Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N9/00—Details of colour television systems
  • H04N9/64—Circuits for processing colour signals
  • H04N9/67—Circuits for processing colour signals for matrixing
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G5/00—Control arrangements or circuits for visual indicators common to cathode-ray tube indicators and other visual indicators
  • G09G5/02—Control arrangements or circuits for visual indicators common to cathode-ray tube indicators and other visual indicators characterised by the way in which colour is displayed
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
  • H04N1/46—Colour picture communication systems
  • H04N1/56—Processing of colour picture signals
  • H04N1/60—Colour correction or control
  • H04N1/6058—Reduction of colour to a range of reproducible colours, e.g. to ink- reproducible colour gamut
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
  • H04N1/46—Colour picture communication systems
  • H04N1/56—Processing of colour picture signals
  • H04N1/60—Colour correction or control
  • H04N1/6077—Colour balance, e.g. colour cast correction
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2320/00—Control of display operating conditions
  • G09G2320/02—Improving the quality of display appearance
  • G09G2320/0242—Compensation of deficiencies in the appearance of colours
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2320/00—Control of display operating conditions
  • G09G2320/06—Adjustment of display parameters
  • G09G2320/0666—Adjustment of display parameters for control of colour parameters, e.g. colour temperature
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2320/00—Control of display operating conditions
  • G09G2320/06—Adjustment of display parameters
  • G09G2320/0673—Adjustment of display parameters for control of gamma adjustment, e.g. selecting another gamma curve

Definitions

• the present invention relates to a system for correcting at least one colour image according to the preamble of claim 1.
• the present invention further relates to a method for correcting at least one colour image according to the preamble of claim 7.
• LCDs show a specific colour characteristic due to their used backlight illumination based on light emitting diode (LED) technology.
• LED light emitting diode
• the primary colours red, green and blue strongly deviate from the coordinates which are standardized by the European Broadcast Union (EBU) or which are defined as sRGB (standard Red Green Blue).
• EBU European Broadcast Union
• sRGB standard Red Green Blue
• the sRGB transfer function (Gamma 0.45) is different from the transfer function standardized for studio video (technical background: CCIR Rec. 709 takes the coordinates from EBU red, EBU blue and the average rounded to an accuracy of two digits from EBU green and S[ociety of]M[otion]P[icture]T[elevision]E[ngineers] green.
• Fig. 1 shows the gamut of a typical mobile display in comparison with the sRGB colour triangle. Obviously a colour conversion (mapping) methodology is required otherwise false colour impression disturbs the picture quality. Especially skin and green tones extremely suffer from wrong colour reproduction due to the capability of the human visual perception to recognize those colours.
• a typical methodology for colour gamut mapping can be accomplished by using a 3 x 3 matrix operation in the RGB domain.
• the CGM matrix operation
• MTX cgm (MTX d ⁇ splay Y - MTX sRGB mput describes the mathematical operation to be performed.
• the CGM input values i. e. the measured display primaries as well as the input sRGB primaries are required:
• measured display primaries input sRGB primaries: x y x y
• Fig. 2A The block diagram of Fig. 2A describes the steps required for proper signal processing.
• Soft clipping is a possible solution for this problem; in this context, soft clipping is realized by an adaptive colour gamut mapping (ACGM) in which the coefficients of the matrix are adaptively calculated for each pixel value.
• ACGM adaptive colour gamut mapping
• Fig. 2B additional measurement blocks are added to adaptively determine the matrix coefficients as realized in Philips' LifePixTM comprising algorithms significantly enhancing the image quality and performance of mobile displays, in particular of liquid crystal displays (LCDs) or of organic light emitting diode (OLED) displays.
• the basis of this advanced colour gamut mapping algorithm includes four steps: the first step is to reduce the brightness of saturated colours in order to create room for the mapping algorithm; the second step is to reproduce all the colours inside the display gamut with the correct hue and saturation; the third step involves clipping the pixels outside the display gamut by adding a certain amount of white; in the final step, pixels which are too bright are reduced without altering the saturation or hue.
• This concise colour mapping algorithm is designed to identify the optimal colour ranges of the primary colours (red, green, blue) and to reprocess the colour information in order to produce more realistic colours; by this, mobile LCDs or OLED displays are complemented without compromising performance or changing the panel. The result is a vividly enhanced colour reproduction on any mobile device with a colour display, without compromising on other areas of screen performance.
