I didn't include it in the website, admittedly because I didn't fully understand it, and I wanted to keep the service as simple as possible.
Would it make sense for this to be the default?
[1] https://i.scaley.io/png-gamma-256x128/i.imgur.com/X9vKa2s.pn...
The ImageWorsener's website as an excellent reading material on image resizing and other transformations. ImageWorsener is a "correctness first" image resizing software.
It is possible to optimize the linearization steps so that they don't detract too much from the performance, especially if you're using one of the better (i.e. slower) interpolation methods.
"The Importance of Being Linear"[1] which just came up on HN a few days ago, discusses some issues with non-linearity, specifically in the context of 3D rendering.
[1] - http://http.developer.nvidia.com/GPUGems3/gpugems3_ch24.html
[1] http://blog.johnnovak.net/2016/09/21/what-every-coder-should...
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.