Bi-level image display signal processing apparatus

Abstract: An image signal processing apparatus for converting step-gradation density values (I xy ) of successive picture elements to bi-level values (P xy ) for display by a bi-level display device, in which a bi-level error (E xy ) between a bi-level value determined for a picture element and an appropriate density value for the element is apportioned among a set of peripherally adjacent picture elements (A to D) by computing a corresponding set of error apportionment values (G A to G D ), and in which a difference between the sum total of these apportionment values and the bi-level error is derived, and error apportionment is modified such as to eliminate the effects of this difference. Improved reproduction is thereby attained of image regions which are of uniformly high density or low density.

Description: A requirement has arisen in recent years for high-quality reproduction of both printed images and step gradation images (for example an image representing a continuous-tone image as a plurality of step gradation density values) by utilizing a bi-level type of display device such as a gas plasma dot-matrix display which is inherently capable of generating only two levels of display density. This is achieved by generating a spatial gray scale display image. However, problems are presented by the prior art methods proposed for implementing such reproduction. With such a bi-level display device, each picture element generated by the display can be set (e.g. as an element of a dot matrix) in either a light or a dark state. There have been various proposals for implementing pseudo-continuous tone reproduction by such display devices by employing a spatial gray scale, which are based on setting a relatively high proportion of picture elements of the display in the light state within a display region which is to represent a light (i.e. low-density) region of the original image, and a high proportion in the dark state in the case of a dark (i.e. high-density) region of the original image. The most well-known method of providing such pseudo-continuous tone reproduction is the dither technique, whereby a step gradation representation of a continuous tone image is reproduced on the basis of numbers of dots within each of predetermined areas of the continuous tone image, by utilizing a dither matrix. Threshold values of the dither matrix are compared with the level of an input signal, one picture element at a time, to thereby execute bi-level image conversion processing. However this method has the disadvantage that the step gradation reproduction characteristic and the resolution of the image that is obtained will both depend directly upon the size of the dither matrix, and have a mutually incompatible relationship. Moreover with the dither technique, it is difficult to avoid the generation of moire patterns, particularly when printed images are reproduced. A method has been proposed in the prior art which is highly effective in overcoming these problems of the dither method. This is the "error diffusion" method, which has been proposed by R. Floyd and L. Steinberg under the title "An Adaptive Algorithm for Spatial Gray Scale", published in the SID 75 Digest, pp 36-37. The basic principles of the proposed method are as follows. To determine whether a picture element of the input signal is to be displayed at the dark or at the light level in the image generated by the bi-level display device, the corresponding level of the input image signal is compared with a threshold value, and a light/dark decision thereby made. The density value (i.e. completely dark or completely light) which is thereby determined for that picture element will in general be in error with respect to the desired density of that element, i.e. with respect to the density of the corresponding picture element in the original image. This error is referred to in the following as the bi-level conversion error. However with the error diffusion method, the effect of this error are substantially reduced by modifying (in accordance with the value of the error) the respective values of desired density to be subsequently used in processing respective ones of a specific set of picture elements which are positioned peripherally adjacent to the object picture element, and have not yet been processed. This modification is performed by apportioning the value of error obtained for that object picture element among these peripheral picture elements, with the error being apportioned in accordance with predetermined fixed factors. Such factors are referred to in the following as apportionment factors. The term "object picture element" as used herein has the significance of a picture element which is currently being processed, to determine a corresponding bi-level display image value, with data being utilized in that processing which have been obtained beforehand during processing of preceding picture elements. The error diffusion method is superior to the dither method with regard to image resolution and the reproduction characteristic for a step gradation source image, and enables the degree of generation of moire patterns to be made very small even when a printed image is being reproduced. However in the case of reproduction of an image which has only small amounts of changes in density, such as a computer-generated image having areas of extremely uniform density, the error diffusion method produces regions of texture in the reproduced image. This texture is inherent to the error diffusion method, and for this reason the error diffusion method has not been widely adopted. The reason for generation of this texture is that a fixed relationship is continuously maintained between an object picture element and the aforementioned set of picture elements which are disposed peripherally adjacent to the object picture element, and fixed