• an object of the present invention is to further develop a system of the kind as described in the technical field as well as a method of the kind as described in the technical field in such way that the amount of calculation cycles is reduced by maintaining most of the clipping preserving effect of the adaptive colour gamut mapping (ACGM).
• ACGM adaptive colour gamut mapping
• the present invention is principally based on the idea of colour gamut mapping without feedback loop on frame-based matrix update.
• the motivation for developing such frame-updated colour gamut mapping is to reduce the amount of calculation cycles by maintaining most of the clipping preserving effect of the adaptive colour gamut mapping (ACGM), preferably by using only matrix per frame and/or per field.
• the ACGM of LifePixTM can be used as a basis in order to transfer the advanced colour gamut mapping algorithm from a pixel-based version into a frame -based version.
• an adaptation to the colour space of the display can be performed in dependence on the data of the colour image; said adaptation is performed by means of only one constant set of coefficients of the colour space per colour image, thus avoiding unwanted clipping artefacts.
• the frame -updated colour gamut mapping (FUCGM) algorithm according to the present invention is designed for mobile use but can also be used with all other applications where the colour space is to be converted.
• the system as well as the method to perform the adaptive colour gamut mapping on frame wise updated matrix coefficients are used to overcome blue- stretched behaviour typically existing in liquid crystal displays (LCDs) or in organic light emitting diode (OLED) displays, for instance of mobile phones.
• LCDs liquid crystal displays
• OLED organic light emitting diode
• the present invention can be applied in a video system, for example for a mobile display on the basis of at least one low-power single chip T[hin]F[ilm]T[ransistor] LCD driver with integrated source driver, gate driver, display R[andom]A[ccess]M[emory], timing control and D[irect]C[urrent]-to-D[irect]C[urrrent] converter.
• the expected quality level is in between a static colour conversion and Philips' pixel based LifePixTM methodology.
• Fig. 1 schematically shows a diagram of mobile display gamut compared to sRGB gamut
• Fig. 2A schematically shows a block diagram of a standard colour gamut mapping
• FIG. 2B schematically shows a block diagram of an adaptive colour gamut mapping
• Fig. 3 schematically shows a block diagram of a measurement system of a frame - updated colour gamut mapping (FUCGM) operation according to the present invention
• Fig. 4 schematically shows a block diagram of a luminance, saturation and hue (LSH) detector according to the present invention, said LSH detector being part of the FUCGM measurement system of Fig. 3;
• Fig. 5 schematically shows a block diagram of a white-point, saturation and hue
• WSH WSH modulator
• said WSH modulator being part of the FUCGM measurement system of Fig. 3
• Fig. 6 schematically shows a block diagram of an average unit / critical colour correction (CCC) unit according to the present invention, said average unit / CCC unit being part of the FUCGM measurement system of Fig. 3;
• CCC critical colour correction
• Fig. 7 schematically shows a block diagram of a colour garmut matrix generator, said colour garmut matrix generator being part of the FUCGM measurement system of Fig. 3;
• Fig. 8 schematically shows a block diagram of a video matrix, said video matrix being part of the FUCGM measurement system of Fig. 3.
• the same reference numerals are used for corresponding parts in Fig. 1 to Fig. 8.
• Fig. 3 shows a block diagram describing the measurement system 100 of a frame - updated colour gamut mapping (FUCGM) algorithm according to the present invention.
• FUCGM colour gamut mapping
• the FUCGM algorithm operates in intra frame mode; consequently, only storage of an actual frame or field is required.
• An adjustable subsampling is performed in horizontal and vertical direction by means of a subsampling unit 10 being provided with the input signal IS.
• a luminance, saturation and hue (LSH) detector 20 derives a luma signal (with reference numeral lum in Fig. 3) and a saturation signal (with reference numeral sat in Fig. 3) from the RGB input.
• LSH luminance, saturation and hue
• Both signals i. e. the luma signal lum as well as the saturation signal sat are fed to a white-point, saturation and hue (WSH) modulator 30 being also provided with the three parameters WLM, SSm, HSm.
• WSH saturation and hue
• white-point (with reference numeral W in Fig. 3), saturation (with reference numeral S in Fig. 3) and hue (with reference numeral H in Fig. 3) represent the three degrees of freedom for colour conversion which calculate the attenuation parameters of the colour matrix from the slope values derived from the display colour coordinates; the slope values are calculated upfront and depend on the display colour coordinates as well as on the colour coordinates of the input signal IS.
