Let's consider purely technical details of icon design
1. Lines, slopes
One of the vital parts of the composition is the outline of the object, i.e. rough border, which limits the object from the rest of the image. When designing normal-sized images, the artist rarely cares about highlighting of objects with additional lines. This is unnecessary because of the scale: even non-contrasted objects do not blur into a single whole. Pixel graphics is different. Two solutions are available: either foreground and background colors have to be from different sides of the color chart, or the foreground must be divided from the background by contrast outlines and shadows. I will discuss lines deeper.
If we think more to the big-size images, we see that in order to highlight contours, we can use any (including the most complex) angles, Bezier splines and borders. Anyway, the line will look ideal thanks to anti-aliasing. When decreasing the image dimensions to the size of an icon, situation changes greatly. When each single pixel is equally valuable and can change the whole look of the icon, anti-aliasing is simply unreasonable. It comes that you have to think about the possible line angles.
The slope you select for the line, determines the step of this line. This means that each line consists of basic elements, the combination of which defines its accuracy and visual appeal.
For example, a 18-degree slope consists of many tiny 3-pixel lines connected using a 1-pixel downshift:
This is how almost each line looks like. Primary element with joining followed by second basic element. However, not every angle makes the line look neet and not annoying.
For example, here are 25 and 20-degree slopes:
It can be easily seen which line is more attractive. The 25-degree slope makes a line consisting of even 2-pixel primary elements. The 20-degree angle has a basic element consisting of three parts: 3-pixel, 3-pixel and 2-pixel. The angle is more complicated, thus, when not combined by other visual effects, it brings a feeling of messiness: the primary elements have contrast which is too strong to hide from the human sight.
Here we can come to the first conclusion. The most "appropriate" angles have to make as simple primary elements as possible. Thus, the ideal angles are 0 and 90 degrees. In those, 1-pixel basic elements are joined without downshift and form even lines. Less perfect are angles which form basic elements of 1, 2, 3 pixel with 1-pixel downshift. Of course, the lines will not be even, but the human brain will process the image and provide to the user what you intended to show to him. Also, the mentioned slopes can be considered correct (but at a stretch): angles with primary elements consisting of unequal lines shifted by more than 1 pixel.
It is the time to discuss another problem. In the previous paragraph, I intentionally defined angles with primary elements of 1, 2 and 3 pixels as acceptable. Such small lines, especially when a great number of them is combined, appear as solid. But what happens if we increase the primary elements to 10 or 20 pixels?
Here is an example:
It can be clearly noticed that the line doesn't have solidness. It is not a continuous line now; it is a set of different lines, located near each other. Rarely any designer wants its composition to look that way. Thus, we have the second conclusion: if you use small slopes, which make long basic elements, it has to be justified and employed with great caution.
And, finally, the last thing I wanted to tell about lines. I did write that the basic element should be as simple as it can.
For example, a 25-degree angle can be produced in the two (sure more of them are possible) following ways:
In the first case the basic element is a 2-pixel line. In the other case, it is made of three elements: 3-pixel, 2-pixel and 1-pixel. The same slope but the look is different. The complex primary element produced the line untidy.
Such examples can be produced for about any angle, so in your projects, make sure to simplify everything.
It can be stated without an exaggeration that the color is the leading element of your work; improper or badly balanced colors can ruin even the best idea. What can we write about the color in miniature graphics?
First, there is a clear rule: if the image looks poor when converted to grayscale, it means the colors that were used are incorrect. This rule is true not only for icons but for the whole graphic design.
Second, you will be unable to apply common RGB. Sure, you can select GIF colors only, but this limits you greatly. Gradients, shadows, dithering and many other effects will not be available to you.
Third, try to increase the areas of "plain" color. The more evenly colored areas without diffusion and jumps your composition has, the more neat it looks. Icons are too small and the over usage of special filters makes the image look messy.
Fourth, gradients. This is a wonderful instrument which can change the look of any picture. Other than visuals, gradients are an ideal way to abolish the "broken line" effect (lines with too long basic elements). But be careful when using them since too much gradients can ruin the plain color, and be not suitable for the gif palette.
Fifth, shadows and highlights. All rules for using shades and highlights in icon making are completely identical to the aspects of the general graphic design. The only ine I have to note: draw everything yourself. Don't apply filters, make all shades and highlights in a different layer and then edit the opacity. When using effects, you rarely can predict what the result will be. It is unfavorable when you can't control the process of creating of a composition. It is worse if you don't control it when drawing miniatures.
And, finally, sixth. Nuancing is the key aspect of icon creation, which greatly affects the esthetic appearance of the project. If the green human eye in the icon has the size of 1 pixel, increase the color saturation. For instance, instead of 0, 131, 159 make 71, 195, 242. It is indifferent that the human eye never has this color; in the first case it will look like a gray dot, and in the second case it will really be a green eye.
Thoroughly control the picture, do not allow the optical illusions to ruin your design. It is OK if objects is not perfect technically, but the entire picture must be flawless.
As an example, I drew the pack of juice located in front of me. The picture has plain color, gradients with broken lines, shadows and highlights and tints.
If I wrote about something bigger than icon creation, this part wouldn't fit into an entire book, but the resolution of compositions restricts fonts too.
The resolution of symbols becomes the main aspect as opposed to their look. Nearly in all cases, only if not the letters are the primary part of the image, the text size must be reduced to the greatest extent possible.
In principle, almost any design studio has its own font with little characters. This font can be made in a few hours. You can browse the web and make your own collection of very nice fonts. Primarily, I advice you to get the DS Pixel and Seventen 7Vedi webpage as well as the complete series of fonts from the Lakmus Lab website.
If you don't want a new font, tiny typesizes of Arial and Verdana will do. As a last option, you can create the necessary letters yourself.
There are quite a few rules there. First, characters cannot be less than 5 pixels high or less than 3 pixels wide.
Second, letters cannot use anti-aliasing. It turns the text into an incomprehensible mixture of pixels.
However, there are no rules without exceptions and the 5-point Arial appears perfectly understandable.