values are also maintained for the respective proportions by which the bi-level conversion error of the object picture element is apportioned among these peripheral picture elements. Furthermore, if it is attempted to produce a practical type of digital computation processing circuit for implementing this prior art error diffusion method, it is found that the sum of the aformentioned error apportionment values which are generated during processing of each picture element is not equal to the corresponding bi-level error which is computed for that picture element. This is due to invevitable inaccuracies which arise in the process of computing these error apportionment values, due to discarding of low-significance bits of certain computation results. As a result, the entire bi-level error is not correctly apportioned among the picture elements which are peripherally adjacent to the object picture element, so that all of the possible step gradation density levels of the input signal cannot be properly reproduced in the final image. This is especially true when the input signal level represents a high value or a low value of image density, and hence the range of step gradation image density values which can be reproduced by the prior art error diffusion method is made narrow. It is a further disadvantage of the prior art is that a plurality of respectively different computations must be executed consecutively in order to process each picture element, so that it is difficult to achieve a high speed of image processing. An error diffusion apparatus is described in EP-A-0 174 721. It is an aim of the present invention to provide an image signal processing apparatus which overcomes the disadvantages of the prior art error diffusion method described above, and which provides a display image having high resolution and an excellent step gradation reproduction characteristic, with only very small amounts of moire pattern being produced even when the source image is a printed image, and with no texture being generated even in image regions which are of highly uniform density. The present invention relates to an image signal processing apparatus for receiving an input signal comprising successive digital level values each representing a step gradation density level of a corresponding picture element of a source image, to produce a bi-level image signal by sequentially converting these level values to corresponding bi-level values in respective processing steps, with an error between the bi-level value derived for each of the picture elements and a required density level for that picture element being apportioned among a set of peripherally adjacent picture elements which have not yet been processed. According to the present invention there is provided an image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, including error memory means for holding an accumulated error for an object picture element in one of a set of memory locations respectively corresponding to said object picture element and to a set of picture elements which have not yet been processed and are peripherally adjacent to said object picture element, input correction means for adding an input level value of said object picture element to said accumulated error for said object picture element to thereby produce a corrected input level value, bi-level conversion means for comparing said corrected input level value with a predetermined threshold value to thereby determine a bi-level value for said object picture element, and difference computation means for obtaining a bi-level error which is a difference between said corrected input level value and said bi-level value determined for said object picture element, characterized by: density modulation means for adding to said input level values respective ones of successively generated values which cyclically vary in amplitude and polarity independently of said input level values, to produce successive modulated input level values, and for supplying said modulated input level values to said input correction means said cyclic variation in amplitude and polarity being predetermined such that a sum total of respective amounts of difference between said modulated input level values and corresponding ones of said input level values is zero, as measured over each of respective sets of said input level values which correspond to respective fixed-size 2-dimensional arrays of picture elements; apportionment factor generating means for generating a set of apportionment factors for apportioning said bi-level error among said set of peripherally adjacent set of picture elements, and for altering said set of apportionment factors at predetermined periodic intervals; and error apportionment and updating means for computing error apportionment values respectively corresponding to said set of picture elements which have not yet been processed and are peripherally adjacent to said object picture element, based on said bi-level error and said set of apportionment factors produced from said apportionment factor generating means, adding said error apportionment values to respective values of accumulated error which have been stored previously at locations in said error memory means respectively corresponding to said set of peripheral picture elements to obtain respective addition results, and storing said respective addition results in said error memory means as updated accumulated error values. With the above configuration, the density modulation means functions to superimpose on the density levels of the picture elements of the source image respective density values which vary in amplitude and polarity in a manner which is independent of the picture element density levels. The resultant variations in displayed image density are determined such as to have a total value of zero when computed over each of successively positioned small regions