Method for smoothing an image

Abstract: This invention relates to a method for forming an image constituted of a dot matrix and is applied to a printer, such as a laser printer of al LED printer, or to a bit map display unit such as a CRT display, and more particularly to a method for smoothing the image by smoothing stairsteps included in the image to obtain a high resolution. The method comprises steps of dividing a pixel (A, B) into a plurality of subpixels (*1...*3) along a vertical scanning line or a horizontal scanning line, and of reversing a signal for a subpixel (*1...*3) in accordance with a reversion signal to smooth the image as desired. The reversion signal may be provided by subjecting a specific pixel to a boolean operation, a conjunction or disjunction operation, with the adjacent pixels next to the specific pixel. The reversion signal may also be provided by steps comprising: generating, in advance, a set of four bordering data which represents the relationship of a specific pixel with bordering pixels, which bordering data are generated by subjecting the specific pixel and the four bordering pixels to a nonequivalent boolean operation, respectively, detecting the location of the stairstep and recognizing the style of the stairstep referring to the relationship among the bordering data for the pixels located along the vertical or horizontal line, and measuring the length of the level pixels adjoining with the stairstep.

Description: BACKGROUND OF THE INVENTION [Field of the Invention] This invention relates to a method for forming images composed of dots in a matrix used for a printer, a light emitting diode (LED) printer, or in a bit map display unit such as a cathode ray tube (CRT), and more particularly to improve the images by smoothing stairsteps included in the dot images. [Description of the Prior Art] The prior art shows that a laser printer forms a picture element pattern or pixel pattern on a photosensitive drum along its coaxial scanning line in a matrix by scanning repeatedly laser beams modulated with video data sent serially from an image controller. Another prior art teaches that a dot printer such as an LED printer forms a pixel pattern in a matrix on a recording member, a photosensitive drum for example, moving relatively vertically, by means for forming pixels such as an LED disposed along the coaxial scanning line, which is controlled to emit simultaneously in the full line or successively in block after block. In either printer, the pixels are arrayed in an n x m matrix to form characters or pictures. Letters such as O and V consisting of curves or diagonals, therefore, are formed with stairstep edges which damage the quality of the picture. To overcome the defects, in the laser printers which form pixel patterns by scanning the modulated laser beams along the scanning line, attempts for smoothing the stairsteps are disclosed to form a smaller dot size by reducing the output energy for the pixels at the diagonal edges (Laid Open Pat. Apln. 60-139072); or to modulate so as to narrow the width of the output laser beams and to print small dots within the normal dot area. In the LED printers, on the other hand, arrayed with the diodes in a line, the diodes are disposed with a fixed distance along the scanning line. The LED's, further, are arranged in a modular tip with n units of diodes which results in to control of the scanning line with the n units or the full line. It is, therefore in the LED printers, hard to adopt the above techniques which change the dot size and shift the smaller dots. In an LED printer, therefore, which outputs a video image with a line unit, a technique for smoothing stairstep edges is constituted by a method for arranging pixels so that each pixel shifts in every scanning line within a suitable distance to dispose the pixels diagonally along the vertical scanning line (Pat. Publn. 62-24987). In the prior technique, however, as the pixels shift in the full scanning line, it is hard not only to obtain a smoothed character, but also to absorb a steep stairstep even if the elements are arranged by logical adjunction. To overcome the defects above, a technique is disclosed to replace the pixel in question with narrower pixels which are made coordinately narrower either along the horizontal scanning line or the vertical scanning line (Laid Open Pat. Aplln. 