The AlphaChannelTypes that alpha
wants are constants within Magick
so you want to use Magick::ActivateAlphaChannel
, Magick::BackgroundAlphaChannel
, ...:
img.alpha(Magick::ActivateAlphaChannel)
I don't my way around CarrierWave but I'd guess it just wants to see one of the Magick::
constants somewhere.