In the current implementation, cryptomatte passes are connected to the node
and elements are picked by using the eyedropper tool on a special pick channel.
This design has two disadvantages - both connecting all passes individually
and always having to switch to the picker channel are tedious.
With the new design, the user selects the RenderLayer or Image from which the
Cryptomatte layers are directly loaded (the type of pass is determined by an
enum). This allows the node to automatically detect all relevant passes.
Then, when using the eyedropper tool, the operator looks up the selected
coordinates from the picked Image, Node backdrop or Clip and reads the picked
object directly from the Renderlayer/Image, therefore allowing to pick in any
context (e.g. by clicking on the Combined pass in the Image Viewer).
Technically there is some loss of flexibility because the Cryptomatte pass
inputs can no longer be connected to other nodes, but since any compositing
done on them is likely to break the Cryptomatte system anyways, this isn't
really a concern in practise.
In the future, this would also allow to automatically translate values to names
by looking up the value in the associated metadata of the input, or to get a
better visualization of overlapping areas in the Pick output since we could
blend colors now that the output doesn't have to contain the exact value.