Please add ability to extract alpha channel

[stevenvachon]

I'm currently stuck using https://github.com/aheckmann/gm for this. I wish I knew C++ so that I could help.

[elad]

This shouldn't be very difficult to add. Could you please show me the command line you'd use to achieve what you want?

Implementation pointers:

• http://stackoverflow.com/a/18918281
• http://www.imagemagick.org/api/statistic.php#GetImageChannelStatistics