Television – Image signal processing circuitry specific to television – Gray scale transformation
Reexamination Certificate
1999-06-02
2001-09-04
Eisenzopf, Reinhard J. (Department: 2614)
Television
Image signal processing circuitry specific to television
Gray scale transformation
C348S671000, C348S673000
Reexamination Certificate
active
06285413
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method and apparatus for dynamic contrast improvement of video pictures.
In the field of video display design there is a strong demand for picture contrast but each video display has a limited dynamic range. Especially the flat display panels like LCD and plasma displays have a lower dynamic range compared to the CRT displays. The picture contrast can't be increased by simply increasing the video signal amplitude is because exceeding the display dynamic range causes unwanted disturbing effects.
SUMMARY OF THE INVENTION
It is an object of the invention to disclose a method and an apparatus which makes efficient use of the display dynamic range to increase the picture contrast and quality. According to the present invention the picture contrast is optimised by analysing the picture contents of the displayed video pictures and adjusting the contrast dependent on the results of the picture content analysis step. More specifically, the basic solution according to the invention is to analyse the video pictures framewise in real time and adjust the parameters of a transfer function for contrast improvement depending on the analysis results for the best subjective picture quality.
Advantageous additional embodiments of the inventive method are disclosed below. It has been found that the use of a dual segment transfer function gives good results. The preferred dual segment transfer function which is used in one embodiment consists of two segments with an adaptive pivot point which separates the two segments. A lower segment for dark samples and an upper segment for light samples.
Each image frame is analysed for different characteristics. The image average brightness, the dark sample distribution and the frame peak value. In a preferred embodiment all the three parameters are used to adjust the transfer function.
The gain of the lower segment is adaptive to the dark sample distribution. A higher gain results from fewer dark samples and a lower gain from a higher number of dark samples. The gain of the upper segment is adaptive to the frame peak value. It is computed in the way that the detected peak value lower than the nominal peak value for full contrast, will be moved to the nominal peak value. If the detected peak value is equal or higher than the nominal peak value then a gain of 1.0 is used (no change). In the other direction the computed theoretical gain is limited to a maximum value in order to avoid unnatural effects.
The third parameter of the transfer function is the pivot point. It is adaptive to the average image brightness and allows to make dark pictures lighter and increase the contrast. Low average brightness values move the pivot point to lower and high average brightness values to higher levels.
A new kind of peak value determination has been developed taking into account the width of the peaks in order to avoid modulation of the segment gain.
In order to smooth the results of the contrast improvement method, the values of the image analysis unit, i.e. average image brightness, dark sample distribution and frame peak value are filtered preferably with an IIR filter.
According to one solution the inventive apparatus comprises an image analysis unit in which the video pictures are analysed regarding to their picture content and a transfer function adaptation unit in which the parameters of the transfer function for contrast improvement are adapted based on the results achieved in the image analysis unit.
Advantageous additional embodiments of the inventive apparatus are also disclosed.
For preparation of the luminance signal adaptive to the signal to noise ratio of the video signal, an adaptive signal splitter is provided in which a noise reduced luminance signal is generated to which the transfer function is applied afterwards.
In particular the adaptive signal splitter may comprise a low pass filter in which the luminance signal is filtered and the low pass component of the signal is fed to the image analysis unit. The low pass filtered signal is used for image analysis in order to reduce the noise influence.
In a more specific embodiment of the inventive apparatus the adaptive signal splitter comprises an adaptive coring unit wherein the high pass component of the luminance signal is reduced from noise and small detail signal components adaptive to an estimate of the signal to noise ratio of the luminance signal by applying a corresponding coring level to the high pass component. To this noise reduced signal component is the low pass component signal added, thus providing the noise reduced luminance signal.
A further improvement can be achieved with the measures disclosed in claim
15
. Adding the noise and small details enclosing signal component which has been cut by the adaptive coring unit to the resulting output signal after applying the transfer function allows for a reduction of the noise amplification without a loss of small detail signals. Since the coring function does not distinguish between noise and image signals, a sharpness loss would have been caused otherwise due to the missing of high frequency small amplitude signal components.
In order to compensate for the effect of the processing of the luminance component on the colour saturation it is advantageous to provide a colour saturation compensation unit which processes the chrominance signals accordingly to compensate for this effect.
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Deutsche Thomson-Brandt GmbH
Eisenzopf Reinhard J.
Kiel Paul P.
Tripoli Joseph S.
Yenke Brian P.
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