Device and method for coding video pictures
AssigneeU.S. Philips Corporation
InventorsGerrit J. Keesman, Willem M. J. Coene, Eduard W. Salomons
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Abstract
The picture quality of an MPEG-coded video signal can be improved considerably by modifying selected coefficients after conventional quantization, for example by rendering them zero. The modification is such that the Lagrangian cost D+.lambda.R (D is distortion, R is bitrate) is minimal for a given value of a Lagrange multiplier .lambda.. A relatively simple process is disclosed in which the value of .lambda. is calculated by means of statistical analysis (6) of the picture to be coded. The statistical analysis comprises the estimation of the RD curve on the basis of the amplitude distribution of the coefficients. The searched .lambda. is the derivative of this curve at the desired bitrate.
Claims20
A device for coding video pictures, comprising:
- a picture transformer (2) for transforming blocks of pixels into coefficients;
- coding means (3,4) for encoding said coefficients, the bit rate R and distortion D of the encoded picture being determined by a coding parameter (r.sub.k);
- means (3) for selecting one from a series of coding parameter values for which the sum D+.lambda.R is minimal for an applied value of .lambda.wherein .lambda. is a Lagrangian multiplier;
- and
- calculation means (6) for calculating the value of .lambda.;
- characterized in that the calculation means (6) comprise:
- means (601) for measuring the amplitude distribution of coefficients;
- means (602) for estimating the bit rate R and distortion D from said amplitude distribution for a plurality of coding parameter values to obtain an estimated RD curve;
- and
- means (603) for computing .lambda.=-dD/dR at a selected point of said RD curve.
A method of coding video pictures, comprising the steps of:
- transforming blocks of pixels into coefficients;
- quantizing the coefficients and modifying the quantized coefficients in order that the sum D+.lambda.R, in which D is the distortion and R is the bitrate, is reduced for an applied value of .lambda.wherein .lambda. is a Lagrangian multiplier;
- and
- coding the modified coefficients;
- characterized in that the method comprises the step of calculating the value of .lambda. by:
- measuring the amplitude distribution of coefficients;
- estimating the bit rate R and distortion D from said amplitude distribution for a plurality of step sizes to obtain an estimated RD curve;
- and
- computing .lambda.=-dD/dR at a selected point of said RD curve.
A method of coding video pictures, comprising the steps of:
- transforming blocks of pixels into coefficients;
- encoding said coefficients, the bit rate R and distortion D of the encoded picture being determined by a coding parameter;
- selecting one from a series of coding parameter values for which the sum D+.lambda.R is minimal for an applied value of .lambda. wherein .lambda. is a Lagrangian multiplier;
- and
- calculating the value of .lambda.;
- characterized in that the step of calculating comprises:
- measuring the amplitude distribution of coefficients;
- estimating the bit rate R and distortion D from said amplitude distribution for a plurality of coding parameter values to obtain an estimated RD curve;
- and
- computing .lambda.=-dD/dR at a selected point of said RD curve.
Dependencies are inferred from the claim text.