Method and apparatus for enhancing a digital image

Abstract

A system and method processes original digital numbers (DNs) provided by a satellite imaging system to produce a set of spectral balanced and contrast enhanced multispectral images. Spectral balancing is achieved based on physical characteristics of sensors of the imaging system as well as compensation for atmospheric effects. The DNs in the multispectral bands may be processed using a relatively small amount of processing resources otherwise required to produce such images. Such images may be processed completely automatically and provide relatively easy visual interpretation. Each image pixel may be, for example, in an 8-bit or 16-bit format, and the image may be displayed and/or printed without applying any additional color correction and/or contrast stretches.

1 . A method for producing an enhanced digital image, comprising: 
 receiving a plurality of pixels of imaging data from an imaging system, each of said pixels of imaging data comprising a digital number;    processing said digital number of each of said pixels to determine a distribution of digital numbers for said plurality of pixels; and    determining a spectral radiance of each of said plurality of pixels based on said distribution.    
   
   
       2 . The method for producing an enhanced digital image, as claimed in  claim 1 , wherein said receiving step comprises: 
 receiving a plurality of bands of digital numbers of imaging data from a plurality of imaging sensors.    
   
   
       3 . The method for producing an enhanced digital image, as claimed in  claim 2 , wherein said plurality of imaging sensors comprises: 
 a blue band imaging sensor;    a green band imaging sensor;    a red band imaging sensor; and    a near-infrared band imaging sensor.    
   
   
       4 . The method for producing an enhanced digital image, as claimed in  claim 1 , wherein said receiving step comprises: 
 receiving a plurality of top-of-atmosphere digital numbers of imaging data from a plurality of imaging sensors of said imaging system; and    adjusting said top-of-atmosphere digital numbers for a first imaging sensor of said imaging sensors based on known sensor characteristics of said first imaging sensor.    
   
   
       5 . The method for producing an enhanced digital image, as claimed in  claim 1 , wherein said processing said digital number step comprises: 
 determining a lower distribution cutoff of said distribution of digital numbers.    
   
   
       6 . The method for producing an enhanced digital image, as claimed in  claim 5 , wherein said lower distribution cutoff is set at 0.1% of the cumulative distribution.  
   
   
       7 . The method for producing an enhanced digital image, as claimed in  claim 5 , wherein said lower distribution cutoff is based on a point in the cumulative distribution at which said digital numbers are indicative of path spectral radiance.  
   
   
       8 . The method for producing an enhanced digital image, as claimed in  claim 5 , wherein said determining a spectral radiance step comprises: 
 subtracting a digital number associated with said lower distribution cutoff from each of said plurality of digital numbers.    
   
   
       9 . The method for producing an enhanced digital image, as claimed in  claim 5 , further comprising: 
 determining the value of a digital number associated with said lower distribution cutoff; and    subtracting said digital number associated with said lower distribution cutoff from each of said plurality of digital numbers when said digital number associated with said lower distribution cutoff is less than a predetermined maximum value.    
   
   
       10 . The method for producing an enhanced digital image, as claimed in  claim 9 , further comprising: 
 subtracting said predetermined maximum value from each of said plurality of digital numbers when said digital number associated with said lower distribution cutoff is greater than said predetermined maximum value.    
   
   
       11 . The method for producing an enhanced digital image, as claimed in  claim 1 , further comprising: 
 determining a median value of said distribution of digital numbers;    determining a target brightness value for said enhanced digital image; and    adjusting at least a subset of said digital numbers to generate a second distribution of digital numbers, said second distribution having a median value that is substantially equal to said target brightness value.    
   
   
       12 . The method for producing an enhanced digital image, as claimed in  claim 11 , wherein said receiving a plurality of pixels step comprises: 
 receiving a plurality of bands of pixels of imaging data from a plurality of imaging sensors.    
   
   
       13 . The method for producing an enhanced digital image, as claimed in  claim 12 , wherein said determining a median value of said distribution of imaging data comprises: 
 processing digital numbers of each of said bands of imaging data to determine a distribution of digital numbers for each of said plurality bands of imaging data;    determining a median value of each of said distributions of digital numbers; and    computing an average of said median values.    
   
   
       14 . The method for producing an enhanced digital image, as claimed in  claim 1 , wherein said determining a spectral radiance step is performed independently of scene content of the digital image.  
   
   
       15 . The method for producing an enhanced digital image, as claimed in  claim 1 , wherein said determining a spectral radiance step is performed on a physical basis of said imaging data.  
   
   
       16 . A satellite image comprising a plurality of pixels each having an output digital number determined by: 
 determining a magnitude of spectral radiance of each a plurality of input digital numbers;    processing said magnitudes of spectral radiance to determine a distribution of input digital numbers; and    calculating the output digital number of each of said plurality of pixels based on said distribution.    
   
   
       17 . The satellite image, as claimed in  claim 16 , wherein said determining a magnitude step comprises: 
 receiving a plurality of bands of input digital numbers from a plurality of imaging sensors, each input digital number representing the spectral radiance received by the associated imaging sensor; and    determining a magnitude of spectral radiance for each of said input digital numbers for each of said bands.    
   
   
       18 . The satellite image, as claimed in  claim 17 , further comprising: 
 compensating said magnitude of spectral radiance for at least a portion of said digital numbers for any given band of said bands of pixels based on a known non-linear response of the imaging sensor associated with the respective band of pixels.    
   
