26-12-2012, 04:44 PM
GENERALIZED HOUGH TRANSFORM
GENERALIZED HOUGH.ppt (Size: 701.5 KB / Downloads: 105)
Recap on classical Hough Transform
In detecting lines
The parameters r and q were found out relative to the origin (0,0)
In detecting circles
The radius and center were found out
In both the cases we have knowledge of the shape
We aim to find out its location and orientation in the image
The idea can be extended to shapes like ellipses, parabolas, etc.
Generalized Hough Transform
The Generalized Hough transform can be used to detect arbitrary shapes
Complete specification of the exact shape of the target object is required
The Shape is specified in the form of the R-Table
Information that can be extracted are
Location
Size
Orientation
Number of occurrences of that particular shape
Creating the R-table for Generalized Hough Transform
Algorithm to create the R-Table
Choose a reference point
Draw a vector from the reference point to an edge point on the boundary
Store the information of the vector against the gradient angle in the R-Table
There may be more than one entry in the R-Table corresponding to a gradient value
Generalized Hough Transform – Size and Orientation
The size and orientation of the shape can be found out by simply manipulating the R-Table
For scaling by factor S multiply the R-Table vectors by S
For rotation by angle q, rotate the vectors in the R-Table by angle q
Spatial decomposition
This technique preserves the localized information
Divide the image recursively into quad-trees, each quad-tree representing a part of the image i.e. a sub-image
The leaf nodes will be voted for feature points which are in the sub-image represented by the leaf node
Spatial Decomposition of Hough Transform
Parameter space is defined from a global origin rather than a local one
Each node contains information about the sub-nodes as well as the number of feature points in the sub-image represented by the node
Pruning of sub-trees is done if the number of the feature points falls below a threshold
An accumulator is assigned for each leaf node