subr pixelmaskarea(pw, window, in, nx, ny)
 ;pw is the pen width to use for the drawn mask
 ;returns indices of area masked
 pencolor,'red'
 ;ty,'to use: put cursor on start position, click quickly once, and move'
 ;ty,'mouse with button UP (this is easier for detailed work).'
 ;ty,'Click again to stop.'
 ;ty,'Do NOT hold button down or it will just give you a dot.'
 xdraw, pw, window
 ;drawn, read back and figure mask
 xq=tvread(window)
 nx = dimen(xq,0)	ny = dimen(xq,1)
 in=sieve(xq eq iq)
 ;now remark the area in green so it doesn't interfere with next area
 xq(in)=!green_pixel
 tv,xq
 endsubr
 ;============================================================================