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opencv3

Fast image segmentation using MaxFlow/MinCut Boykov-Kolmogorov algorithm

Opencv grabcut algorithm is pretty good, but slow for large images. One bottleneck is the function GCGraph::MaxFlow, which implements the max-flow/min-cut Boykov-Kolmogorov algorithm. We propose a simple parallel version of this algorithm, providing a significant speedup.

  • We extend the class GCGraph (cf. file gcgraph.hpp) with an overloaded function maxFlow(int r). It computes the max flow corresponding to a subgraph of the initial graph.

  • We implement a function constructGCGraph_slim (cf. grabcut.cpp) building a partially reduced graph. The reduction is based on a paper of Scheuermann and Rosenhahn : https://pdfs.semanticscholar.org/92df/9a469fe878f55cd0ef3d55477a5f787c47ba.pdf

  • We implement a mulithreaded version of the function estimateSegmentation() (cf. file grabcut.cpp), using our overloaded function maxFlow(). Threads run on disjoint subgraphs, corresponding to disjoint subregions of the image, thus no synchronization is needed. The residual graph is updated and the partial flows are added. A second parallel run with slightly shifted regions is performed to process inter-region edges. A last call to maxFlow() on the whole residual graph achieves the segmentation.

The final segmentation is an exact min cut.

Tests with a 24 M pixel image :

 maxFlow()                                      48 s           grabcut()                63 s
 
 parallel maxFlow() (64 regions, 8 threads)     18 s           parallel grabcut()       34 s

History

Most recent branch is workers

Last version is a test-only version

files

dist/sources/modules/imgproc/include/opencv2/imgproc.hpp

dist/sources/modules/imgproc/src/gcgraph.hpp

dist/sources/modules/imgproc/src/grabcut.cpp