The solution to these problems is to produce patterns in a vector program. They range from Adobe Illustrator (expensive) to Inkscape (free and open source). This is not meant to be a tutorial, but a guide to tracing patterns. First, create a new document in the vector software of your choice the same size as your image. Then insert your image into the document. This will be layer 1. Set the opacity to about 50%. Next make a new layer on top of layer 1. Select the pen or pencil with the stroke width of your choice and trace your pattern. When the design is completed layer 1 can be deleted and the tracing saved as a svg (scaleable vector graphic) file. This format will allow you to increase the size of the drawing without any loss of quality. Once the size has been agreed upon a copy can be exported as a jpeg and printed.
Most vector programs have embedded in them automatic tracing capabilities, but exhibit the same weaknesses as those earlier mentioned. They are useful, however, in producing simple patterns from plain images as shown below. The distortion is inherent in raster images.
Another feature that is useful (and available in most graphic programs) is to paint the elements of the pattern with colors that represent the various species of veneer. This is a way to preview your design in real-time much in the same way as the masters would create a watercolor before executing their marquetry. Tone and contrast are easily tackled in this manner. All that has to be done is to make a color palette that represents the veneers that are available. Be sure to save this palette. Below is a screenshot of a pattern that has had the color added. I use a palette of about 200 colors that represent both natural and dyed veneers. I find it indispensable.