of the displayed image, so that no visible effects are produced as a result of this input density level modulation operation, on the displayed image which is produced using the output bi-level signal from the apparatus. As a result, the generation of a texture pattern in the displayed image is effectively prevented, even in image regions which are of extremely uniform density. The present invention will be further described hereinafter with reference to the following description of an exemplary embodiment, and the accompanying drawings, in which: Fig. 1 is a conceptual block circuit diagram of an embodiment of an image signal processing apparatus according to the present invention; Fig. 2 is a diagram to illustrate an arrangement of sequentially positioned image regions for assistance in describing the embodiment of Fig. 1; and Fig. 3 is a conceptual block circuit diagram of an example of a density modulation section in the embodiment of Fig. 1. Fig. 1 is a general block diagram showing the essential components of a first embodiment of an image signal processing apparatus according to the present invention. Numeral 104 denotes an input terminal to which is applied an input image signal I xy consisting of successive digital values constituting respective level values, each representing a step gradation density level of a corresponding picture element of a source image. The input image signal is derived by successively sampling the source image in units of picture elements, i.e. by successively sampling picture elements along each of a successively selected lines of picture elements, each line being aligned in a direction referred to in the following as the x-direction, the lines being successively arranged along a direction perpendicular thereto (referred to in the following as the y-direction), so that each picture element can be defined by x and y-coordinates. The apparatus functions to produce a a bi-level output value P xy in response to each input level value I xy , from an output terminal 106 by sequentially converting the input level values in respective processing steps. The bi-level output values that are thus produced can each represent either of two values, designated as 0 and R in the following, corresponding to a low and a high density (i.e. light and dark state) respectively of a bi-level display image produced on the basis of these bi-level values. An error between the bi-level value derived for a picture element and the density level which is actually appropriate for that picture element (defined as described hereinafter) is apportioned among a set of peripherally adjacent picture elements which have not yet been processed. In Fig. 1, the display coordinates of an object picture element (i.e. a picture element for which image processing is currently being executed to generate a corresponding bi-level output value) are designated as (x,y). Reference numeral 101 denotes an error memory section, used to store values of accumulated error as described hereinafter. Numeral 103 denotes a location in the error memory section 101 in which is stored the accumulated error value for the object picture element, the object picture element position being indicated by the * symbol, and numeral 102 denotes a storage region of error memory 101 containing a set of memory locations respectively corresponding to positiones of a set of picture elements (designated as A, B, C and D) that have not yet been processed and are positioned adjacent to the periphery of the object picture element. Each picture element is processed during an interval of fixed duration, synchronized with a sync signal. The picture element at position A is processed to generate a corresponding bi-level value immediately following processing of the object picture element, i.e. processing of successive picture elements takes place (in the x-direction) from left to right as seen in Fig. 1, and picture element positions D, C and B will be successively processed during scanning of the next line of picture elements, i.e. after one line shift in the y-direction has occurred. Numeral 105 denotes an adder which amends the input signal level I xy by adding thereto an accumulated error S xy (whose value is determined by a combination of accumulated error values as described hereinafter) to produce a corrected input level I' xy for the object picture element, i.e. I' xy = I xy + S xy . Numeral 107 denotes a terminal to which is applied a threshold level which in this embodiment is equal to R/2, and 108 denotes a device such as a comparator, for executing bi-level conversion of the input level I' xy , by comparing the level I' xy with the fixed threshold value R/2, to produce as output a bi-level value P xy at the R level if I' xy > R/2, and otherwise producing P xy at the 0 level. Numeral 109 denotes a subtractor, for computing a difference between the corrected input level I' xy and the output bi-level value P xy to thereby derive a bi-level conversion error Exy (i.e. E xy = I' xy - P xy ) for the object picture element. The components described above, and their functions, might be envisaged on the basis of the published matter concerning the error diffusion method described hereinabove. It is an inherent disadvantage of the prior art error diffusion method that any regions in the source image which have extremely uniform density will result in corresponding regions of the displayed bi-level image containing a texture pattern. An embodiment of an image signal processing apparatus according to the present invention will be described whereby such a texture pattern can be substantially completely eliminated. This embodiment differs from the prior art with respect to error apportionment and updating section 920, apportionment factor generating section 910, and density modulation section 928, which are described in detail in the following. The density modulation section 