-Normally, the output image will be made to be the size of the screen.
-This lets you specify the desired size.
+Normally, the output image will be made to be the size of the
+screen (or target window.) This lets you specify the desired size.
+.TP 8
+.B \-opacity \fIratio\fP
+How transparently to paste the images together, with 0.0 meaning
+"completely transparent" and 1.0 meaning "opaque." Default 0.85.
+A value of around 0.3 will produce an interestingly blurry image
+after a while.