2-112966). In the previous prior technique, the replacement of the specific pixel in question is made on the basis of a reference pixel pattern having four compensation sub-cells which are narrower in a coordinate direction. The prior technique, in the LED printer, is able to provide narrower dots in the vertical direction by controlling the emitting time of the LED array. Because the LED is disposed in a fixed distance, the light intensity has to be controlled to make the pixel narrower in the scanning direction. As described above, because the LED's are combined in a tip with a plurality of diodes, it is substantially hard to control the light intensity of individual diodes. U.S. Pat. 4,437,122 shows a smoothing technique in which a center pixel is replaced with a 3 x 3 matrix of subpixels, referring the surrounding pixels. The technique is hard to adopt in LED printers, because an LED can not be divided along the scanning line. Further, the center pixel has to be referred with eight surrounding pixels, and to be matched with the 3 x 3 subpixel combinations, that is, 20 subpixel patterns including their rotation which results in preparing a sophisticated circuit, and in a prolonged processing time. Thus, the technique does not satisfy the needs for faster processing. Each of the prior techniques consists of the steps of firstly appointing the central pixel in question, secondly comparing the configuration of n x m neighboring pixels with many templates prepared in advance, and lastly smoothing the central pixel according to the template pattern matched. To compare the configuration of the neighbouring pixels means that one has to prepare a lot of templates corresponding to the combinations in the n x m matrix. Further, there is a meaningful stairstep to figure such corner as in letters E and F, which corner must be kept sharp. To achieve effective smoothing and not to smooth the meaningful stairsteps, one has to prepare other templates for the stairsteps not to be subjected to the procedures which results in providing another memory for the meaningful stairsteps, and in constructing more sophisticated circuits. The large scale of the circuit and the big amount of work for matching delay the process and thus, fail to satisfy the requirement for faster processing. Further, EP-A-0 393 980 discloses an image printing method in which an image is composed of a pixel matrix wherein a plurality of dots are printed in each pixel, the black and white dot information expressing the print density degrees. SUMMARY OF THE INVENTION Considering the defects of prior techniques, it is an object of the present invention to provide an image smoothing method which is able to smooth images easily and accurately without a sophisticated circuit, and with a simple processing. It is a further object of the present invention to provide an image smoothing method which is able to smooth images, and also to improve pixels acceptable for enlargement of the smoothed data with enhanced resolution. It is a further object of the present invention to provide an image smoothing method which is able to accomplish the objects, usable with a linear dot printer such as an LED printer. These objects are met by a method according to claim 1 and a method according to claim 6. Preferred embodiments are disclosed in the depending claims. The method includes dividing each pixel into P subpixels in one scanning direction following the teachings of Pat. Publn. 62-26626, Laid Open Pat. Appln. 60-134660 and so forth, and further, providing a subpixel signal for each subpixel, either an "on" signal for a dot, or black pixel, or an "off" signal for a blank, unprinted pixel area. Switching between on and off signal values is referred to as reversing the signal. The present embodiment will be described in detail referring to attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 are explanatory views for describing the image smoothing procedure of the present invention, in which Figs. 1(a) and (b) illustrates the basic concept of the procedure; and Fig. 2(a) and (b) illustrate a modified procedure. Figs. 3 through 5 are explanatory views for describing first to third embodiments of the image smoothing procedure of the present invention, in which Fig. 3 illustrates the procedure reversing especially subpixel(s) of white pixel alone; Fig. 4 illustrates the procedure reversing subpixel(s) of both white and black pixels; and Fig. 5 illustrates another modified procedure reversing subpixel(s) next to the specific pixel. Figs. 6 through 9B are explanatory views for describing a fourth embodiment of the present invention; in which Fig. 6 illustrates the procedure to generate bordering data and those examples; Fig 7A illustrates types of horizontal stairsteps and those bordering data; Fig. 7B illustrates types of vertical stairsteps and those bordering data; Fig. 8 illustrates reversion data tables for pixels divided vertically or horizontally into subpixels, respectively; Figs. 9A and 9B are views for describing a practical procedure of the present invention applied for an LED printer; in