Method and Apparatus for Enhancing Images

Abstract

A method and an apparatus for enhancing images are provided. An input image generated by an imaging device, such as a short-wavelength infrared (SWIR) camera, sensor, or any other imaging device, is processed to produce an input histogram representing the distribution of pixel intensities in the input image. Histogram start and end points are determined, and a variable plateau profile is calculated depending upon the type of enhancement desired. The plateau profile could take on any desired shape or size, and two or more plateau regions could be included in the profile. The input histogram is clipped along the variable plateau profile. A cumulative histogram is constructed from the clipped histogram, and is normalized. The input image is then transformed into an enhanced output image using the normalized cumulative histogram as a look-up table.

1 . A method for enhancing an image, comprising the steps of:
 constructing an input histogram corresponding to the image, the input histogram representing a pixel intensity distribution corresponding to the image;   calculating a midpoint based upon an offset and a threshold value of the input histogram;   establishing first and second horizontal plateaus to be applied to the input histogram, the first horizontal plateau extending to the midpoint and the second horizontal plateau extending from the midpoint;   clipping the input histogram at the first and second horizontal plateaus to produce a clipped input histogram;   constructing a cumulative histogram from the clipped input histogram;   normalizing the cumulative histogram to produce a normalized cumulative histogram; and   transforming the input image using the normalized cumulative histogram to produce an enhanced output image corresponding to the input image, wherein at least a portion of the input image is enhanced in the output image.   
     
     
         2 . The method of  claim 1 , further comprising excluding saturated pixels in the image from the input histogram. 
     
     
         3 . The method of  claim 2 , further comprising excluding low level pixels in the image from the input histogram. 
     
     
         4 . The method of  claim 1 , further comprising performing non-linear operations on the normalized cumulative histogram prior to transforming the input image. 
     
     
         5 . The method of  claim 1 , further comprising interpolating between bins to produce the enhanced output image. 
     
     
         6 . The method of  claim 1 , further comprising selectively adjusting gain in the output image. 
     
     
         7 . The method of  claim 1 , wherein the step of establishing the first and second horizontal plateaus further comprises establishing a dark plateau for enhancing dark regions in the image and a light plateau for enhancing light regions in the image. 
     
     
         8 . The method of  claim 1 , further comprising identifying a start bin of the input histogram when cumulative pixels are at least a first percentage of a total number of pixels collected. 
     
     
         9 . The method of  claim 8 , further comprising identifying an end bin of the input histogram when cumulative pixels are at least a second percentage. 
     
     
         10 . An apparatus for enhancing an image, comprising:
 an imaging device for capturing an image;   a contrast enhancement module for receiving and processing the image, wherein the contrast enhancement module constructs an input histogram corresponding to the image, calculates a midpoint based on an offset and a threshold value of the input histogram, constructs first and second horizontal plateaus for clipping the input histogram, the first horizontal plateau extending to the midpoint and the second horizontal plateau extending from the midpoint, and clips the input histogram along the first and second horizontal plateaus and generates a normalized cumulative histogram; and   means for generating an output image based upon the normalized cumulative histogram.   
     
     
         11 . The apparatus of  claim 10 , wherein the first horizontal plateau enhances dark areas of the input image and the second horizontal plateau enhances bright areas of the input image. 
     
     
         12 . The apparatus of  claim 10 , further comprising an automatic gain control module for adjusting gain of the input image. 
     
     
         13 . The apparatus of  claim 12 , wherein the automatic gain control modules includes user-selectable gain control modes for providing additional enhancement of the input image. 
     
     
         14 . The apparatus of  claim 10 , wherein the contrast enhancement module excludes saturated pixels in the image from the input histogram. 
     
     
         15 . The apparatus of  claim 10 , wherein the contrast enhancement module excludes low level pixels in the image from the input histogram. 
     
     
         16 . The apparatus of  claim 10 , wherein the contrast enhancement module performs non-linear operations on the normalized cumulative histogram. 
     
     
         17 . The apparatus of  claim 10 , further comprising means for interpolating between bins to produce the enhanced output image. 
     
     
         18 . The method of  claim 10 , wherein the contrast enhancement module identifies a start bin of the input histogram when cumulative pixels are at least a first percentage of a total number of pixels collected. 
     
     
         19 . The method of  claim 18 , wherein the contrast enhancement module identifies an end bin of the input histogram when cumulative pixels are at least a second percentage.