Update Tracing an Image

Bernd Kamperequested to merge
kaleru/manuals:tracing-1.1 into master

Change casing of options. Ok button is labelled Apply in 1.1. Update image with Trace Bitmap dialogue. Add description of Autotrace options including an image showing centerline tracing vs. tracing with brightness cutoff.

Images created with Inkscape 1.1.2. Files have been optimised for size. Part of #16 and #48 (closed)

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