CHROMINANCE SIGNAL INTERPOLATION

Abstract: In a method of chrominance signal interpolation, in which output chrominance signals (OUTUV) are interpolated (MED) from mutually corresponding input chrominance signals (INUV(N-1), INUV(N)) from mutually neighboring input images, a color saturation value of at least one of the output chrominance signals (OUTUV) is smaller than an average of color saturation values of the mutually corresponding input chrominance signals (INUV(N-1), INUV(N)) corresponding to the one output chrominance signal (OUTUV) if the mutually corresponding input chrominance signals (INUV(N-1), INUV(N)) meet a predetermined criterion. The expression corresponding means that the chrominance signals concerned (INUV(N-1), INUV(N)) correspond both as to spatial positions in the mutually neighboring input images (N, N-1) and as to chrominance values, i.e. either U/I or V/Q. The criterion may be that the reference value (REF) lies between the corresponding input chrominance signals (INUV(N-1), INUV(N)), or that differences between the corresponding input chrominance signals (INUV(N-1), INUV(N)) exceed a predetermined threshold.

Description: Chrominance signal interpolation. The invention relates to a method and device for chrominance signal interpolation. The invention also relates to a display apparatus comprising such a device. It is known from literature [3] that the Human Visual System (HVS) is more sensitive to luminance than to chrominance stimuli. Therefore, in many video-processing algorithms, the algorithm applied to the luminance signal is much more advanced then the one applied for the chrominance signal. In scan-rate-conversion algorithms, like the PROZONIC [1] and MELZONIC [2], color processing is non-motion compensated. Motion artifacts in the chrominance signal caused by the conversion from e.g. 50 Hz interlaced to 100 Hz interlaced are to a significant extend masked by the fact that the HVS is less sensitive to the chrominance signal as stated before. However, for very saturated colors, the artifacts can become visible and annoying. If the luminance signal is motion compensated (MC) while the chrominance signal isn't, a misalignment in moving objects occurs for temporal interpolated images. This misalignment is especially annoying for very saturated dark colors, where the constant luminance principle is jeopardized most severely due to the gamma correction of individual R, G, and B signals. It is, inter alia, an object of the invention to provide an improved chrominance signal interpolation. To this end, the invention provides a chrominance signal interpolation method and device, and a display apparatus comprising such a device, as defined in the independent claims. Advantageous embodiments are defined in the dependent claims. In a method of chrominance signal interpolation in accordance with a primary aspect of the present invention, in which output chrominance signals are interpolated from mutually corresponding input chrominance signals from mutually neighboring input images, a color saturation value of at least one of the output chrominance signals is smaller than an average of color saturation values of the mutually corresponding input chrominance signals corresponding to the one output chrominance signal if the mutually corresponding input chrominance signals meet a predetermined criterion. The expression corresponding means that the chrominance signals concerned correspond both as to spatial positions in the mutually neighboring input images and as to chrominance type, i.e. either U/I or V/Q. The criterion may be that the reference value lies between the corresponding input chrominance signals, or that differences between the corresponding input chrominance signals exceed a predetermined threshold. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows an embodiment of a display apparatus in accordance with the invention; and Fig. 2 shows a second embodiment of a chrominance signal interpolation device in accordance with the invention. The invention eliminates the color misalignment, thereby reducing an annoying artifact. The essential insight on which the current idea relies is that it is better to have no color, or a color with a decreased saturation than a saturated color at the wrong place, which may lead to luminance errors. In short: 'Better less or no color than a wrong color'. As the error only occurs in (fast) moving image parts, a very simple motion detector could provide a signal to switch-off the chrominance in places where it changes significantly. We shall elaborate this basic thought in two more or less equally performing embodiments. Consider the situation of the conversion of 50 Hz interlaced to 100 Hz interlaced. This is sketched in Fig. 1. In the invention, the chrominance signal can be created in various ways. A first proposal for creating U(x, n — j ) , the intuitive notation for the temporally inteφolated field between U(x, ή) and [/(3c, n - 1) , and V(x, n - j is defined by: U(x,n - ) = MED(U(x, n - 1), ref, U(x, n)) (1) V(x, n - 1) = MED(V(x, n - 1), ref, V{x, ή)) with MED being the median filter operator and ref being a reference signal level, e.g. zero, indicating no-color. U(x, n - 1) and V(x, n -1) may be vertically inteφolated samples. Hereinafter, U(N-1) will be used as a shorthand notation for U{x, n - 1) , etc. As a consequence, if U(N-1) and U(N) are on one side of the reference ref, the one closest to the reference is directed to the output. Therefore, temporal inteφolation will result in the same chrominance component only if both samples (from the previous and current field) are about equal, which is the case in still picture parts. In moving picture parts, the chrominance component closest to the reference is chosen, i.e. the one that yields the lowest saturation, which eliminates the annoying artifact of misalignment mentioned above. However, the temporally inteφolated images contain less color information in moving areas compared to the non-temporally inteφolated ones. Although the output chrominance signals contain less color information than in the prior art, it still