Gray scale processing using error diffusion

Abstract: Coupled to an error variance circuit 11 is an emission luminance characteristic acquisition circuit 20 that counts up, at a display number counter 21, the display number in the single or plural frames of the respective bits of image data by the counters, M in number, corresponding to said bits, then solves for display area percentage (Sk) dividing, at a display area percentage operation part 22, the display dot number as counted at a display number counter 21, by total dot number, and acquires the luminance deviation characteristic for each bit by means of an emission luminance deviation characteristic measuring part 24. The luminance deviation thus obtained is renewed for each frame and transferred to the error variance circuit 11, and processed for error variance on the basis of the emission luminance characteristic to be output at PDP. At low level, on the other hand, the luminance deviation is rendered either fixed type luminance deviation or emission luminance level more or less higher than the actual one to reduce the diffusion noise particularly at the low level image portion thereby obtaining a more natural image.

Description: This invention relates to that error variance processing equipment for display device which displays false half tone by error variance. Recently PDP (Plasma Display Panel) has been attracting a good deal of public attention as a thin, light-weighted display device. Totally different from the conventional CRT drive, the drive method of this PDP is a direct drive by means of digitalized image input signal. Consequently, the luminance and tone of the light emitted from the panel face depends on the bit number of the signal to be processed. PDP may be divided into two types: Ac and DC types whose basic characteristics are different from each other. AC type PDP features satisfactory characteristics as far as is concerned the luminance and durability, As for the tonal display, maximum 64 tones only have reportedly been displayed at the level of trial production. It is however proposed to adopt in future a technique for 256 tones by address/display separate type drive method (ADS subfield method). One frame consists of 8 subfields whose relative ratios of luminance are 1, 2, 4, 8, 16, 32, 64 and 128 respectively. Combination of these 8 luminances enables a display in 256 tones. The respective subfields are composed of the address duration that writes in one screen of refreshed data and the sustaining duration that decides the luminance level of the corresponding fields. In the address duration, first wall charge is formed initially at each pixel simultaneously over all the screens, and then the sustaining pulse is given to all the screens for display. The brightness of the subfield is proportional to the number of the sustaining pulse to be set to predetermined luminance. Two hundred and fifty-six tonal display is thus realized. In such an AC drive method, the more the number of tones, the more the number of bits of the address duration as the preparation time for lighting up and making the panel luminescent within one frame of duration increases. The sustaining duration as light emitting duration becomes therefore relatively short reducing thus the maximum luminance. Because the luminance and tone of the light emitted from the panel face depends upon the number of bits of the signal to be processed, increased number of the bits of the signal improves the picture quality, but decreases the emission luminance. If conversely the number of the bits of the signal to be processed is decreased, the emission luminance increases but decreases the tone to de displayed thereby causing the degradation of the picture quality. The error variance intended to minimize the color depth difference between the input signal and emission luminance rendering the number of bits of the output drive signal smaller than that of the input signal is a process to express false half tone used when the maximal shade of color is desired to be manifested with lesser tone. FIGURE 1 shows a conventional, general error variance circuit, where an image signal with the original picture elements or pixels Ai, j of p (8, for example) bits is input into the error variance circuit 11 from an image signal input terminal 10. This image signal is processed in a processing circuit 13 and reduced to q (4, for example) in bit number before emitting light from PDP. On the other hand, an emission luminance characteristic operation part 14 consisting of ROM and other components measures and stores the emission luminance characteristic of the PDP from, for instance, the representative input data (solid line) as far approximate as possible to the equation y = x (dotted line) shown in FIGURE 2. The emission luminance characteristic is sent to the error operation part 12 to calculate out the error, which is added to the input image signal in a processing circuit 13 where it is diffused. False half tone was thus displayed. As a result, a corrected luminance line as y = = x (dotted line) was obtained despite the instantaneous emission luminance in steplike form (solid line), which in fact was recognised as a smoothed-out shape. The emission luminance characteristic of such a display device as PDP varies however in