which Fig. 9A illustrates the specification of the type of stairstep, and to count the length of level pixels; and Fig. 9B illustrates to apportion the reversion code to respective pixels according to the type of stairstep and the length of level pixels referring to a reversion code table, and which subpixels of a stairstep image are reversed to smooth the stairsteps. Figs. 10 and 11 are explanatory views for describing an LED printer to which the present invention is applied; in which Fig. 10 illustrates a circuit block diagram of the printer head of an LED printer; and Fig. 11 illustrates the output pixels formed by the circuit. Fig. 12 is an illustration describing a circuit block diagram for the first image smoothing procedure. Figs. 13 and 14 are illustrations describing circuit block diagrams for the first to third embodiments of the image smoothing procedure; in which Fig. 13 illustrates the whole circuit block diagram; and Fig. 14 illustrates a detailed circuit block diagram for apportioning the reversion data, and reversion of the subpixels. Figs. 15 through 17 are illustrations describing circuit block diagrams for the fourth embodiment of the image smoothing procedure; among which Fig. 15 illustrates the whole circuit block diagram; Fig. 16 illustrates a detailed circuit block diagram for generating the bordering data; and Fig. 17 illustrates detailed circuit block diagrams for boolean logic unit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figs. 1 and 2, present a method for smoothing an image by the subpixel method for a line dot printer, an LED printer for example, of which picture elements are arrayed along the scanning line. A pixel is constituted by a number of subpixels P=3 or more, preferably 5 as shown in Fig. 1, either prior to or without image smoothing, in which a smaller number N (N<P) of subpixels are serially generated by subpixel signals to form the pixel for the full line or a unit of n bits to be output; and in the stage of image smoothing, the subpixel signals are applied not only to said subpixels, but also to other pixel or pixels to reverse its signal(s) referring the specific pixel in question with the preceding and succeeding pixels to form the output pixel. The smoothing procedure, as shown in Fig. 1, is made by reversing the subpixel signal(s) so that the black pixel(s) seemingly may be added or reduced vertically within the specific pixel; or as shown in Fig. 2(a), so that the black pixels seemingly may be shifted vertically to the next pixels located on the preceding or succeeding lines, or beyond the specific pixel; or as shown in Fig. 2(b), so that the black pixel(s) seemingly may be reduced vertically within the specific pixel, or may be enlarged so that the pixel(s) seemingly shift to the next pixel to form a bold pattern. The smoothing procedure, as shown in Figs. 1 and 2, comprises the following steps. Supposing the printer outputs a pixel with 5 subpixels P1 through P5 divided vertically as seen in Fig. 1(a), the normal pixel b2 consists of 3 subpixels in the middle as a black dot, the upper and bottom subpixels remaining as blank or white pixels. The 3 subpixels among 5 subpixels will be supplied with on subpixel signals, and be considered as a 100% intensity pixel. To achieve the image smoothing following the pixel information next to the specific pixel, the signals for subpixels P1 and P4 may be reversed to shift seemingly the black dot close to the preceding pixel as in b1, or subpixels P2 and P5 may be reversed to shift seemingly the black dot close to the succeeding pixel as in b3. Thus, 3 dot patterns b1, b2 and b3 are available. If the reversion of subpixel P2 alone, or subpixels P2 and P4 takes place, reduced pixels b4 (67% intensity) and b5 (33% intensity) are obtainable. The arrangement of subpixels b1 through b5 allows to print enhanced characters with a smoothed stairsteps for diagonal or curved lines. For the letter X, for example, it is possible to avoid a bold crossing, if the pixel b4 (67% intensity) is chosen for the central crossing. The instance above shows an example where P is taken as 5, and N is taken as 3 which provides vacant subpixels at the top and bottom. The vacant subpixels allow the image smoothing procedure with reversion. The vacant subpixels, on the other hand, make the pixel thin when the pixel does not need to be subjected to the image smoothing. Further, the vacant subpixels provide a clearance between preceding and succeeding pixels, which clearance prohibits to paint all over a certain area. The clearance, in some cases, makes the pixels vertically apart, and makes characters coarse which may interfere with the image smoothing. In the events above, the reversion of subpixels may not