produces a subjectively better image quality. The misalignment in the prior art simple temporal averaging is much more annoying than the loss of color contrast for half of the images in the 100Hz interlaced sequence. Fig. 1 shows an embodiment of a display apparatus in accordance with the invention. An input luminance signal INγ is applied to a first luminance image memory MEM- Y-l to generate a luminance signal INγ(N) having a doubled field frequency. The luminance signal INγ(N) is applied to a second luminance image memory MEM-Y-2 to generate a delayed luminance signal INγ(N-l). The luminance signal INγ(N) and the delayed luminance signal INγ(N- 1 ) are applied to respective inputs of a motion vector estimator ME and a motion-compensated spatio-temporal inteφolation circuit MCI. Motion vectors MV estimated by the motion estimator ME are applied to the motion-compensated spatio-temporal inteφolation circuit MCI. The motion-compensated spatio-temporal inteφolation circuit MCI furnishes an output luminance signal OUTγ. An input chrominance signal INuv is applied to a first chrominance image memory MEM-UV-1 to generate a chrominance signal INuv( ) having a doubled field frequency. The chrominance signal INuv(N) is applied to a second chrominance image memory MEM-UV-2 to generate a delayed chrominance signal INuv(N-l). The chrominance signal INuv(N) and the delayed chrominance signal INuv(N-l) are applied to inputs of a median filter MED that also receives a reference signal REF. The median filter supplies an output chrominance signal OUTuv- The output luminance signal OUTγ and the output chrominance signal OUTuv are applied to a matrix circuit MX to obtain RGB signals that are applied to a display D. A second proposal for creating the chrominance signal is defined by: ref (2) V(x,n -±) = > Th) G(V(x, n - 1), V(x, «)), (otherwise) with G() an inteφolation function, e.g. temporal averaging, and Th a threshold. If the difference between the chrominance components exceed a certain threshold, the reference signal, indicating no color, is chosen. If the difference is small compared to the threshold, an average or any other inteφolation algorithm can be applied to calculate the output chrominance sample. Therefore, for still picture parts, the chrominance information remains within the image part, and for moving colored image parts, detected by a large difference in the chrominance signals, the chrominance component is reset to 'no-color'. Fig. 2 shows a second embodiment of a chrominance signal inteφolation device in accordance with the invention, operating in accordance with the second proposal. An input U chrominance signal INu is applied to a first U chrominance image memory MEM-U-1 to generate a U chrominance signal U(N) having a doubled field frequency. The U chrominance signal U(N) is applied to a second U chrominance image memory MEM-U-2 to generate a delayed chrominance signal U(N-1). The U chrominance signal U(N) and the delayed U chrominance signal U(N-1) are applied to inputs of a temporal averaging circuit AVu, the output of which is connected to a first switching contact of a switch SWu- A second switching contact of the switch SWu receives the reference signal REF. The switch SWu supplies an output U chrominance signal OUTu. The U chrominance signal U(N) and the delayed U chrominance signal U(N-1) are also applied to inputs of a subtraction circuit SI J for obtaining the absolute value of their difference. A comparison circuit CMPu compares this absolute difference to a threshold Th. An input V chrominance signal INγ is applied to a first V chrominance image memory MEM-V-1 to generate a chrominance signal V(N) having a doubled field frequency. The chrominance signal V(N) is applied to a second V chrominance image memory MEM-V-2 to generate a delayed chrominance signal V(N-1). The V chrominance signal V(N) and the delayed V chrominance signal V(N-1) are applied to inputs of a temporal averaging circuit AVy, the output of which is connected to a first switching contact of a switch SWy. A second switching contact of the switch SWy receives the reference signal REF. The switch SWv supplies an output V chrominance signal OUTy. The V chrominance signal V(N) and the delayed V chrominance signal V(N-1) are also applied to inputs of a subtraction circuit Sy for obtaining the absolute value of their difference. A comparison circuit CMPv compares this absolute difference to a threshold Th. Outputs of the comparison circuits CMPu and CMPv are applied to a logical OR circuit LOR, an output of which is applied to control inputs of the switches SWu and SW V . A primary aspect of the invention can be summarized as follows. Future television systems apply motion compensated scan rate conversion to reduce the large area and line flicker without introducing motion artifacts. Although both luminance and chrominance information needs to be converted, the advantage of motion compensation for the chrominance is comparatively small. This disclosure describes an innovative color processing that is cheaper than motion compensation, but better than linear inteφolation or simple repetition. A method, and apparatus realizing this method, for video format conversion in which the color signals for a pixel in an inteφolated output image (U I V(x, n - )) depends on the value of these signals in the neighboring original fields (U I V(x, n - 1) and U IV(x, n)), characterized in that, in case the values in the neighboring fields are different, at least sometimes the smallest of the two, or the value zero (neutral color), is fed to the output. In alternative embodiments, chrominance signals range from 0 to 255 with the neutral reference value being 127. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. References: [1] Datasheet SAA4990, PROZONIC, 1995. [2] G. de Haan, J. Kettenis, A. Lδning, and B. De Loore. IC for Motion- Compensated 100 Hz TV with natural-motion movie-mode. IEEE Tr. on Consumer Electronics, Vol. 42, No. 2, May 1996, pages 165-174. [3] A.K. Jain. Fundamentals of Digital Image Processing. Prentice-Hall International Inc., Englewood Cliffs, NJ, 1989.