928 functions to modulate the input image signal, i.e. the input level I xy , such as to superimpose periodically positioned regions of modulated density upon the density levels of the source image, to produce an input level I' xy . Each of these modulation regions can for example consist of an array of 8 x 8 picture elements, as illustrated in Fig. 2. The function of this modulation of the source image density levels is to impart variations in density to any portions of the source image which are of highly uniform density, prior to bi-level conversion processing, and to thereby eliminate the generation of texture patterns in the resultant bi-level displayed image, which is an inherent feature of the prior art error diffusion method as described hereinabove. However the modulation within each of these regions is both positive and negative-directed, i.e. density is added to and subtracted from respective picture elements within each region, with this density addition and subtraction being executed such that the total resultant change in density within each region is zero. Thus, since each of the modulation regions is of extremely small size by comparison with the overall display image area, there will be no visibly perceptible effect upon the display image produced by utilizing the output bi-level signal from by the apparatus. Fig. 2 illustrates an arrangement of such modulation regions, for the case in which each region consists of a rectangular array of 8 x 8 picture elements. Numeral 940 denotes the set of picture elements of the source image (only part of which are shown), numeral 941 a first modulation region, numeral 951 a modulation region positioned immediately below region 941, and numeral 961 a modulation region positioned immediately to the right of region 941. As shown, each modulation region consists of a rectangular array of picture elements which are respectively numbered from I x0,y0 to I x7,y7 (i.e. by coordinates which define position within each modulation region). Numeral 942 denotes the first picture element I x0,y0 of modulation region 941, numeral 943 denotes picture element I x0,y0 of region 951, and numeral 944 denotes picture element I x0,y0 of modulation region 961. Fig. 3 shows an example of a configuration for the density modulation section 928, in which an address signal generating circuit 962 is coupled to receive an x-direction sync signal (i.e. a signal which is synchronized with successive picture element processing intervals along the x-direction) and a y-direction sync signal (i.e. a signal which is synchronized with selection of successive lines of picture elements, along the y-direction), and produces address signals which are supplied to a memory 963. The memory 963 can be configured as a RAM (random access memory) or a ROM (read-only memory), and has a predetermined set of density values stored therein, for thereby producing output signals representing successive values which respectively vary in amplitude and polarity in response to successive address signals supplied from the address signal generating circuit 962. The values produced from the memory 963 are predetermined such that the sum total of these values within each of the modulation regions is zero. Each of these values is added to an input density level signal I xy in an adder 964, which produces an output density-modulated value I xy ' to be supplied to the adder 105 in Fig. 1. Thus, during processing of the first line of picture elements of the source image, a set of 8 values varying respectively in amplitude and polarity are successively produced from memory 963 and added to successive input level values in the adder 964 corresponding to picture elements I x0,y0 to I x7,y0 of the first modulation region 941, the same set of 8 values are then once more read out from memory 963 for the second modulation region 961, and so on. The same process thereafter occurs, with a different set of 8 values being repetitively read out from memory 963, during processing of the second line of picture elements. When the 9th line of picture elements is reached, the second row of modulation regions begins to be modulated, by the same sequences of output values from the memory 963 as were produced for the first row. It should be noted that it would also be possible to achieve the effects of the address signal generating circuit 962 and the memory 963 by utilizing a shift register circuit. Referring again to Fig. 1, the apportionment factor generating section 910 has stored beforehand therein one set of apportionment factors (i.e. corresponding in number to a set of unprocessed picture elements positioned at the periphery of the object picture element ) and functions to randomly select from that set of apportionment factors a set of apportionment factors K A to K D for respectively apportioning the bi-level error E xy among a plurality of picture element positions A to D within the peripheral picture element region 102, and supplies the selected apportionment factors to the error apportionment and updating section 900. The error apportionment and updating section 900 operates in synchronism with the sync signal 931a (i.e. the x-direction sync signal as described hereinabove) to store in the error memory section 103 the bi-level error E xy of an object picture element (produced from difference computation section 109) in accordance with the apportionment factors K A to K D , at memory locations corresponding to the picture element positions A, C and D respectively within the peripheral picture element region 102. The error apportionment and updating section 920 also reads out accumulated errors S A ', S C ' and S D ' (which have been obtained in previously executed picture element processing operations) corresponding to the picture element positions A, C and B in the peripheral picture element region 102, and derives new accumulated errors S A to