One of the most effective methods of creating a technical illustration is the photo-tracing technique.

In order to create a line-art representation from a photo, you take a photo of the object or situation and use it as a template in your graphics program. Most graphics programs offer a background layer on which you can place your photo. A block function allows you to lock the layer in order to prevent accidental selection, moving or deletion of the photo.

You can then trace the photo on a second layer. This means that you trace all outer lines with the elements that are offered by your graphics program. The graphic shows you how the elements are allocated on layers A and B.

The resolution (dpi) of the photo is an important aspect: If you choose a resolution that is too high, your graphics program will process your commands rather slowly. If your resolution is too low, you will not be able to recognize the details anymore. A resolution between 180 and 250 dpi has been proven to be reasonable. Also, the color information of the original photo is memory-consumptive. Convert color photos into grayscale and you will notice an increase in the performance of your graphics program!

When tracing a photo in your graphics program, there are some more things to keep in mind:

Draw only as few details as necessary
When tracing a photo, mind the display size of the future illustration. It does not make sense to trace every little detail if it will not be visible in the end.

Mind the correct position of the elements
Very often, misplaced ellipses can be found in technical illustrations. Only one of these ellipses is sufficient to destroy the overall impression of the illustration. An ellipse has a so-called "thrust-axis," which runs along the imaginary drilling direction. The small diameter of the ellipse lies on the thrust-axis, resp. in a right angle to the big diameter of the ellipse:

Photos do not have a parallel perspective
All elements in a photo follow the rules of the vanishing point perspective. This means, that it is not possible to apply already drawn elements in another part of the same illustration.