I have run into several effects that return an image with spectrum=2 (instead of spectrum=3 for RGB or spectrum=4 for RGBA images like most other effects).
So this is basically an image with only a red and green channel. How am I supposed to be treating this when being output to RGB? From what I assume while testing with the GMIC CLI and writing to a PNG image, only the first (red) channel will be taken and copied to green and blue, with alpha being set to opaque/full. Can anyone confirm this?
An example of such an effect would be fx_tk_infrared.
Or does spectrum=2 maybe mean one color and one alpha channel?
When using a GUI filter, you need to know the intent of the author and the intended host app (e.g. GIMP). Often, the output can be ambiguous. E.g. RGBA or CMYK? G’MIC can handle any number of spectra or slices. If you end up with 2 channels, assuming it is GA (Grey+Alpha), make it GGGA before saving it as a PNG.
Yes, that makes sense.
In case of the video rendering hosts, I will always have to return a 4 channel RGBA pixel buffer, so I need to define a default mapping. Funnily, I found out I dealt with the same issue 5 years ago already, I just had a bug in my code for spectrum=2, which is why I stumbled across this now. So my default will be that spectrum=2 will be treated as GA and converted to GGGA (i.e. grayscale with alpha), which is also what the output writer in the CLI seems to do.
Of course you are right that any spectrum can occur and different meanings may be associated with the channels, but that is ok, I will just go with the lowest denominator.