Strange that I've never tried this, but I thought it would be pretty simple. I've cookie cuttered images with drawing objects, and I want to do the reverse, i.e. crop a polygon in the drawing (or create a new object, doesn't matter) to the selection in the image. I read the help and tried to follow it but I get nada. See the attached image.
Save the selection as a mask/channel, then use trace areas on the white pixels in the result.
Thanks, I got the mask part but I can't see the Tracing toolbar. It looks like it should already be visible.
From memory, I think you need to have a drawing to trace into in a map with the image.
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AND you have to be on the drawing not the image, I didn't think of that. Doh!