Video contrast enhancement through partial histogram equalization
AssigneeSharp Laboratories of America, Inc.
InventorLouis Joseph Kerofsky
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Abstract
Methods and apparatus for video contrast enhancement are disclosed. A pixel level threshold is set for an input video frame in a video sequence. For pixel levels in the input video frame that are below the threshold, an adaptive contrast-enhancing function is applied. For other pixel levels, a scene-stable mapping function is applied. This contrast enhancement method can improve the contrast in darker areas of a scene depicted in the video sequence, without destroying the intended light levels for a scene or causing temporal brightness fluctuations in the enhanced video sequence.
Claims23
A method of video contrast enhancement comprising:
- setting a first pixel level threshold for an input video frame in a video sequence, the input video frame including a plurality of pixels having corresponding pixel values within a pixel value range, where the first pixel level threshold is set to one of the pixel values within the pixel value range;
- when a given input-video-frame pixel's value is below the pixel level threshold, remapping that pixel according to an adaptive contrast-enhancing function;
- when the given input-video-frame pixel's value is above the pixel level threshold, remapping that pixel according to a scene-stable mapping function;
- detecting substantial changes in scene histogram content from one frame of the video sequence to a following frame;
- and
- when a substantial change in scene histogram content is detected, allowing the adaptive contrast-enhancing function to change more rapidly than it is otherwise allowed to change.
A video contrast enhancer comprising:
- a contrast-enhancing function generator capable of accepting a target histogram specification and a set of histogram bins derived from one or more frames of a video sequence, the bins representing a histogram at least for pixel levels below a selected pixel level threshold, the function generator capable of generating a remapping function for input pixel levels below the threshold based on the target histogram specification and the set of histogram bins;
- a scene-stable mapper to control the remapping function for input pixel levels above the threshold;
- a histogram calculator capable of constructing a frame histogram for the input video frame;
- a temporal histogram filter to supply the set of histogram bins for use by the contrast-enhancing function generator, each bin comprising an exponentially time-filtered combination of sequential frame histograms from the histogram calculator;
- and
- a scene change detector capable of detecting scene chances based on a comparison of successive frame histograms from the histogram calculator and supplying a scene change signal when a scene change is detected in the video sequence, the temporal histogram filter applying a different time constant when the scene change signal is asserted.
Dependencies are inferred from the claim text.