Convert a raster image into a polygon using QGis (or another method)

I want to convert several images into polygon shp files using QGis (Quantum GIS 1.6).

I need to do edge detection AND differentiate between several different colors of lines (red, green, yellow and black). I need good edge detection as my images are scanned in at 200 DPI.

I'm open to other suggestions t开发者_StackOverflow中文版hat don't involve QGis. Could I use Photoshop or would Arcgis do a better job of this?

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Inkscape has a rather functional vectorizer (Trace Bitmap, IIRC)

http://inkscape.org/doc/tracing/tutorial-tracing.html

Inskscape's native format is SVG (fully vector based). It allows simplifcation of the resulting paths as well. Also, you might use the resulting XML to process automatically.