need help with output extacted edges

[Ning Zhong]

Hi, Grass Users,

I have a gif image file depicting some solid regions. I want to get the
boundary edges of the regions and output the file with only boundary edges
as an image file(tiff). I used r.poly, and then v.to.rast. But the result
is still an image with solid regions. Could anyone inform how to get
an raster image file with only boundaries?
Thanks in advance,

Ning