be limited to within a single pixel, but may be extended to the pixels adjoining vertically to the specific pixel as shown on Fig. 2(a). Up to two bottom subpixels of a pixel above a subject pixel may be reversed to seemingly extend across the pixel boundary (b6), or up to two super subpixels of a pixel below the subject pixel may be reversed to seemingly extend across the pixel boundary (b12), while other 4 pixels (b6, b7, b11 and b12) provide a dense and fine image smoothing. Further, it is worthwhile to expand, by reversing any subpixel(s) of the pixels above, the pixels up to 35 patterns in all including such patterns as 33% dots (b13, b14), 67% dots (b15, b16), 100% dots (b17 or b6, b18), 133% dots (b19, b20), and 167% dots (b21, b22). Figs. 3 through 5 show specific procedures for image smoothing for the apparatus which adopts the reversion process described above. According to these procedures, a specific pixel next to a boundary where a pixel changes from white to black, or from black to white along a desired scanning line, and the reference pixels next to the specific pixel on the preceding or succeeding scanning line are subjected to boolean operations, conjunction and disjunction (AND and OR), to smooth the specific pixel, the reference pixel(s) and adjacent pixel(s) thereof. The specific pixel next to the boundary, therefore, is not limited to a black pixel, but also includes a white pixel. The present invention also includes the case that the number of subpixels P in a pixel is equal to the number of the normal subpixels N in the subpixels P, and the case previously described that N < P as well. Fig. 3 shows a first procedure for replacing black pixel(s); in which Fig. 3(a) shows a stairstep to be smoothed, and Fig. 3(b) smoothed pixels. The specific pixel A is detected where a series of white pixels (0) turn to a black pixel (1). The specific pixel A is subjected to a conjunction operation (AND operation) with reference pixels E and F next to the specific pixel A and located on a preceding or succeeding scanning line to assign subpixels *1 and *2 to be reversed. A subpixel *1' is produced simultaneously with the subpixel *1. Following the steps above, the specific pixel F is detected where a series of black pixels (1) turn to a white pixel (0). The specific pixel F is subjected to a conjunction operation with the reference pixels A and G next to the specific pixel F located on a preceding or succeeding scanning line to decide subpixels *3 and *4 to be reversed. A subpixel *4' is produced simultaneously with the subpixel *4. The boolean operation is described as follow: *1 : A (0→1)∧E∧F *2 : A (0→1)∧E∧F *3 : F (1→0)∧A∧G *4 : F (1→0)∧A∧G Fig. 4 shows a second procedure of the present invention; in which Fig. 4(a) shows stairsteps to be smoothed, and Fig. 4(b) smoothed pixels. Following the steps above, the specific pixels A and A' are detected where a series of white pixels (0) turn to a black pixel (1). The specific pixels A and A' are subjected conjunction operation with the reference pixels B and B' next to the specific pixel A and A' and located on a preceding or succeeding scanning line to assign subpixels *2 to be reversed. Correspondingly, the specific pixels B and B' are detected where a series of black pixels (1) turn to a white pixel (0). The specific pixels B and B' are subjected to a conjunction operation with the reference pixels A, A' and D next to the specific pixels B and B' located at a preceding, succeeding or the same scanning line to assign subpixels *3 to be reversed from black to white subpixels. The boolean operation is described as follow: *1 : A, A' (0 →1)∧B *2 : B, B' (1→0)∧A (A') *3 : B, B' (1→0)∧D (A)∧A (A') Fig. 5, further, shows a third procedure of the present invention which reverses subpixels a2 to g2 located at subpixel lines P13 and P14 of the lower subpixels on the same scanning line I, and subpixels c through g located at subpixel lines P21 and P22 of the upper subpixels on the succeeding scanning line II. The specific pixel A is detected where a series of white pixels (0) turn to a black pixel (1) on the same scanning line I. The specific pixel A is subjected to conjunction operation with the reference pixels E and F next to the specific pixel A located on the succeeding scanning line II. The specific pixel C is detected where a series of black pixels (1) turn to a white pixel (0) on the same scanning line I. The specific pixel C is subjected to conjunction operation with the reference pixels G and H next to the specific pixel C located at succeeding scanning line II to provide subpixels to be reversed, preceding subpixel b1 and succeeding subpixel b2 on the subpixel line P13, and 2 bits of subpixels on the line P14, preceding