terms of the data to be displayed, sometimes resulting in that emission luminance characteristic diverging from the y = x (dotted line) which is shown by the solid line in FIGURE 3. The prior art was problematic in that the method of convergence into representative emission luminance characteristic as shown in FIGURE 2 was not well applicable to any tonal characteristic of the data other than that when such a representative characteristic was acquired, thereby eliciting the false contour caused by the tonal inadequacy. EP-A-0264302 discloses a processing circuit for diffusing error in a digital display where a bi-level conversion error at each pixel is randomly diffused to adjacent pixels. EP-A-0488891 discloses a method for controlling the brightness gradation of an image on a flat panel display by altering the number of sustain pulses at each subframe of the image. It is an object of this invention to prevent the false contour apt to appear when an error is dispersed on the basis of a representative luminance deviation characteristic. According to the present invention an error variance processing apparatus in accordance with that claimed in claim 1. The present invention calculates the emission luminance characteristic for each frame on the basis of the luminance deviation characteristic as obtained from the load factor of the input data of a display device, like a PDP, instead of the convention emission luminance characteristic that is given from a ROM, dispersing the error by renewal of the emission luminance characteristic for each frame to prevent the appearance of the false contour. Attaining this primary object of the present invention will permit to renew the tonal characteristic in response to the emission luminance characteristic that may vary in terms of the data to be displayed as shown by solid, dotted, and chain lines in FIGURE 4. Since the conventional convergence into representative emission luminance characteristic is thus avoided, the error variance can adapt itself well to the tonal characteristic of any data which may change moment by moment. Preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which:- FIGURE 1 is a block diagram of a conventional error variance circuit. FIGURE 2 is a characteristic diagram that illustrates a representative example of emission luminance characteristic. FIGURE 3 is another characteristic diagram that illustrates another example of emission luminance characteristic. FIGURE 4 is still another characteristic diagram that illustrates another example of emission luminance characteristic. FIGURE 5 is a block diagram that shows up the first embodiment of the error variance processing equipment for display device according to this invention. FIGURE 6 is a characteristic diagram that depicts the relationship between the emission luminance deviation and display area percentage. FIGURE 7 is a block diagram that shows up the second embodiment of the error variance processing equipment for display device according to this invention. FIGURE 8 is another characteristic diagram that illustrates another example of emission luminance characteristic. FIGURE 9 is a block diagram that shows up the third embodiment of the error variance processing equipment for display device according to this invention. FIGURE 10 is a characteristic diagram that illustrates an example of emission luminance characteristic by the third embodiment of this invention. FIGURE 11 is a block diagram that shows up the fourth embodiment of the error variance processing equipment for display device according to this invention. Referring now in particular to the drawings, there are illustrated the embodiments of this invention. This invention will be understood more readily with reference to the following examples; however these examples are intended to illustrate the intention and are not to be construed to limit the scope of this invention. In an exemplary PDP used as a display device that is driven by the aforesaid address/display separate type driving method (ADS subfield method), the display tone number N enabled by the ADS subfield is determined by M, the number of subfields; N = 2 M . An ideal luminance level Yn of a given input level n may be expressed by: (α:the referential emission luminance level) where n is binary converted, the respective bits are b N - 1 , ··· b 2 , b 1 and b 0 . Since in fact there exists a luminance deviation that depends upon the display area percentage (Sk) for every subframe, where δ represents the emission luminance deviation characteristic as obtained from the load factor of input data, and Display area percentage (Sk) = Display dot number of the subfield K Dot number on the screen First Embodiment: FIGURE 5 represents the first embodiment of this invention, which consists of the conventional error variance circuit 11 and the emission luminance characteristic acquisition circuit 20. As has been already explained referring to FIGURE 1, the conventional error variance circuit 11 consisting of error operation part 12 and processing part 13 performs the error variance on the basis of given emission luminance characteristic to display the false half tone. Referring now to FIGURE 5, the emission luminance characteristic acquisition circuit 20 by this invention, which consists of the display number