Visibility against an unknown background

[Fredrik Lundh]

Sergei Organov wrote:

> There is a better way to do it. Check if the toolkit has line drawing
> mode that XORs the line color with the background, then draw using this
> mode and white color of the line.

which means that the selection looks like crap most of the time, and 
doesn't work at all on medium gray or b/w dithered patterns.

for most cases, "marching ants" is a much better approach (usually done 
by drawing alternating stippled patterns).  see e.g.

     http://en.wikipedia.org/wiki/Marching_ants

for some background.

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