Patent ID: 8159616

Claim:
A method for improving the perceived visual quality of images for instance displayed on a colour television screen, the method comprising the steps of: provision of an input video signal (Y′, U, V); carrying out a histogram processing on said incoming video signal, whereby a luminance transfer function T (k) is obtained and based on said luminance transfer function T (k) calculating a global chrominance gain g c ; for individual pixels of the image, carrying out a chrominance processing that associates a local gain g(U p ,V p ) for each individual pixel having the colour components (U p , V p ); provision of an output video signal (Y′, U, V) OUT2 for individual pixels wherein: the luminance component Y′ OUT2 is determined by said luminance transfer function T (k); the first chrominance component U OUT2 is given by the expression: U OUT2 =g c g ( U p , V p ) U p ; and the second chrominance component V OUT2 is given by the expression: V OUT2 =g c g ( U p , V p ) V p ; whereby the chrominance components of the individual pixels of the image are modified by a global gain factor g c relating to the characteristics of the entire image and a local gain factor g relating to the individual pixels; wherein an average luminance level Y AVG over all histogram bins k is determined and wherein said chrominance gain g c is determined as: g c = T ( Y AVG )/ T ( Y AVG ) where T(Y AVG ) is an identity mapping minus an offset bl; i.e., T(Y AVG ) is given by the expression: T ( Y AVG )= Y AVG −bl; wherein said offset bl is calculated as the sum of two offsets bl 1 and bl 2 , where bl 1 is determined from a second cumulative histogram p c (k) determined by the expression: p C ⁡ ( k ) = ∑ i = o k ⁢ p ⁡ ( i ) , k ∈ { 0 , … ⁢ , N - 1 } where p(i) is the unprocessed histogram and where said bl 1 is then calculated as the luminance level, for which the cumulative proportion exceeds a predetermined, preferably small and fixed, percentage of the sample counts and where bl 2 is calculated according to the expression: bl 2 =c 1 Y AVG β where c 1 and β are constants and where said total offset bl is then calculated according to the expression: bl =min[ bl 1 +bl 2 , b max ] where b max predetermined threshold value.