subpixels a1 and a2, and succeeding subpixels g1 and g2. Similarly, the specific pixels F and G are detected on the succeeding scanning line II. The specific pixels F and G are subjected to conjunction operation with the reference pixels A, B, and B, C respectively next to the specific pixels F and G located on the scanning line I to provide subpixels to be reversed, one each bit of subpixels d and f at the subpixel line P22, 2 each bits of subpixels c, c/e, and c/e, g at the subpixel line P21 respectively. Since the subpixels c and e overlap, they are subjected to the conjunction operation twice, that is they are subjected to a disjunction operation (OR operation) to reverse the subpixel c/e. Following the present invention, firstly, a specific pixel is detected, where a series of white (black) pixels turn to a black (white) pixel on the scanning line, and, secondly, is subjected to a boolean operation, conjunction or disjunction, with reference pixels next to the specific pixel located at a preceding or succeeding scanning line to produce subpixel(s) to be reversed. Thus, the image smoothing is achieved easily with a simple boolean operation under simple instructions, to provide higher resolution. The reversion process is effective especially for an LED printer having a writing head arrayed in a line. A fourth procedure of the present invention is shown on Figs. 6 through 9B for smoothing an image providing boundary data which specify the relationship between every pixel and pixels having a common boundary. According to the prior techniques described above, a specific pixel is smoothed by matching the n x m neighboring pixels with a number of patterns prepared in advance. The first to third procedures of the present invention detect the specific pixel, and then the specific pixel is subjected to the boolean operation, conjunction or disjunction, to reverse subpixels for smoothing. In the fourth procedure of the present invention shown in Figs. 6 through 9B, the reversion process is similar to the preceding procedures of the present invention; firstly to provide bordering data which specify the relationship with four pixels having a common horizontal or vertical boundary (called as bordering pixels, hereinafter) for all pixels forming the image pattern; secondly, to recognize the location and type of stairstep subjecting to boolean operation with bordering data to find out a certain relationship; thirdly, to count the length of level pixels; and finally, to smooth the pixels by reversing subpixels using those data. This procedure of the present invention is quite novel for the image smoothing which provides the bordering data for every pixel specifying the relationship with bordering pixels, without firstly identifying the specific pixel nor the reference pixels. For providing the bordering data, all pixels forming the image pattern are subjected to a boolean operation (preferably a nonequivalence operation, or exclusive-OR), with the four bordering pixels next to the specific pixel horizontally and vertically, to obtain four bits of binary data for every pixel. It is, further, preferable, but not limited thereto, for recognizing the stairstep, that a pair of bordering data of adjoining pixels located on the horizontal or vertical line with a common boundary are extracted to verify if there is a stairstep between the pixels, or whether or not three equations described below are satisfied, and if so, to classify what type of the stairstep is to be modified. Subpixels to be reversed are determined from the length of level pixels counted, which is preferably used for selecting a reversion code specifying which of the subpixel(s) is reversed. The reversion code table shows that the reversion code, 1 or 2, is apportioned for each pixel apart from the stairstep, and for each stairsteps having the length of level pixels. The reversion code table offers a simpler circuit for faster processing with higher accuracy. This procedure of the invention will be described as follows: 1) Generating Bordering Data The pixels on the (n-1)th, (n)th and (n+1)th lines, as shown in Fig. 6(a), are shifted to be read one after the other. As shown on Fig. 6(b), a specific pixel Bn, for example, is subjected to nonequivalent or exclusive-OR (E-OR in short) operation with the bordering pixels, An, Bn-1, Cn and Bn+1, having a common vertical or horizontal border line with the pixel Bn, which generate 4 bits of binary values denoted as do, d1, d2 and d3. The boolean values, do, d1, d2, d3, will be generated for all pixels, a through g, for example as shown on Fig. 6(c). The E-OR boolean value is "1" if the two operands (pixels) differs from each other (white-black, or black-white pixels), and otherwise, are "0" for the same operands (white-white, or black-black pixels). It is notable, therefore, that it makes no difference if a pixel is a dot or blank, but the value tells the relationship between the pixels. Taking f and g as an example, though the pixels are of different colors, white for f, and black for g, the values, d0, d1, d2, d3, for the pixels are the same (0, 0, 0, 0), because each pixel f or g has the same value as its respective bordering pixel. That is, the value tells us the relationship with bordering pixels. 