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.

Description

FIELD OF THE INVENTION [0001] The present invention is directed to enhancing digital images, and, more specifically, to enhancing color and contrast in a multispectral digital image obtained from a remote sensing platform. BACKGROUND [0002] In a generalized way, a digital image is a set of one or more two-dimensional numeric arrays in which each array element, or pixel, represents an apparent brightness measured by an imaging sensor. The brightness value at each pixel is often represented by an integer number called a digital number (DN). Digital images are commonly generated by remote imaging systems that collect imagery in the visible and infrared regions of the electromagnetic spectrum. Images collected from such systems are used in numerous applications by both commercial and government customers. [0003] When collecting digital imagery, specific bands of electromagnetic energy from the area being imaged are commonly collected at several imaging sensors. For example, in many imag...

AI Technical Summary

Benefits of technology

[0007] The present invention provides a system and method to process original DNs provided by a satellite imaging system to produce a set of color balanced and contrast enhanced images. The present invention enhances a multispectral digital image in a fully-automatic, systematic, and universal way, allows for the production of optimized enhanced digital image products that are cost effective and profitable, and also produces a set of quantitative surface-reflectance estimates that can be further processed by other automatic algorithms to yield specific kinds of information about the objects that have been imaged. The set of color balanced and contrast enhanced images is referred to herein as Dynamic Range Adjusted (DRA) images. The DNs in the multispectral bands may be processed using a relatively small amount of processing resources otherwise required to produce such images. Such DRA products provide relatively easy visual interpretation, and may include images in which each pixel is in an 8-bit format, four-band 8-bit or 24-bit RGB. The image may be displayed and / or printed without applying any additional contrast stretches.

[0010] In another embodiment, the contrast of a digital image is also enhanced by determining a median value of the input DNs, determining a target brightness value for the enhanced digital image, and adjusting each DN to generate an output DN, the median value of the output DNs being substantially equal to the target brightness value. When multiple bands of imaging data are present, the DNs of each band of imaging data are received from the plurality of imaging sensors. The median value of the received DNs is determined by processing the DNs of each of the bands of imaging data to determine a distribution of DNs for each of the plurality bands of imaging data, determining a median value of each of the distributions of DNs, and computing an average of the median values.

Problems solved by technology

Many contrast stretch algorithms change the color content of the imagery resulting in questionable color content in the scene.

For example, if the distribution of DNs is not centered within the range of possible DN values, such a contrast stretch can skew the DNs resulting in a color offset and, in an image containing structures, a house may appear as being the wrong color.

In addition it is often difficult to decide what stretch to apply to a given image.

While useful in applications where users may be accustomed to color distortion, in applications where a user is not accustomed to such a color skew, it may result in customer dissatisfaction for applications where users are not accustomed to such a color skew.

For example, an employee of a firm specializing in the analysis of digital imagery may be accustomed to such a color skew, while a private individual seeking to purchase a satellite image of an earth location of interest to them may find such a color skew unacceptable.

While such methods preserve color, they are generally quite computer intensive and require significant amounts of additional processing as compared with a contrast stretch as described above.

This can be a very time-consuming, labor-intensive process.

This technique is reliable, however, it requires significant additional processing as compared to a contrast stretch performed on RGB data as previously described.

A major drawback to this type of stretch involves the significant amounts of computer processing time involved in converting from RGB to HIS space and back.

Images