S D by using the following equation (1 ): Updating processing is then executed by the error apportionment and updating section 920, by writing the new values of the accumulated errors S A to S D into the error memory section 101 at locations which respectively correspond to the picture element positions A to D. With this embodiment of the invention, due to the fact that level values varying in amplitude and polarity in a manner which is independent of the source image are added to the density levels of respective picture elements of the source image, a texture pattern which is produced in the processed output image when using prior art error diffusion method in the case of a source image containing regions of highly uniform density is effectively suppressed. Since computer-generated images frequently contain regions of extremely uniform image density, this embodiment of the invention provides a practical apparatus for display of computer-generated images by a bi-level display device. In the embodiment of the present invention described above, it is necessary to conscutively execute three basic operations within each picture element processing period (i.e. within each period of the x-direction sync signal). These operations are, (1) Read out the stored accumulated error S xy for the object picture element, and apply this to compensate the output density modulated value I xy for the object picture element, to produce a corrected level value I xy ''. (2) Compare this corrected level value with a threshold level, to determine a bi-level output value P xy for the object picture element, and subtract that output value from the corrected level value I xy '' to obtain a bi-level error value E xy for the object picture element. (3) Utilize this error value E xy to derive respective values of accumulated error for the set of peripherally adjacent picture elements to the object picture element (A, B, C and D). The above three basic operations must be performed consecutively, since a result obtained in each of the basic operations (1) and (2) is required for the succeeding operation. The total time required to successively execute all of these basic operations during each picture element processing period determines the speed at which image processing can be performed by such an apparatus.

Claims: An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, including error memory means (101) for holding an accumulated error (S XY ) for an object picture element in one of a set of memory locations respectively corresponding to said object picture element and to a set of picture elements which have not yet been processed and are peripherally adjacent to said object picture element, input correction means (105) for adding an input level value of said object picture element to said accumulated error (S XY ) for said object picture element to thereby produce a corrected input level value, bi-level conversion means (108) for comparing said corrected input level value with a predetermined threshold value to thereby determine a bi-level value (P xy) for said object picture element, and difference computation means (109) for obtaining a bi-level error (E XY ) which is a difference between said corrected input level value and said bi-level value determined for said object picture element, characterized by: density modulation means (928) for adding to said input level values (I XY ) respective ones of successively generated values which cyclically vary in amplitude and polarity independently of said input level values, to produce successive modulated input level values (I' XY ), and for supplying said modulated input level values to said input correction means (105) said cyclic variation in amplitude and polarity being predetermined such that a sum total of respective amounts of difference between said modulated input level values (I' xy ) and corresponding ones of said input level values (I xy ) is zero, as measured over each of respective sets of said input level values which correspond to respective fixed-size 2-dimensional arrays of picture elements; apportionment factor generating means (910) for generating a set of apportionment factors for apportioning said bi-level error among said set of peripherally adjacent set of picture elements, and for altering said set of apportionment factors at predetermined periodic intervals; and error apportionment and updating means (920) for computing error apportionment values respectively corresponding to said set of picture elements which have not yet been processed and are peripherally adjacent to said object picture element, based on said bi-level error and said set of apportionment factors produced from said apportionment factor generating means, adding said error apportionment values to respective values of accumulated error which have been stored previously at locations in said error memory means respectively corresponding to said set of peripheral picture elements to obtain respective addition results, and storing said respective addition results in said error memory means as updated accumulated error values. An image signal processing apparatus according to claim 1, wherein said density modulation means comprises means (962, 963, 964) for superimposing successive density levels upon respective input level values of successive picture elements by cyclically generating predetermined sets of density levels, each of said sets of density levels including density levels of positive and negative polarity, said sets of density levels being generated in synchronism with successive picture element processing intervals.

Inventors: KUROSAWA TOSHIHARU, MARUYAMA YUJI, NAKAZATO KATSUO, TAKAHASHI KIYOSHI, TSUCHIYA HIROYOSHI, KUROSAWA, TOSHIHARU, MARUYAMA, YUJI, NAKAZATO, KATSUO, TAKAHASHI, KIYOSHI, TSUCHIYA, HIROYOSHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1987
Main classification: H04N1
IPC codes: H04N1