2) Detecting the Stairstep and Recognizing the Type To detect a stairstep, at least three sets of the bordering data for the adjoining pixels on a horizontal or vertical line are extracted, and it is verified if the data satisfy the Equs. 1) through 3). If so, a determination is made of the type of stairstep represented by the data. The types of stairsteps to be recognized are illustrated in Figs. 7A and 7B, types (1) to (4), and (5) to (8), respectively. The boolean values, d0 through d3, for the pixels beyond the concerned pixel line, for the pixels on the (n+1 or n+2)th line for example, are not extracted for the stairstep detection. Because the relationship with adjoining pixels on the same line is used to detect the stairstep, and because the verification of the bordering data of pixels on the concerned line results in considering the states of pixels on three lines which includes the upper and lower lines, the bordering data for one line, therefore, are enough to detect the stairstep. As this procedure of the present invention generates firstly the bordering data as for the relationship with the bordering pixels, and detects the stairstep together with its type by the relationship in the bordering data of adjoining pixels on the same horizontal or vertical line, the present invention is able to smooth the images with a simpler circuit, faster processing, and higher resolution than the prior techniques which must match the n x m matrix data with a number of patterns. The boolean operation for detecting the stairstep is, firstly, performed to extract a pair of bordering data for the pixels adjoining horizontally or vertically, denoted d0A, ..., d3A and d0B, ..., d3B, and assuming that (α and β) is any one of the combinations among (0, 1), (1, 2), (2, 3) and (3, 0), if (dαA, dαB) = (dβA, dβB) = (0, 1), and assuming τ or δ are any other numerals designating the bordering data for the pixel A taken in Equation 1), dτA = dδA = 1. Secondly, the operation is subjected to extract the bordering data for the pixel C, which is opposite to the pixel B next to the pixel A, and assuming τ or δ are any other numerals taken in the Equation 1), and to see if (dτC, dδC) = (1, 0) or (0, 1). If the relationship among the bordering data for the three pixels, A, B, and C, satisfy simultaneously at least the three equations, Equs. 1), 2) and 3), the boundary between A and B is deciphered as a stairstep. In the case of letters E or F, therefore, two or more adjacent dots form a rectangular pattern which will not be erroneously recognized as a stairstep. Further, as seen in Fig. 9B (A), the stairstep (a) is not deemed a stairstep, because there is no level pixel succeeding the stairstep. The stairstep (a), therefore, will not be subjected to the smoothing procedure. Therefore, there is no need to consider beyond the types as shown on Figs. 7A and 7B, which will be four types for each direction of horizontal and vertical ways, totaling to eight types. The detecting and the recognizing procedures are so simple to subject four bits of binary values of the bordering data for the adjoining pixels to analyze relationship therebetween by the boolean operation that the procedures are able to be treated with a simple circuit and processing without worrying about the color of the pixels, white or black, without handling in a block unit as is done in the prior techniques. 3) Assigning Reversion Code The length of level pixels is counted. One of the reversion codes, 1 or 2, as show on Fig 9B (A), is assigned for each pixels referring to the distance from the stairstep, and the length of level pixels. To count the length, as shown on Fig. 9B (A), the level pixels succeeding or preceding the stairstep are counted. The pixel at the stairstep is not included in the length. The length for the stairstep (e): L = 1; for the stairsteps (c) and (b): L = 4; for the stairstep (d): L > 5; and the stairstep (a): L = 0 which is not subjected to the smoothing procedure. Referring to the reversion code table shown on Fig. 9B (A), for the stairstep (d) of which the length of level pixel is more than five, the reversion code for the pixel at the stairstep for which the distance from the stairstep is zero is assigned as 2, and the reversion code for the pixels located at the distance 1 pixel and 2 pixels from the stairstep are respectively assigned as 1. 