• the attenuation parameters are collected separately for average calculation (with reference numeral 6Ow for average white-point, reference numeral 60s for average saturation, and reference numeral 6Oh for average hue in Fig. 3).
• the average is calculated over the number of pixels taken into measurement.
• a critical colour correction (CCC) area detector 40 is connected behind the subsampler 10 and in parallel to the luminance, saturation and hue (LSH) detector 20.
• this CCC area detector 40 has indicated the pixel value as a "critical colour" (with such critical colour being set by parameters and describing a colour area of highly saturated greenish-yellow to redish-yellow) the value triplet will be additionally stored in a respective separated average routine 62w for white-point W, 62s for saturation S, and 62h for hue H.
• the critical colour average calculates over the number of critical colour pixels, which are stored separately.
• colour gamut matrix generator 80 is provided with three parameters WRGB, SRGB, HRGB-
• the horizontal/vertical subsampling unit 10, the luminance, saturation and hue (LSH) detecting unit 20, the white-point, saturation and hue (WSH) modulating unit 30, the average calculation routine 6Ow for white -point W, the average calculation routine 60s for saturation S, the average calculation routine 6Oh for hue H, the mix routine 70, and the colour gamut matrix generating unit 80 are assigned to the adaptive processing.
• the preloaded conversion matrix 90 will be attenuated.
• colour-mapping matrix coefficients S 1 , gi, bi, r l5 S 2 , b 2 , r 2 , g 2 , S 3 derived from that calculation are loaded once per frame/per field into the matrix 90, which enables the colour mapping operation to be performed.
• a simple hard clipper 92 in the post processing, in particular in the post video processing, ensures the predefined value accuracy.
• LSH detector 20 A more detailed disclosure of the luminance, saturation and hue (LSH) detector 20 is given by way of Fig. 4.
• This LSH detector 20 is designed to derive the luma value lum as well as the saturation value sat from the RGB signal Ri, Gi, Bi.
• the RGB square routine 22 passes the data Rs, Gs, Bs, i. e. the squared input values Ri 2 , Gi 2 , Bi 2 to a RGB shuffle operation 24
• the uncorrected saturation value Si is corrected also by way of the determined hue value Cs according to the equation in a saturation corrector 28 resulting in the corrected saturation value Sd, i. e. in the saturation signal sat.
• WSH modulator 30 A more detailed disclosure of the white point, saturation and hue (WSH) modulator 30 is given by way of Fig. 5.
• This WSH modulator 30 is connected behind the luminance, saturation and hue (LSH) detector 20 and is provided with the luma signal lum
• the WSH modulator 30 uses the luma signal Ld and the saturation signal Sd in order to calculate the attenuators of the following matrix processing according to the following equations for the determination of the correction value or reduction value Wc for white-point W, of the correction value or reduction value Sc for saturation S, and of the correction value or reduction value Hc for hue H:
• H c (l- S d )+ (HS m - HS m - S d )
• AVG/CCC critical colour correction
• the average unit 60, 62 comprises in principal six bins 6Ow, 62w, 60s, 62s, 6Oh, 62h, namely three bins 6Ow, 60s, 6Oh for the standard average (AVG) determination and three additional bins 62w, 62s, 62h for the "critical colour correction (CCC)" average (AVG) calculation.
• AVG standard average
• CCC critical colour correction
• the mixing routine 70 following the average bins 60, 62 is implemented by respective mixing units 7Ow (for white-point W), 70s (for saturation S), and 7Oh (for hue H); each of these mixing units 7Ow, 70s, 7Oh selects the amount of influence from which average bin 6Ow, 62w, 60s, 62s, 6Oh, 62h the value will be taken.
• the mixer 70 is controlled by the amount of pixels, which are detected as a critical colour.
• the average (AVG) unit / critical colour correction (CCC) unit AVG/CCC works as follows:
• the attenuation values Wc, Sc, Hc previously derived in the white-point, saturation and hue (WSH) modulator 30 (cf. Fig. 5) are collected in the standard average unit 6Ow, 60s, 6Oh. In case of measuring a critical colour these attenuation values Wc, Sc, Hc are additionally collected in the critical colour correction (CCC) average (AVG) bin 62w, 62s, 62h.
• CCC critical colour correction
• AVG critical colour correction
• the critical colour correction (CCC) area detector 40 makes the decision whether the value is stored in the CCC AVG bin 62w, 62s, 62h.
