After ages of thinking over it, I finally started (again) with the prototype of my magic software project. It is not related to vfx but is heavily influenced by node-based compositing software and uses the dataflow programming approach as the main principle. I use C++ and Qt framework for GUI, because it is very easy to use and has lots of good examples.
I started with implementing some very basic node and edge manipulation where one can move, add and remove nodes and edges. In addition it has simple functions for node graph sorting - in what order must the nodes be executed. Sorting is not based on any certain algorithm (although it most probably matches one). Idea is based on node stack, where sorting starts from bottom (Viewer node). Active node is put to evaluation list (that holds evaluated nodes), gets removed from stack, and all it's inputs get pushed to stack. Topmost node in stack becomes active node, is put to evaluation list, removed from stack, and questioned for inputs. If it has inputs, all of these get pushed to stack and so on. If stack is empty, we have evaluated all nodes. After that we reverse the evaluation list and remove items that are already visited through joining graph branches. It seems to work nicely and logic is easy to understand, and what is more important, it avoids evaluating branches that do not contribute to output.
When creating new edge, it also detects circular dependencies. First it sorts the graph beginning from starting node of new edge and then searches the sorted list for end node. If end node is in that list, it means that there is circular dependency and edge is not created.
I started testing the plugin interface but could not find a good way for creating an arbitrary number of parameter knobs (as specified in plugin) and exchanging the parameters between UI and plugin. I can create widgets that are specified in plugin but don't have a good way to pass parameters back and forth when number and type of parameters is set from inside the plugin.
Little teaser also :)