counter 21, the display area percentage operation part 22, emission luminance deviation characteristic measuring part 23, and the luminance deviation operation part 24, acquires the emission luminance characteristic for every single or plural frames from the image data driven by PDP, and transfers the emission luminance characteristic thus obtained to the error variance circuit 11 during the vertical synchronization of the image. More specifically, the display number counter 21 consisting of M counters counts up the display number of the in single or plural frames using the respective counters corresponding to the respective M bits of the image data. The display area percentage operation part 22 gives the display area percentage (Sk) dividing, by "total dot number," the "display dot number of subfield K" as counted at the display number counter 21. The emission luminance deviation characteristic measuring part 23, which consists of such LUT (lookup table) as ROM seeks after the luminance deviation characteristic of respective bits. The luminance deviation operation part 24 solves for the luminance deviation at each level. The operation of the above described mechanism and further constructional features and advantages will be best appreciated from a description of a complete cycle of operation. The display number counter 21 counts up the "display dot number of subfield K," that is, the display number in single or plural frames of respective bits by M counters corresponding to the respective M bits of image data. The display area percentage operation part 22 gives the display area percentage (Sk) dividing, by "total dot number," the "display dot number of subfield K" as counted at the display number counter 21. The emission luminance deviation characteristic measuring part 23 gives the luminance deviation characteristic of each bit, based on which the luminance deviation operation part 24 solves for the luminance deviation at each level. The emission luminance characteristic acquisition circuit 20 calculates the emission luminance level Yn at a given input level n as taking into consideration the luminance deviation that depends on the display area percentage (Sk) of each subframe, where the luminance deviation characteristic (δ) obtained from the load factor of input data gives in general such characteristic line as shown in FIGURE 6. The function to solve for this δ has been stored in the emission luminance deviation characteristic measuring part 23. The luminance deviation at each level can be calculated by the following equation: The deviation is renewed for every single or plural frames to be transferred to the error variance circuit 11, where error is diffused on the basis of the emission luminance characteristic to be output at the PDP. Since the convergence into representative emission luminance characteristic is thus avoided by this configuration, the error variance can adapt itself well to the tonal characteristic of any data which may change moment by moment contribute to the prevention of the noise at low level. Second Embodiment: The second embodiment of this invention will be explained referring to FIGURE 7. It differs from the first embodiment of this invention in that the maximum luminance operation part 25 has been inserted between the emission luminance deviation characteristic measuring part 23 and the luminance deviation operation part 24. Consequently the display number counter 21 counts up the "display dot number of subfield K," which is the display number in a single or plural frames of respective bits by means of M counters corresponding to the respective bits of M bit image data. The display area percentage operation part 22 gives the display area percentage (Sk) dividing, by "total dot number," the "display dot number of subfield K" as counted at the display number counter 21. The operation up to the emission luminance deviation characteristic measuring part 23, namely up to the stage where the luminance deviation characteristic of each bit is given by the emission luminance deviation characteristic measuring part 23 is the same as in the first embodiment. Next, in the second embodiment, the maximum luminance operation part 25 calculates the luminance at the maximum input level. That is, in the case of such emission luminance characteristic as shown by the dotted lines in FIGURE 8, the maximum luminance operation part 25 computes by the following formula: where max = 2 M - 1 . Based upon this data, the luminance deviation operation part 24 calculates the luminance deviation of each level by the following equation: Δn = Yn - n 2 M Yma x The luminance deviation of each level as obtained by this luminance deviation operation part 24 is transferred to the error operation part 12 of the error variance circuit 11. When the luminance at the maximum input level breaks away from the line y = x, overall correction is applied so that the maximum input level should come nearer to the line y = x. As a result, the characteristic as represented by the dotted lines in FIGURE 8 is converted into that expressed by the solid lines. Then the error operation part 12 and the processing circuit 13 perform the processing of error variance based on given emission luminance characteristic to display the false half tone. The first and second