[Arnt]

Averaging two RGB numbers isn't at all similar to how the human eye averages two colours.

The colour experts have a variety of ways to do much better, almost any of them will give you much better results. Pick one and be done. Your undocumented -gamma- option is fine.

If you want the best option you should be prepared to invest a lot of time and attention, in return for which you'll get a result that's perhaps a little better than your -gamma- and a lot better than plain RGB averaging.