问题
Good morning/afternoon/evening.
So our data structures course gave us an assignment to segment a grayscale image in java using the following algorithm:
Input: A gray-scale image with P pixels and number R
Output: An image segmented into R regions
1. Map the image onto a primal weighted graph.
2. Find an MST of the graph.
3. Cut the MST at the R – 1 most costly edges.
4. Assign the average tree vertex weight to each vertex in each tree in the forest
5. Map the partition onto a segmentation image
The thing is, they just threw us in the dark. They gave us the jgraph package which we had absolutely no experience with (we never studied it) practically saying "go teach yourselves". Nothing new there.
The way I'm going about doing this is by making a class for vertix objects that contains the coordinates of the pixel in addition to its value so that I can add each one both to the graph and a 2D array. Afterwards, I used the array to add weighted edges between adjacent vertices because java can't tell where in the graph a vertix actually is without edges.
Afterwards, I used Kruskal's packaged method for minimum spanning trees and an arraylist to get around the protected status of edge weights in the tree like so:
ArrayList<WeightedEdge> edgeList = new ArrayList<>(height*width*3);
KruskalMinimumSpanningTree mst4 = new KruskalMinimumSpanningTree(map4);
Set<DefaultWeightedEdge> edges = mst4.getSpanningTree().getEdges();
for (DefaultWeightedEdge edge : edges) {
edgeList.add(new WeightedEdge(edge, map4.getEdgeWeight(edge)));
}
edgeList.sort(null);
for (int i = 0; i < n; i++) {
map4.removeEdge(edgeList.get(edgeList.size()-1).getEdge());
}
So now that I cut the (R-1) most costly edges in the graph, I should be left with a forest. And that's where I hit another dead end. How do I get the program to traverse each tree? The way I'm understanding this, I need a general traversal algorithm to visit every tree and assign the average value to each vertix. The problem? There isn't a general traversal algorithm in the package. And there isn't a way to identify individual trees either.
The idea is easy to understand and implement on paper, really. The problems only lie in actually coding all of this in java.
Sorry if this was messy or too long, but I'm just at my wit's end and physical limits. Thank you in advance.
回答1:
I am a big fan of JGraphT and honestly I think it is pretty good that you're given it for your task assignment. It takes a bit of time to get started, but then it proves to be a very good tool. But you also need to understand the CS behind implemented algorithms, using JGraphT without knowing the theory is somewhat difficult.
From your task assignment I don't really understand step 1 (building the primal weighted graph). The rest should work with JGraphT quite well.
You did step 2 with KruskalMinimumSpanningTree. Now you can sort the edges by weight and remove R-1 top edges from the graph.
I would, however, suggest that you first build a new graph which would represent the calculated MST. And then remove remove R-1 top edges from that graph. Effectively truning it into a forest.
How do I get the program to traverse each tree?
With the forest from the previous step you can use the ConnectivityInspector to get a list of sets of connected vertices. Each set will contain vertices from one of the trees of the forest. Sets of vertices are easy to work with, you don't need any traversal, just iterate over the set.
来源：https://stackoverflow.com/questions/50009602/jgraph-general-traversal-forest-traversal