embodiments of this invention have the following actions and effects. (1) According to this invention, the emission luminance characteristic for every single or plural frames is computed on the basis of the luminance deviation characteristic as obtained from the load factor of input data instead of the conventional representative emission luminance characteristic as given from ROM, when the emission luminance deviation characteristic is changed by the data to be displayed. Since further the emission luminance characteristic is renewed for every single or plural frames to diffuse the error, the false contour can be kept from appearing. (2) Because the maximum luminance operation part 25 is provided for total correction so that the maximum input level should come nearer to the line y = x when the luminance at the maximum input level deviates from y = x, the error variance can be performed more exactly to prevent the appearance of the false contour. (3) Because this invention uses the low order bits (n-m = 4) after variance when variance processing is made by adding reproduced error to the image signal with original pixels of n (for example, 8) bits by the vertical adder 31 and horizontal adder 32, this can be identified as the use of a luminance correction line connecting the starting points of the emission luminance levels of 2m tones. In consequence the image after the error variance manifests a smooth change. Third Embodiment: Referring now to FIGURE 9, the equipment according to this invention consists of the error variance circuit 11 and the emission luminance characteristic acquisition circuit 20. The emission luminance characteristic acquisition circuit 20 comprising the display number counter 21, the display area percentage operation part 22, the emission luminance deviation characteristic measuring part 23, and the luminance deviation operation part 24, is intended to acquire the emission luminance characteristic for every single or plural frames from the image data driven by the PDP and transfers the luminance characteristic to the error variance circuit 11 during the vertical synchronization of the image. More specifically in the third embodiment of this invention the low level output terminal 26 of the luminance deviation operation part 24 is not connected to the error operation part 12, but a fixed constant generating part 27 is connected to the low level input of the error operation part 12 to fix the data into the representative input data as near as possible to the preset y = x. More particularly, the display area percentage operation part 22, the emission luminance deviation characteristic measuring part 23 are like those in the first embodiment. The luminance deviation operation part 24 is intended to solve for the luminance deviation at each level for renewing the tonal characteristic of the data other than the low level data for every single or plural frames. The actions and effects are now described of the constructional configuration as above. The third embodiment is identical with the first one in that the luminance deviation at each level in the emission luminance characteristic acquisition circuit 20 is renewed for each single or plural frames and that those at levels other than the low level are transferred to the error variance circuit 11. In the third embodiment of this invention the preset data, particularly at levels other than low level is transferred from fixed constant generating part 27 to the error variance circuit 11. The error variance circuit 11 processes the error variance based on the luminance deviation at the levels other than the low level renewed momentarily by the luminance deviation operation part 24 and on the emission luminance characteristic of the fixed type data for low level preset from the fixed constant generating part 27 to output it to the PDP. The configuration as above can cope perfectly with the tonal characteristic of the ever changing data preventing thus the generation of the low level noise caused by changeover of the luminance deviation, because the convergence is not made into representative emission luminance characteristic even if this characteristic varies with the data to-be displayed. The actions and effects of the insertion of the maximum luminance operation part 25 between the emission luminance deviation characteristic measuring part 23 and the luminance deviation operation part 24 in this third embodiment are the same as in the second embodiment. As for the low level, the preset data is transferred from the fixed constant generating part 27 to the error variance circuit 11 in this case too. The error variance circuit 11 processes the error variance to output its luminance characteristics on the basis of the emission luminance characteristic of the luminance deviation at the levels other than the low level momentarily renewed by the luminance deviation operation part 24 and that of the fixed type luminance deviation preset by the fixed constant generating part 27. The configuration as above can cope perfectly with the tonal characteristic of the ever changing data preventing thus the generation of the low level noise caused by changeover of luminance deviation,because the convergence is not made into representative emission luminance characteristic even if this characteristic