Claims: CLAIMS: 1. A method of chrominance signal inteφolation, the method comprising the steps of: furnishing (MEM-UV-2; MEM-U-2, MEM-V-2) mutually corresponding input chrominance signals (INuv(N-l), LNuv(N); INu(N-l), LNu(N), IN V (N-1), IN V (N)) from mutually neighboring input images, the expression corresponding meaning that the chrominance signals concerned (IN UV (N-1), LNuv(N); INu(N-l), INu(N), IN V (N-1), TN V (N)) correspond both as to spatial positions in the mutually neighboring input images and as to chrominance type, i.e. either U/I or V/Q; and inteφolating (MED; SWu, SWv) output chrominance signals (OUTuv; OUTu, OUTv) from the mutually corresponding input chrominance signals (INuv(N-l), INuv(N); INu(N-l), INu(N), INv(N-l), INv(N)), a color saturation value of at least one of the output chrominance signals (OUTuv; OUTu, OUTv) being smaller than an average of color saturation values of the mutually corresponding input chrominance signals (INuv(N-l), INuv(N); INu(N-l), LNu(N), INv(N-l), INv(N)) corresponding to the one output chrominance signal (OUTuv; OUTu, OUTv) if the mutually corresponding input chrominance signals (INuv(N-l), INuv(N); INu(N-l), INu(N), IN V (N-1), IN V (N)) meet a predetermined criterion. 2. A method as claimed in claim 1, wherein the color saturation value of the one output chrominance signals (OUTuv; OUTu, OUTv) does not exceed any of the color saturation values of the mutually corresponding input chrominance signals (INuv(N-l) > INuv(N); LNU(N-1), LNU(N), INv(N-l), INV(N)) corresponding to the one output chrominance signal (OUTuv; OUTu, OUTv) if the mutually corresponding input chrominance signals (INuv(N-l), INuv(N); INU(N-1), INU(N), IN V (N-1), IN V (N)) meet the predetermined criterion. 3. A method as claimed in claim 2, wherein the inteφolating step (MED) includes the step of calculating each output chrominance signal (OUTuv) by taking a median of both mutually corresponding input chrominance signals (INuv(N-l), INuv(N)) corresponding to the output chrominance signal (OUTuv) and a reference value (REF) representing no saturation, said criterion being that the reference value (REF) lies between the corresponding input chrominance signals (INuv(N-l), INuv(N)). 4. A method as claimed in claim 2, wherein the inteφolating step includes the steps of: determining (CMPu, CMPv) whether differences (Su, Sv) between the mutually corresponding input chrominance signals (INu(N-l), INu(N); INv(N-l), INv(N)) exceed a predetermined threshold (Th), said criterion being that the differences (Su, Sv) between the corresponding input chrominance signals (INuv(N-l), INuv(N)) exceed the predetermined threshold (Th); outputting (SWu, SWv) a reference value (REF) representing no saturation if the differences (Su, Sv) exceed the predetermined threshold (Th); and outputting (SWu, SWv) a temporal average (AVu, AVv) of the corresponding input chrominance signals (INuv(N-l), INuv(N)) if said differences (Su, Sy) do not exceed the predetermined threshold (Th). 5. A device for chrominance signal inteφolation, the device comprising: means for furnishing (MEM-UV-2; MEM-U-2, MEM-V-2) mutually corresponding input chrominance signals (INuv(N-l), INuv(N); INu(N-l), INu(N), INv(N-l), INv(N)) from mutually neighboring input images, the expression corresponding meaning that the chrominance signals concerned (INuv(N-l), INuv(N); INu(N-l), LNu(N), INv(N-l), INv(N)) correspond both as to spatial positions in the mutually neighboring input images and as to chrominance type, i.e. either U/I or V/Q; and means for inteφolating (MED; SWu, SWy) output chrominance signals (OUTuv; OUTu, OUTv) from the mutually corresponding input chrominance signals (INuv(N-l), INuv(N); INu(N-l), LN U (N), IN V (N-1), IN V (N)), a color saturation value of at least one of the output chrominance signals (OUTuv; OUTu, OUTv) being smaller than an average of color saturation values of the mutually corresponding input chrominance signals (INuv(N-l), INuv(N); INU(N-1), INU(N), IN V (N-1), TN V (N)) corresponding to the one output chrominance signal (OUTuv; OUTu, OUTv) if the mutually corresponding input chrominance signals (INuv(N-l), INuv(N); INu(N-l), INu(N), IN V (N-1), IN V (N)) meet a predetermined criterion. A display apparatus, comprising: a chrominance signal inteφolation device as claimed in claim 5; and a display device (D) coupled to said chrominance signal inteφolation device.

Inventors: BELLERS ERWIN B, DE HAAN GERARD, BELLERS, ERWIN, B., DE HAAN, GERARD
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H04N9
IPC codes: H04N5, H04N9