Custom narrowband filters for RGB stacks

[LTDakin]

Allow for users to designate which files will be in which channels R,G,B so that we can create rgb images from narrow bands.

Currently the UI does a check for the Red, Green, and Blue inputs that only allows users to select inputs that have the corresponding filter. Simply removing this check will allow any filter to be selected for the R, G, and B stacking channels which allows for the use of non color filters as well as more freedom (with the chance to make really weird looking images)

• remove code that sorts inputs by filter
• update dictionary to include additional color defaults

[zemogle]

At present we don't need to add a 4th image into the stack. We need to be able to make an RGB from narrow band images.

Ideally the user would have control over which filter image went in the R, V and B slots. The fits2image library assigns the colours RGB to images 0, 1 and 2 in the stack. The thumbnail services designed for automatic images.

Adding a UI component to specify the order of images in the stack would get us most of the way with this one. At present the operation needs the correct filters. I would like to request that this feature is modified so that any 3 images can be added and the user must assign each image to a channel. Maybe DataLab could guess the desired assignment from the filters.

Incidentally, I have already written code to automatically assign the RVB and narrowband (or a mixture) to a sensible order in the RGB stack, for the thumbnail service. It's in a PR on OCS that's been forgotten about ...

The function is called rvb_frames.

[LTDakin]

I updated the description with your recommendations but left out the idea for desired assignments since that idea could be approached/thought about for all operations.

[jnation3406]

@zemogle Do they need to be able to assign any filter to any channel, or do you just need support for narrow band filters with the mapping you had in that fits2image PR?:

 FILTERS = {
        'red': ['r', 'rp','ip','h-alpha'],
        'visual': ['v','oiii'],
        'blue': ['b','gp','sii'],
 }

Update: allow any image to be choosen for each channel, but sort the images by the above filters first for each channel to make it simpler for the normal use case.

[zemogle]

I think that dict is a good guide for defaults but the user should be free to assign any filter to any colour.