Many thanks for the replies.
Getting a nice looking output is proving a real mission.
Quote:
Originally Posted by mark_k
A simple average isn't very good, because the human eye is much more sensitive to green than blue. A good-enough way to calculate the luminance is 3×red + 6×green + 1×blue
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In the end I went for 0.21 R + 0.72 G + 0.07 B, though there are so many different suggestions on the net (should have googled first!) its kind of try it and see.
I suspect I'm either going to force the output background to be white (or as close to white as the user has) or force the percentage background to be white - playing around at present to see which looks best.