Auto-focus image system

Abstract

An auto-focus image system that includes a pixel array coupled to a focus signal generator. The pixel array captures an image that has a plurality of edges. The generator generates a focus signal that is a function of a plurality of edge-sharpness measures, each being measured from a different one of the plurality of edges. The edge-sharpness measure is a quantity that has a unit that is a power of a unit of length. It may be a distance in the edge. It may be an area. It may be a central moment. The generator may reduce a relative extent to which an edge contributes to the focus signal on basis of detecting that the edge does not have sufficient reflection symmetry in a sequence of gradients of an image signal across the edge according to a predefined criterion. The edge may be prevented from contributing altogether.

1 . A method for generating a focus signal from a plurality of edges of an image of a scene to indicate a degree of image sharpness, comprising:
 rejecting or de-emphasizing a contribution from an edge towards the focus signal in a computing device depending on a quantity that measures a degree of reflection asymmetry in a sequence of gradients across the edge.   
     
     
         2 . The method of  claim 1 , wherein any edge that makes a contribution towards the focus signal contributes an edge-sharpness measure that is a quantity computed from a plurality of samples of image data within a predetermined neighborhood of said any edge,
 wherein the edge-sharpness measure has a unit that is a power of a unit of length, given that distance between gradients and count of pixels both have a unit of length, gradient value has unit of a unit of energy divided by a unit length and normalized gradient values are unitless.   
     
     
         3 . The method of  claim 2 , wherein the power is neither +1 nor −1. 
     
     
         4 . A method for generating a focus signal from a plurality of edges of an image of a scene to indicate of a degree of image sharpness, comprising:
 computing an edge-sharpness measure for an edge, the edge-sharpness measure being a quantity computed from a plurality of samples of image data within a predetermined neighborhood of the edge; and   computing the focus signal using the edge-sharpness measure,   wherein the edge-sharpness measure has a unit of a power of a unit of length, given that distance between gradients and count of pixels both have a unit of length, gradient value has unit of a unit of energy divided by a unit length and normalized gradient values are unitless.   
     
     
         5 . The method of  claim 2 , wherein said edge-sharpness measure of any edge is a width of a predefined undivided portion of said any edge, said predefined portion being defined in a predetermined manner with respect to said plurality of samples of image data of said any edge. 
     
     
         6 . The method of  claim 5 , wherein said predefined undivided portion is a narrowest undivided portion of said any edge that contributes a predetermined fraction of a contrast across said any edge. 
     
     
         7 . The method of  claim 5 , wherein said predefined undivided portion consists of all pixels of said edge where gradient magnitudes are larger than a predetermined threshold. 
     
     
         8 . The method of  claim 5 , wherein said predefined undivided portion is a widest undivided portion between a positive peak (or interpolated peak) and a negative peak (or interpolated peak) of a sequence of second order derivatives across said edge. 
     
     
         9 . The method of  claim 2 , wherein said edge-sharpness measure of any edge is a peak gradient value of a sequence of gradients across said any edge divided by a contrast across said any edge. 
     
     
         10 . The method of  claim 2 , wherein said edge-sharpness measure of any edge is a function of distances of a plurality of gradients of a sequence of gradients across said any edge from a predefined position relative to said plurality of gradients. 
     
     
         11 . The method of  claim 10 , wherein said predefined position is a center of gravity among said plurality of gradients, gradient values being treated as weights. 
     
     
         12 . The method of  claim 10 , wherein said function measures a k-th central moment of said plurality of gradients about said predefined position, k being a positive even integer. 
     
     
         13 . The method of  claim 2 , further comprising:
 detecting that the edge is associated with a sequence of gradients across itself for which a quantity indicating a degree of reflection asymmetry in the sequence has a value that exceeds an asymmetry threshold or that is outside a predetermined tolerance region.   
     
     
         14 . The method of  claim 13 , wherein the edge is rejected or de-emphasized from contributing to the focus signal if the quantity indicating a degree of reflection asymmetry exceeds a threshold. 
     
     
         15 . The method of  claim 2 , wherein a quantity indicating a degree of reflection asymmetry in a sequence of gradients across the edge is used to reduce a relative weight for the edge in a weighted average of the edge-sharpness measures of edges that contribute to the focus signal. 
     
     
         16 . The method of  claim 14 , wherein the quantity indicating of degree of asymmetry is computed from a distance from a midpoint of a pair of positive and negative peaks of a sequence of second-order derivatives across the edge to a interpolated position of zero-crossing of the sequence of second-order derivatives. 
     
     
         17 . The method of  claim 14 , wherein the quantity indicating of degree of asymmetry is computed from a center-to-center distance between a first narrowest undivided portion of the edge that has a first contrast and a second narrowest undivided portion of the edge that has a second contrast. 
     
     
         18 . A non-transitory computer-readable medium that comprises computer-executable instructions that, when executed by a computing device, cause said computing device to execute a method according to  claim 2 . 
     
     
         19 . A circuit that generates a focus signal from a plurality of edges of an image of a scene to indicate a degree of image sharpness, comprising:
 an edge detection and width measurement (EDWM) unit; and,   a focus signal calculator,   wherein the edge detection and width measurement unit detects edges in image data of the image, determines for the edges respective relative weights of their respective contributions to the focus signal, and evaluates edge-sharpness measures for edges that contribute to the focus signal,   wherein the focus signal calculator generates a focus signal from the edge-sharpness measures, taking into account the respective relative weights,   wherein the edge detection and width measurement (EDWM) unit implements a method according to  claim 2 .   
     
     
         20 . An image capture system, comprising:
 a focus lens;   an image sensor comprising an image sensing pixel array;   a focus lens motor means; and,   a circuit according to  claim 19 .   
     
     
         21 . The method of  claim 2 , wherein the edge-sharpness measure is a distance measured between two positions of the edge defined according to a predetermined manner except as positions where (interpolated) gradients have same values. 
     
     
         22 . The method of  claim 21 , where in one of the two positions is a midpoint between another pair of positions defined according to another predetermined manner.