varies with the data to be displayed. The third embodiment of this invention has the following actions and effects. Because this invention allows to perform the error variance by calculating the emission luminance characteristic for every single or plural frames at the levels other than the low one based on the luminance deviation characteristic as got from the load factor of input data and renewing the emission luminance characteristic for every single or plural frames, the false contours can be kept from appearing. Since at the same time the fixed type data is used at low level the noise by changeover of luminance deviation at low level caused by the calculation for every single or plural frames may be avoided. Fourth Embodiment: Referring now to FIGURE 11, the error variance processor in the equipment by this invention consists of the error variance circuit 11 and the emission luminance characteristic acquisition circuit 20. The emission luminance characteristic acquisition circuit 20, which comprises the display number counter 21, the display area percentage operation part 22, the emission luminance deviation characteristic measuring part 23, and the luminance deviation operation part 24, is intended to acquire, from the image data driven by the PDP, the emission luminance characteristic for every single or plural frames and to transfer the characteristic thus obtained to the error variance circuit 11 all while the image undergoes the vertical synchronization. More particularly in this fourth embodiment the adder 28 is inserted between the luminance deviation operation part 24 and the error operation part 12, by which the emission luminance levels of the luminance deviation operation part 24 can be set uniformly higher to rather darker image for reducing the noise in particular at low level. More in detail, the display number counter 21, the display area percentage operation part 22, the emission luminance deviation characteristic measuring part 23 are respectively the same as those in their first embodiment (FIGURE 5). The luminance deviation operation part 24, which solve for the emission luminance deviation at each level and renews the tonal characteristic of the data at levels other than the low level for every single or plural frames, is the same as that in the third embodiment (figures 9). The adder 28 adds indiscriminately a constant value (1 for instance) as input at input terminal 29 to the output of the high level line 30 to low level line 31 of the luminance deviation operation part 24. Otherwise the high-level line 30 may be connected directly with the error operation part 12 as shown by the doted line so that a constant value (1 for instance) input at the input terminal 29 may be added to the low level line 31 only by the adder 28. Explained now will be the actions of the configuration as above. The luminance deviation at each level of the emission luminance characteristic acquisition circuit 20 is renewed for every single or plural frames, added a constant value at the adder 28 to be transferred to the error variance circuit 11. Because the error of the error variance may be expressed by error = = input level - emission luminance, a uniform addition, by the adder 28, of a constant value over the high-level line 30 to the low level line 31, will intensify the emission luminance as a whole as shown by the chain line in FIGURE 4, and the error lessens all the more. Since, in this case, the addition ratio at the low level is large enough, though the added value is constant from low to high levels, the effect of the noise reduction is greater at low level that at high level. Nearly the same noise reduction effect can be had even if a constant value is added only to the low level line 31. The calculation of the emission luminance into the rather higher value and the error into rather lower value will thus darken more or less the image after the error variance rendering the image more natural by the diffusion reduction effect at the low level image. The error variance circuit 11 processes the error variance based upon the emission luminance deviation by the data renewed moment by moment by the luminance deviation operation part 24, to which a constant value is further added to output the error into the PDP. Since the convergence into representative emission luminance characteristic is thus avoided by this configuration, the error variance can adapt itself well to the tonal characteristic of any data which may change moment by moment contribute to the prevention of the noise at low level. The fourth embodiment by this invention has the following actions and effects. Because the emission luminance characteristics for every single or plural frames are calculated out on the basis of the luminance deviation characteristic as obtained from the load factor of input data, and that the error variance is performed renewing the emission luminance characteristics for every single or plural frames, the false contour can be kept from appearing. At the same time, a calculation of the emission luminance into rather high value and the error into rather smaller value makes the image after the error variance darker thereby affording an effect of reducing the diffusion noise in particular at the low level image, which will become more natural.

