Generalizing the Hough transform to detect arbitrary shapes

-The Hough transform is a method for "~.~tecting curves by exploiting the duality between points on a curve and parameters of that curve. The initial work showed how to detect both analytic curves (1'2) and non-analytic curves, (3) but these methods were restricted to binary edge images. This work was generalized to the detection of some analytic curves in grey level images, specifically lines, 14) circles 15) and parabolas/6) The line detection case is the best known of these and has been ingeniously exploited in several applications/7'89) We show how the boundaries of an arbitrary non-analytic shape can be used to construct a mapping between image space and Hough transform space. Such a mapping can be exploited to detect instances of that particular shape in an image. Furthermore, variations in the shape such as rotations, scale changes or figureground reversals correspond to straightforward transformations of this mapping. However, the most remarkable property is that such mappings can be composed to build mappings for complex shapes from the mappings of simpler component shapes. This makes the generalized Hough transform a kind of universal transform which can be used to find arbitrarily complex shapes. Image processing Parallel algorithms Hough transform Shape recognition Pattern recognition 1. I N T R O D U C T I O N In an image, the pertinent information about an object is very often contained in the shape of its boundary. Some appreciation of the importance of these boundary shapes in human vision can be gained from experiments performed on the human visual system, which have shown that crude encodings of the boundaries are often sufficient for object recognition (1°) and that the image may be initially encoded as an 'edge image', i.e. an image of local intensity or color gradients. Marr (~ 1) has termed this edge image a 'primal sketch' and suggested that this may be a necessary first step in image processing. We describe a very general algorithm for detecting objects of a specified shape from an image that has been transformed into such an edge representation. In that representation, sample points in the image no longer contain grey level information, but instead each sample point contains a magnitude and direction representing the severity and orientation of the local grey level change. Operators that transform the image in such a way are known as edge operators, and many such operators are available, all based on different models of the local grey level changes. Two of the most used are the gradient operator (for example, see Prewitt ~12)) and the Hueckel operator/t3J which model local grey level changes as a ramp and a step respectively. * The research described in this report was supported in part by NIH Grant R23-HL-2153-01 and in part by the Alfred P. Sloan Foundation Grant 78-4-15. Our generalized Hough algorithm uses edge information to define a mapping from the orientation of an edge point to a reference point of the shape. The reference point may be thought of as the origin of a local co-ordinate system for the shape. Then there is an easy way of computing a measure which rates how well points in the image are likely to be origins of the specified shape. Figure 1 shows a few graphic examples of the information used by the generalized Hough transform. Lines indicate gradient directions. A feature of the transform is that it will work even when the boundary is disconnected due to noise or occlusions. This is generally not true for other strategies which track edge segments. The original algorithm by Hough (2) did not use