   
       19 . The satellite image, as claimed in  claim 18 , wherein said portion of said digital numbers are digital numbers which are greater than a digital number associated with a sensor roll-off for said imaging sensor.  
   
   
       20 . The satellite image, as claimed in  claim 18 , wherein said compensating step comprises: 
 firstly determining that a digital number for a pixel of said band is greater than a predetermined digital number;    secondly determining digital numbers associated with said pixel from the remaining bands; and    calculating a compensated digital number based on digital numbers from said secondly determining step.    
   
   
       21 . The satellite image, as claimed in  claim 16 , wherein said determining a magnitude step comprises: 
 receiving an input top-of-atmosphere digital number associated with each of said pixels of imaging data; and    adjusting said input top-of-atmosphere digital numbers based on known imaging sensor characteristics.    
   
   
       22 . The satellite image, as claimed in  claim 16 , wherein said calculating the value step is performed independently of scene content of the digital image.  
   
   
       23 . The satellite image, as claimed in  claim 16 , wherein said calculating the value step is performed on a physical basis of said imaging data.  
   
   
       24 . A method for transporting a color enhanced image towards an interested entity, comprising: 
 conveying, over a portion of a computer network, an image that includes a plurality of pixels each having an output digital number that has been determined based on a distribution of spectral radiance values associated with the plurality of input digital numbers.    
   
   
       25 . The method, as claimed in  claim 24 , wherein said output digital numbers have further been based on a median brightness value of the distribution of the input digital numbers.  
   
   
       26 . The method, as claimed in  claim 25 , wherein said image includes a plurality of bands of pixels, each of said bands having a plurality of pixels, each pixel having an output digital number that has been determined based on a distribution of input digital numbers associated with the band of pixels.  
   
   
       27 . The method, as claimed in  claim 26 , wherein a band includes a plurality of digital numbers that have been compensated based on a known non-linearity of an imaging sensor associated with said band.  
   
   
       28 . The method, as claimed in  claim 24 , wherein said output digital numbers are determined by subtracting a path spectral radiance from the value of each of said input digital numbers.  
   
   
       29 . The method, as claimed in  claim 28 , wherein said path spectral radiance is determined based on said distribution of input digital numbers.  
   
   
       30 . The method, as claimed in  claim 24 , wherein said image comprises a plurality of pixels each having an output digital number that has been determined based on a distribution of spectral radiance values associated with the plurality of input digital numbers and that have been contrast enhanced by applying a stretch to the value of each of said plurality of input digital numbers.  
   
   
       31 . The method, as claimed in  claim 30 , wherein said stretch is determined based on a median value of said distribution of input digital numbers, a target brightness value for the digital image, and upper and lower percentile limits for said distribution of input digital numbers.  
   
   
       32 . A method for producing an enhanced digital image, comprising: 
 receiving a plurality of digital numbers imaging data from an imaging system;    determining a magnitude of spectral radiance of each said plurality of digital numbers;    processing said magnitudes of spectral radiance to determine a distribution of magnitudes of spectral radiance;    firstly adjusting each of said plurality of digital numbers based on said distribution to produce an adjusted value spectral radiance digital number;    processing the adjusted value spectral radiance digital number for each of said plurality of digital numbers to determine a first adjusted distribution for said plurality of spectral radiance digital numbers;    assessing said first adjusted distribution to determine a range of adjusted values and a median value of said range of adjusted values; and    secondly adjusting the value of at least a subset of said plurality of spectral radiance digital numbers to create a second adjusted distribution wherein the median of said second adjusted distribution corresponds with a target median and the range of said second adjusted distribution corresponds with a target range.    
   
   
       33 . The method for producing an enhanced digital image, as claimed in  claim 32 , wherein said receiving step comprises receiving a plurality of bands of digital numbers from a plurality of imaging sensors within said imaging system, each digital number representing the spectral radiance received by the associated imaging sensor; and 
 said determining step comprises determining a magnitude of spectral radiance for each of said digital numbers for each of said bands of pixels.    
   
   
       34 . The method for producing an enhanced digital image, as claimed in  claim 33 , further comprising: 
 compensating said magnitude of spectral radiance for at least a portion of said digital numbers for a first band of said bands of pixels based on a known non-linear response of an imaging sensor of said imaging sensors associated with said first band.    
   
   
       35 . The method for producing an enhanced digital image, as claimed in  claim 34 , wherein said portion of said digital numbers are digital numbers which are greater than a digital number associated with a sensor roll-off for said first imaging sensor.  
   
   
       36 . The method for producing an enhanced digital image, as claimed in  claim 34 , wherein said compensating step comprises: 
 firstly determining that a first digital number for a first pixel of said first band is greater than a predetermined digital number;    secondly determining digital numbers associated with said first pixel from the remaining bands; and    calculating a compensated first digital number based on digital numbers from said secondly determining step.    
   
   
       37 . The method for producing an enhanced digital image, as claimed in  claim 32 , wherein said firstly adjusting step is preformed independently of scene content of the digital image.  
   
   
       38 . The method for producing an enhanced digital image, as claimed in  claim 32 , wherein said firstly adjusting step is performed on a physical basis of said imaging data.  
   
   
       39 . The method for producing an enhanced digital image, as claimed in  claim 32 , wherein said firstly adjusting step is based on a point in the cumulative distribution of magnitudes of spectral radiance at which said digital numbers are indicative of path spectral radiance.