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Image Scissors

	Today’s Readings
		Intelligent Scissors, Mortensen et. al, SIGGRAPH 1995

Extracting objects

	How could this be done?
		hard to do manually
		hard to do automatically (“image segmentation”)
		easy to do semi-automatically

Intelligent Scissors (demo)

Intelligent Scissors

	Approach answers a basic question
		Q:  how to find a path from seed to mouse that follows object boundary as closely as possible?
		A:  define a path that stays as close as possible to edges

Intelligent Scissors

	Basic Idea
		Define edge score for each pixel
			edge pixels have low cost
		Find lowest cost path from seed to mouse

Intelligent Scissors

	Basic Idea
		Define edge score for each pixel
			edge pixels have low cost
		Find lowest cost path from seed to mouse

Path Search (basic idea)

	Graph Search Algorithm
		Computes minimum cost path from seed to all other pixels

Path Search (basic idea)

	Graph Search Algorithm
		Computes minimum cost path from seed to all other pixels

Path Search (basic idea)

Let’s look at this more closely

	Treat the image as a graph

Defining the costs

Defining the costs

Defining the costs

	c can be computed using a cross-correlation filter
		assume it is centered at p
	A couple more modifications
		Scale the filter response by length of link c.  Why?
		Make c positive
			Set c = (max-|filter response|*length)
			where max = maximum |filter response|*length over all pixels in the image

Defining the costs

Dijkstra’s shortest path algorithm

Dijkstra’s shortest path algorithm

Dijkstra’s shortest path algorithm

Dijkstra’s shortest path algorithm

Dijkstra’s shortest path algorithm

	Properties
		It computes the minimum cost path from the seed to every node in the graph.  This set of minimum paths is represented as a tree
		Running time, with N pixels:
			O(N2) time if you use an active list
			O(N log N) if you use an active priority queue (heap)
			takes fraction of a second for a typical (640x480) image
		Once this tree is computed once, we can extract the optimal path from any point to the seed in O(N) time.
			it runs in real time as the mouse moves
		What happens when the user specifies a new seed?

Results

Help session—Ryan