Let's discuss purely technical aspects of icon design
1. Lines, slopes
One of the most important parts of the composition is the outline of the object, i.e. rough line, which limits the object from the background. When designing large-scale graphics, the designer rarely thinks about defining objects with additional outlines. This is not needed because of the scale: even low contrast objects do not mix into a single whole. Icon graphics is other story. Two variants are available: either foreground and background colors have to be from opposite sides of the color wheel, or the foreground must be separated from the background by visible outlines and shadows. I would like to discuss lines deeper.
If we think more to the large scale images, we can note that in order to define edges, we can use all (including the most complex) angles, Bezier curves and borders. Anyway, the line will look ideal thanks to anti-aliasing. When shrinking the image scale to the small icon size, situation changes greatly. When each particular pixel is equally important and can change the overall appearance of the icon, anti-aliasing is just not applicable. It means that you should consider the possible line angles.
The slope you choose for the line, specifies the step of this line. This means that each line is made of basic elements, the union of which determines its accuracy and visual appeal.
For instance, a 18-degree slope contains many small 3-pixel lines connected using a 1-pixel downshift:
This is how almost every line looks like. Primary element followed by joining followed by another primary element. Unfortunately, not each angle creates lines that look look attractive and not messy.
For example, here are 25 and 20-degree slopes:
It can be easily seen which line is more appealing. The 25-degree slope produses a line consisting of even 2-pixel basic elements. The 20-degree angle has a basic element consisting of three lines: 3-pixel, 3-pixel and 2-pixel. The slope is more complex, therefore, when not supported by additional visual effects, it brings a feeling of untidiness: the basic elements have contrast which is too obvious to escape the human eye.
Here we can see the first conclusion. The most "appropriate" angles must make as plain basic elements as possible. Thus, the ideal slopes are 0 and 90 degrees. In them, 1-pixel primary elements are combined without any shift and form smooth lines. Less perfect are slopes which form primary elements of 1, 2, 3 pixel with 1-pixel downshift. Of course, the lines can not be smooth, but the human brain will process the picture and provide to the user what you intended to show to him. Also, the following 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 move to another issue. In the last paragraph, I intentionally defined slopes with basic elements of 1, 2 and 3 pixels as acceptable. Such small lines, especially when a great amount of them is combined, appear as a single whole. But what do we see if we change the primary elements to 10 or 20 pixels?
Here is an example:
It can be clearly seen that the line doesn't have integrity. It is not a single line now; it is a combination of several lines, situated near one another. Rarely any artist wants its composition to look inconsistent. Thus, we come to the second rule: if you use small slopes, which make long basic elements, it must be reasoned and used with great caution.
And, finally, the last aspect I have to mention about lines. I already wrote that the primary element has to be as simple as possible.
For example, a 25-degree slope can be drawn in the two (of course more of them exists) following ways:
In the first case the primary element is a 2-pixel line. In the other picture, it consists of three elements: 3-pixel, 2-pixel and 1-pixel. The same angle but the image is different. The complicated basic element produced the line untidy.
Such examples can be produced for almost any angle, so in your projects, try to simplify everything.
It can be said without an overstatement that the color is the leading aspect of your work; not suiting or poorly matched colors can kill even the most creative idea. What can we say about the color in icon graphics?
First, there is a certain rule: if the image looks poor when converted to monochrome, it means the colors that were used are wrong. This rule is valid not only for icons but for the whole graphic design.
Second, you will be unable to use common RGB. Sure, you may select GIF colors only, but this ties you hand and foot. Gradients, shadows, dithering and many other tricks will not be available to you.
Third, try to enlarge the areas of "flat" color. The more consistently colored areas without diffusion and jumps your picture has, the more neat it appears. Icons are too small and the over usage of special effects makes the image look messy.
Fourth, gradients. This is a great tool which can change the appearance of any composition. Besides esthetics, gradients are a perfect way to abolish the "broken line" look (lines with too long primary elements). But be attentive when applying them since too many gradients can kill the plain color, and be unable to fit into the gif palette.
Fifth, shadows and flares. All rules for applying shades and highlights in icon creation are totally the same to the rules of the overall graphic design. The only thing I would like to mention: draw everything yourself. Don't use effects, make all shadows and highlights in a separate layer and after that edit its transparency. When using filters, you rarely can predict what will it look like. It is bad when you don't control the making of a project. It is awful if you can't control it when drawing miniatures.
And, finally, sixth. Tinting is the main aspect of pictogram design, which largely affects the visual look of the project. If the green human eye in the image is sized 1 pixel, increase the color intensiveness. For example, instead of 0, 131, 159 make 71, 195, 242. It is indifferent that the human eye can't have this color; in the first case it will look like a gray dot, but in the second one it will definitely be a green eye.
Thoroughly control the image, do not let the visual tricks to spoil your work. It is OK if objects is not perfect from the technical point of view, but the whole composition must be perfect.
As an example, I drew the packet of juice standing before me. The image has plain color, gradients with broken lines, shades and flares and nuances.
If I wrote about something other than icon design, this chapter wouldn't fit into an entire book, but the resolution of pictures ;limits fonts too.
The size of letters is the leading issue as opposed to their beauty. Nearly in any case, unless the letters are the primary part of the composition, the text size has to be decreased to the largest extent possible.
In principle, virtually any design studio has its unique font with little letters. Such a font can be created in a couple of hours. You can search the web and collect your own collection of very interesting fonts. First of all, I would recommend you to get the DS Pixel and Seventen 7Vedi website and the complete series of fonts from the Lakmus Lab website.
If you don't want a new font, small typesizes of Arial and Verdana will do. As a last resort, you can draw the needed characters by hand.
There are not many rules there. First, characters 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 unrecognizable set of pixels.
However, there are no rules without exceptions and the 5-point Arial appears perfectly understandable.