4) Assigning of Subpixel(s) to be reversed Referring to the reversion data table (a) for a laser beam printer (LBP in short), and table (b) for LED shown in Fig. 8, reversion data is selected as per the stairstep type and the reversion code. The reason why the tables are provided is to collect the cases as per the reversion data, without generating the reversion data for each stairstep type and reversion code, to make the circuit simple. The reversion data illustrates which subpixel(s) will be reversed. The LBP, which is able to divide the beam along the scanning line, takes the reversion data of which subpixels are divided vertically as shown on Table (a) of Fig. 8. The horizontal stairstep types (1) through (4) will be collected in a category for each reversion code, 1 and 2. And vertical stairstep types (5), (7) and (6), (8) will be collected in other categories for each reversion code. The table (a) illustrates six reversion data for three categories of stairstep types, and for each reversion code. The hatched subpixel(s) are the subpixel(s) to be reversed. The reversion data table (b) is prepared for an LED printer in which a pixel can be divided into horizontal subpixels along the vertical scanning line. The eight types of the stairsteps drop in three categories for each reversion code, totaling six reversion data, as well. The tables show that in the stairstep types (1) or (3) for the LED printer, for example, if the pixel is assigned the reversion code 1, the bottom subpixel of the pixel will be reversed, and if the code is 2, the lower two subpixels will be reversed, and so forth. 5) Reversion of Subpixel Following the signal obtained by the reversion data table, the subpixel(s) of the pixel are reversed to form the output video data. In the LED printer, for example, which output the image repeatedly four times for a pixel according to the video data, it is quite enough to reverse the subpixels corresponding to the reversion data just before delivering the video data to the printer, which results in smoothing with a simple smoothing circuit, and in a shorter processing time than with known, complex circuits. Fig 10 is an explanatory view illustrating a circuit block diagram of the printer head of an LED printer for implementing the present invention, in which the circuit receives n bits of pixels of images to be delivered sequentially in time sharing attitude to the LED head elements; reference numeral 1 denotes an LED array composed of a plurality of LED tips 1A, 1B, ..., 1M arranged in a line. Each tip consists of n bits of LED elements 1a, numeral 10 denotes driving integrated circuits (IC's) for the LED array 1, which driving IC's include a shift register 11 having a storage capacity corresponding to the number of LED elements n bits, a latch 12, and a switch 13 having switch elements corresponded to the number of LED elements n bits, wherein switch elements and the LED elements are connected with a printed wiring in a matrix formation. Reference numeral 4 denotes a block apportioner which shifts sequentially the connection between the switch 13 and the LED tips 1A, ..., 1M on receiving every n bits of pixel data in the shift register 11. During operation of the circuit, firstly, the shift register 11 receives and stores serially the first n bits of video data of the pixels from an image smoothing control 53 (Fig. 12(a)) on receiving a clock signal, upon receiving a latch signal from a control 5, the latch 12 receives and holds the n bits pixel data signals in parallel, and then transfers the signals to the switch 13 for delivery to the LED elements la of the LED tip 1A. Secondly, the shift register 11 receives and stores the second n bits pixel data following to transfer of the first data bits to the latch 12, the latch 12 receives the second n bits data upon receiving the latch signal from the control 5, and a block signal is sent from the control 5 to the block apportioner 4 to indicate that the latch 12 is ready to transfer to the switch 13, then the block apportioner 4 shifts the connection to the next LED tip 1B. The circuit, thus, outputs pixel data repeatedly m times up to the LED tip 1M to cover the full length of a scanning line. In the same manner, the circuit controls each of LED tips 1A, ..., 1M (m x 4) or (m x 5) times for the scanning line divided into 4 or 5 subpixels to develop the image as shown in Fig. 11. Fig. 12 is an illustration describing a circuit block diagram for the first image smoothing procedure. The circuit is connected with an output terminal of a video memory (not