Method for improving contrast in picture sequences

Assignee: Nokia Technology GmbH

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Filing: 1998-01-24

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Granted: —

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Expires: 2018-01-24

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Abstract

Abstract The invention relates to adjusting contrast in television pictures and other natural picture sequences. In the method according to the invention, the pictures of the picture sequence are gathered into histograms which are processed in certain fashion in order to improve contrast in the pictures, whereafter there are formed mapping functions on the basis of the processed histograms, and the pixel values of the pictures in the picture sequence are then altered on the basis of said mapping functions. The histograms are processed so that from each class where the magnitude of values surpasses a given, predetermined limit, there are cut off all values surpassing said limit, which are then redistributed in the histogram, to classes nearest to the class under observation. The weighting of the distribution controlling the redistribution of the cut-off values advantageously varies according to the class under observation, so that near the low-value end of the histogram, the values are redistributed mainly to classes with higher values than in the class under observation, and near the high-value end of the histogram, the cut-off values are redistributed mainly to classes with lower values than in the class under observation. In the middle region of the histogram, the values are distributed essentially evenly on both sides of the class under observation. Moreover, the cut-off limit can advantageously be defined to be dependent on the class, so that the limit at the extreme ends of the histogram is lower than in the middle region thereof. By applying this procedure, it is possible to avoid a situation where the picture details are covered under areas that are seen nearly black or nearly white. The thus preprocessed histograms are filtered in an adaptive edge-preserving filter, so that flickering is effectively prevented and at the same time there is avoided a distortion of the pictures successive to the cut-off spots in the picture sequence.

Inventors

👤 Petri Nenonen

Bibliographic Details

Application Number —

Jurisdiction EP

Data Source google_patents

Last Synchronized 2026-11-09 11:07

CPC Classifications

H; H04; H04N; H04N5/00; H04N5/14; H04N5/20; G; G06; G06T; G06T5/00; G06T5/40; G06T5/90; G06T5/92; G06T2207/00; G06T2207/10; G06T2207/10016

Claims Analysis

Total Claims: 7

Claim 1

Independent Claim 142 words

A method for improving contrast in picture sequences, in which method pictures of the picture sequence are gathered into histograms, the gathered histograms are processed in order to alter the picture contrast, the gathered histograms are filtered, on the basis of the processed histograms, there are formed mapping functions, and on the basis of the mapping functions, the pixel values of the pictures in the picture sequences are altered, said method being characterized in that the method also comprises the following steps, where from each histogram class where the value magnitude surpasses a given predetermined limit, there are cut off the values surpassing said limit, the cut-off values are redistributed in the histogram according to a distribution weighted towards the classes lying near to said class, and the histograms gathered from chronologically successive pictures are filtered by an adaptive edge-preserving filter structure.

Claim 2

Independent Claim 140 words

A method for improving contrast in picture sequences, in which method pictures of the picture sequence are gathered into histograms, the gathered histograms are transformed in order to alter the picture contrast, on the basis of the transformed histograms, there are formed mapping functions, said method being characterized in that the method also comprises steps, where from each class of the histogram surpassing a given predetermined limit, there are cut off the values surpassing said limit, the cut-off values are redistributed to the histogram according to a given distribution weighted towards the classes lying near to said class, the mapping functions formed on the basis of histograms gathered of chronologically successive pictures are filtered in an adaptive edge-preserving filter structure, and by means of the filtered mapping functions, the pixel values of the pictures in the picture sequence are modified.

Claim 3

Dependent Claim 22 words

A method according to claim 1 or 2, characterized in that said distribution weighted towards the nearest classes is a triangular distribution.

Claim 4

Dependent Claim 29 words

A method according to claim 1 or 2, characterized in that the distribution used in the redistribution of values cut off from a given class depends on said class.

Claim 5

Dependent Claim 67 words

A method according to claim 4, characterized in that at the low-value end of the histogram, the distribution is weighted towards classes with higher values than said class, at the high-value end of the histogram, the distribution is weighted towards classes with lower values than said class, and in the middle region of the histogram, the distribution is weighted essentially equally towards both sides of said class.

Claim 6

Dependent Claim 62 words

A method according to claim 1 or 2, characterized in that the method also comprises a step where after the first value cutting and redistribution, from each class where the value magnitude surpasses a given, predetermined limit, there are cut off the values surpassing said limit, and the cut-off values are redistributed back to the histogram according to an essentially even distribution.

Claim 7

Dependent Claim 24 words

A method according to claim 1 or 2, characterized in that said given, predetermined limit depends on the class of the histogram under observation.

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