what happens if we increase the basic elements to 10 or 20 pixels?
Here is an example:
It can be obviously noticed that the line doesn't have integrity. It is not a single line now; it is a set of different lines, located near each other. Hardly any artist wants its creation to look that way. So, we have the second conclusion: if you use minimal slopes, which make long basic elements, it has to be reasoned and used with maximum caution.
And, finally, the last aspect I wanted to mention about lines. I did write that the primary element should be as plain as it can.
For instance, a 25-degree slope can be drawn in the two (sure more of them exists) following ways:
In the first case the basic element is a 2-pixel line. In the second case, it consists of three lines: 3-pixel, 2-pixel and 1-pixel. The same slope but the image is different. The complex basic element produced the line messy.
Such examples can be produced for almost any angle, so in your projects, make sure to simplify everything.
It can be said without an overstatement that the color is the leading aspect of your work; improper or badly matched colors can kill even the best idea. What can we write about the color in miniature graphics?
First, there is a clear rule: if the picture looks bad when reduced to monochrome, then the colors that were chosen are incorrect. This rule is valid not only for icons but for the entire graphic design.
Second, you will be unable to apply common RGB. Sure, you may select GIF colors only, but this ties you hand and foot. Gradients, shadows, dithering and many other effects will not be available to you.
Third, try to enlarge the areas of "plain" color. The more evenly colored parts without blurring and jumps your composition has, the more clear it looks. Pictograms are too small and the over usage of special effects makes the picture look dirty.
Fourth, gradients. This is a wonderful instrument which can change the look of any composition. Besides visuals, gradients are an ideal way to get rid of the "broken line" look (lines with too long basic elements). But be attentive when applying them since too much gradients can ruin the flat color, and be unable to fit into the gif palette.
Fifth, shadows and highlights. All aspects of using shadows and highlights in icon creation are completely identical to the aspects of the general graphic design. The only thing I would like to mention: create everything by hand. Don't use effects, make all shades and highlights in a separate layer and after that edit the opacity. When using effects, you rarely know what the result will be. It is bad when you can't control the process of creating of a project. It is awful if you don't control it if drawing icons.
And, lastly, sixth. Nuancing is the key aspect of pictogram design, which largely results the esthetic look of the image. When the green human eye in the icon has the size of 1 pixel, increase the color intensiveness. For example, instead of 0, 131, 159 choose 71, 195, 242. It is indifferent that the human eye never has this color; in the first example it will look like a gray dot, and in the second one it will definitely be a blue eye.
completely control the picture, do not let the optical tricks to spoil your work. It is acceptable if objects is not perfect from the technical point of view, but the whole image must be flawless.
As an example, I drew the pack of juice located in front of me. The picture has flat color, gradients with broken lines, shades and flares and tints.
If I talked about something other than icon design, this part wouldn't fit into a whole book, but the resolution of compositions restricts fonts too.
The size of symbols becomes the main issue as opposed to their beauty. Almost in any case, only if not the letters are the main part of the image, the font size has to be reduced to the greatest extent possible.
In general, virtually any design company has its exclusive font with tiny letters. This font can be crafted in several hours. You can browse the web and make your own library of very nice fonts. First of all, I advice you to get the DS Pixel and Seventen 7Vedi website as well as the complete set of fonts from the Lakmus Lab website.
If you don't need a new font, tiny sizes of Arial and Verdana may work. As a last resort, you can create the necessary letters yourself.
There are not many rules there. First, letters cannot be less than 5 pixels high or less than 3 pixels wide.
Second, letters cannot use anti-aliasing. It transforms the font into an incomprehensible set of pixels.
However, there are no rules without exceptions and the 5-point Arial appears rather readable.