• the critical colour on which the mapping operation shall take special attention is a greenish yellow.
• This colour There are two reasons to take special care of this colour: - the high luma fraction of greenish yellow colour, which makes any disturbance in the colour reproduction especially visible; and the incapability of mobile displays to reproduce this greenish yellow colour area in its full saturation.
• a static colour mapping simply clips this greenish yellow colour at the gamut border.
• the critical colour correction (CCC) area detector 40 is a simple comparison block and is designed to determine the colour area to which a critical colour is assigned.
• the CCC area detector 40 takes the following three criteria (i), (ii), (iii) for critical colour decision into account. In this context, it is to be taken into account that the following exemplary values are dependent on quantization as well as on implementation; consequently, for cases of different application other criteria for determining the critical colour value can be employed. (i) absolute difference of the primaries red and green should be below a certain threshold and defines the colour area width (for instance, for RGB in exemplary quantization of eight bit an exemplary threshold for the colour area width is 35);
• a minimum luma value can be defined for the critical colour area.
• the average will be calculated on every CCC AVG bin 62w, 62s, 62h, and the mixer 70, 72 will be controlled as well.
• the number of pixels detected as critical pixels control the mixing routine 70, 72.
• the critical colour correction average value is directed to the conversion matrix 90.
• a linear attenuation mix is calculated from the standard AVG and from the CCC AVG between the slope start value and the slope stop value.
• the matrix generator 80 is connected behind the mixing routine 70, 72 and performs the determination of the coefficients S 1 , gi, bi, r l5 S 2 , b 2 , r 2 , g 2 , S 3 of the conversion matrix
• K 21 S g S c +H g H c
• the generated coefficients K O o, Ko 1 , K 02 , Ki 0 , Kn, Ki 2 , K 20 , K 21 , K 22 (corresponding to S 1 , gi, bi, ri, s 2 , b 2 , r 2 , g 2 , S3) are loaded once per frame into the conversion matrix 90, and the whole frame is processed with these settings.
• the conversion matrix or video matrix 90 performs the colour mapping which is loaded once per frame with the coefficients K O o, Ko 1 , K 02 , Ki 0 , Kn, Ki 2 , K 20 , K 21 , K 22
• R 0 R, + K 00 R 1+ K 01 (G 1 -R) + K 02 [B 1 -R 1 )
• G 0 G 1+ K n G 1+ K n [B 1 -G) + K 10 [R 1 -G 1 )
• B 0 B t +K 22 B 1 +K 20 .[R 1 -B) + K 21 [G 1 -B)
• the video matrix 90 performs in the nonlinear light domain.
• subsampling unit in particular horizontal and/or vertical subsampling unit 20 luminance, saturation and hue (LSH) detector
• white -point, saturation and hue (WSH) modulator 32 white -point modulating module of white -point, saturation and hue (WSH) modulator 30 34 saturation modulating module of white-point, saturation and hue (WSH) modulator 30
• CCC critical colour correction
• CRC critical colour correction
• CCC critical colour correction
• AVG critical colour correction
• CCC critical colour correction
• AVG/CCC average (AVG) unit / critical colour correction (CCC) unit B third primary colour blue Bi blue input signal, in particular blue part of RGB input signal IS
• Bo blue output signal in particular blue part of RGB output signal OS
• G second primary colour green Gi green input signal in particular green part of RGB input signal IS
• Gs squared green input signal, in particular squared green part of RGB input signal gi first colour-mapping matrix coefficient of second primary colour green G g 2 second colour-mapping matrix coefficient of second primary colour green G
• H hue in particular hue signal or hue value, representing third degree of freedom for colour conversion Hc attenuation value or correction value or reduction value for hue H H RGB third coefficient or third parameter, in particular hue coefficient or hue parameter, provided to colour gamut matrix generator 80, namely Hb hue coefficient or hue parameter for third primary colour blue B
• Ld lum luma signal or luma value, in particular determined luminance signal or determined luminance value Max maximum of squared input signals Rs, Gs, Bs

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• 2007
  • 2007-05-25 CN CNA2007800320671A patent/CN101513036A/zh active Pending
  • 2007-05-25 WO PCT/IB2007/051983 patent/WO2008001239A2/en active Application Filing
  • 2007-05-25 EP EP07736019A patent/EP2039145A2/de not_active Withdrawn

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