Claims: An error variance processing apparatus for a display device that displays a false half tone via a diffusion output signal provided by an error variance circuit (11) which adds a reproduced error to an input image signal, wherein original pixels of the input image signal are digitised and input to the error variance circuit, each pixel of said digitised input image signal comprising p bits, characterised by : an emission luminance characteristic acquisition circuit (20) receiving a processed image signal from the error variance circuit (11), said processed image signal being output to the display, each pixel of said processed image signal comprising M bits, M < p, said emission luminance characteristic acquisition circuit including: a display number counter circuit (21) having M number counters for counting a display dot number of each bit of a subfield in a frame or plural frames of processed image data, said M number counters corresponding respectively to the M number of subfields; a display area percentage operation part (22) providing a respective display area percentage for each subfield by dividing the display dot number of a subfield counted by the display number counter circuit (21) by the total dot number of a frame or plural frames; an emission luminance deviation characteristic measuring part (23) comprising a ROM, which provides a luminance deviation characteristic for each bit, based on the display area percentage; and a luminance deviation operation part (24) for providing a respective luminance deviation for each luminance level capable of being generated by said M subfields, said respective luminance deviations being provided for each luminance level based on the data of the emission luminance deviation characteristic measuring part, the luminance deviations for each level being input to the error variance circuit (11) every single or plural frames; wherein the error variance circuit (11) calculates and diffuses the reproduced error based on the luminance characteristic for each pixel to be output at the display device. An error variance processing apparatus according to claim 1, further including a maximum luminance operation part (25) for calculating the luminance at a maximum input level, wherein the luminance deviation of each input level is calculated based on a function of said maximum input level. An error variance processing apparatus according to claim 1 or 2, wherein the error variance circuit comprises: and error operation part (12) for calculating the error; and a processing part (13) for diffusing said error, wherein error variance based on an emission luminance characteristic provided by the luminance deviation operation part (24) is performed. An error variance processing apparatus according to any one of the preceding claims, wherein the luminance deviation operation part (24) provides at the error variance circuit (11) a luminance deviation at a level other than a low level, or a plurality of low levels, that is calculated based on the data from the emission luminance deviation characteristic measuring part, the error variance processing apparatus further comprising a fixed constant generating part (27) which outputs a fixed constant at a pre-set level as the luminance deviation instead of the luminance deviation at said low level or plurality of said low levels. An error variance processing apparatus according to claim 1 or 2, further comprising an adder (28) for outputting a level of emission luminance higher than the actual level of emission luminance by adding a pre-determined value to the luminance deviation obtained at the luminance deviation operation part (24). An error variance processing apparatus according to claim 5, wherein the adder (28) adds a pre-determined value to any display level. An error variance processing apparatus according to claim 5, wherein the adder (28) adds a pre-determined value only to a display level lower than a predetermined level.

Inventors: DENDA HAYATO, KOBAYASHI MASAYUKI, KOSAKAI ASAO, MATSUNAGA SEIJI, NAKAJIMA MASAMICHI, ONODERA JUNICHI, DENDA, HAYATO, KOBAYASHI, MASAYUKI, KOSAKAI, ASAO, MATSUNAGA, SEIJI, NAKAJIMA, MASAMICHI, ONODERA, JUNICHI, Denda, Hayato, c/o Fujitsu General Ltd., Kobayashi, Masayuki, c/o Fujitsu General Ltd., Kosakai, Asao, c/o Fujitsu General Ltd., Matsunaga, Seiji, c/o Fujitsu General Ltd., Nakajima, Masamichi, c/o Fujitsu General Ltd., Onodera, Junichi, c/o Fujitsu General Ltd.
Applicants: FUJITSU GENERAL LTD, FUJITSU GENERAL LIMITED
Application year: 1995
Main classification: G09G3
IPC codes: G09G3