High-level API for backdrops in custom node tree

Archive Python API
1

I am developing a custom node tree, and my intent is to incorporate a ‘Viewer’ node that would behave similarly to the ‘Viewer’ node in the Compositor. It seems, though, that the ‘Backdrop’ image functionality is hidden for all other node trees.

I am aware of the backdrop callback (as discussed here: Drawing a background image in my custom node tree), but a higher-level API would be nice, as this would expose relevant SpaceImageEditor properties (e.g. SpaceImageEditor.backdrop_channels) along with the backdrop operators (e.g. node.backimage_move).

My suspicion is that this would be a simple change, and that somwhere in the code, bl_idname == CompositorNodeTree is the condition under which the backdrop functionality is exposed.

My proposed implementation would be to add a bl_options property to the bpy.types.NodeTree class. When bl_options is set to {"COMPOSITOR"}, the backdrop functionality would be exposed. There would also need to be an API path for getting/setting the backdrop image (e.g. NodeTree.backdrop_image)

Example code:

import bpy
from bpy.types import NodeTree

class MyCustomTree(NodeTree):
    '''A custom node tree type that will show up in the node editor header'''
    bl_idname  = 'CustomTreeType'
    bl_label   = 'Custom Node Tree'
    bl_icon    = 'NODETREE'
    bl_options = {'COMPOSITOR'}

    def update(self):
        active_viewer_node = get_viewer_node()
        update_preview_image(self, active_viewer_node)
        self.backdrop_image = bpy.data.images["Custom Viewer Node"]

def update_preview_image(node_tree, viewer_node):
    ### update bpy.data.images["Custom Viewer Node"] based on data fed into
    ### Color/Alpha inputs of viewer_node
2

I’d expect that the kind of backdrop that is necessary highly depends on the node tree type. I’m not sure if it makes sense to have a more high level API for that. It seems very likely that this would be misused for things that are similar to the backdrop but not quite the same.
For these cases, the backdrop API would probably be a wrong abstraction.

I can’t tell if it would work in your case without more information. Personally, I’d probably try to implement my own solution instead of trying to use the existing backdrops.