illustrated) which stores the unfolded image data, and is constituted to receive the video data for a plurality of scanning lines from the video memory, to subject the video data to smoothing stairsteps, and to send the data to the shift register 11 of the LED head circuit shown in Fig. 10. The circuit consists of a pattern memory 51, a decoder 52, and an image smoothing control 53. The pattern memory 51 receives serially the unfolded video data for the scanning line on receiving a video clock signal, and stores the specific video data ready for transferring to the shift register 11 together with the video data for preceding and succeeding scanning line(s). The decoder 52 includes a boolean operation unit. The video data in the pattern memory 51 adjacent to the specific pixel, or more particularly, in a matrix RX1 consisting of R pixels in the horizontal scanning direction and X pixels in the vertical scanning direction, are subjected to logical conjunction on receipt of the SL signal from the control 53; and some of the subpixels are reversed as desired to form the output subpixel data. The smoothed data subjected to the reversion procedure are transferred upon receiving the standard clock signal to the n bits shift register 11 of the LED head circuit shown on Fig. 10. In every transferring of the subpixel data to the shift register 11, the decoder 52 on receipt of the standard clock signal subjects the subpixel data in the pattern memory 51 to reversal as desired to smooth the image, shifting the RXn matrix sequentially one after the other, while shifting the specific subpixels *1, *2, ..., *n. The reversion procedure is repeated up to (m)th RXm matrix for the first line P1, and then returns to the first n pixels of the second subpixel line P2, and so forth, until the fifth subpixels line P5 to accomplish the smoothing procedure for the first pixel line as shown Figs. 1 and 2. Figs. 13 and 14 are illustrations of circuit block diagrams for the image smoothing according to an embodiment of the present invention for the second and third image smoothing procedure. Referring to Fig. 13 illustrating the whole circuit block diagram, video data (VDATA) unfolded in an image random access memory (RAM) (not illustrated) are transferred serially to a shift register 104 synchronized with a video clock (VCLK) signal generated in a VCLK 101, then, the VDATA are converted into N+1 bits of parallel data with the shift register 104, and the VDATA are stored in the first address of the first bank of a static RAM (SRAM) 105. The succeeding N+1 bits of converted parallel data are stored in the second address, repeating the same until the VDATA for the first scanning line are stored in the first bank of the SRAM 105. Repeating the same procedure, VDATA for the second line are stored in the second bank, VDATA for the third line are stored in the third bank, and so forth. All of VDATA for the N lines are stored in the 1st to (N)th banks of the SRAM 105. During the time for preparation of VDATA for the (N+1)th line to be stored in the first address of the (N+1)th bank of the SRAM 105, an SRAM control 103 reads sequentially the VDATA stored in the first address of the 1st to (N)th bank on receiving a latch signal, and stores that data in latches of latch & shift circuits 106, ..., 112. Following a signal from the SRAM control 103, the VDATA stored in the latches of the latch & shift circuits 106, ..., 112 are loaded into shift registers of the latch & shift circuits 106, ..., 112. Repeating the same procedure, VDATA for the (N+1)th line are stored in the (N+1)th bank of the SRAM 105 synchronized with VCLK, and simultaneously to the above, VDATA for the 1st to (N)th lines stored in the SRAM 105 are transferred to latches of the latch & shift circuits 106, ..., 112, and then, VDATA in the shift register of the latch & shift circuits 106, ..., 112 are loaded sequentially in an N line shift register 113. When the SRAM 105 has stored the VDATA for the (N+1)th line in the (N+1)th bank, the SRAM 105 stores, following signals from the SRAM control 103, VDATA for the (N+2)th line in the first bank, VDATA for the (N+3)th line in the second bank, and so on, until the 1st to (N)th banks are updated. Finally, a pixel map (N x 7) consisting of preceding and succeeding adjacent line(s), and 7 pixels including each three preceding and succeeding adjacent pixels is arrayed and stored in the N line shift register 113, as the VDATA transferred from the SRAM 105 updates sequentially the pixel map. Thus, with the pixel map, the image smoothing procedure in an image smoothing unit 120 is ready for boolean operation. The image smoothing unit consists of selectors 121A and 121B for selecting the specific pixel, an AND logic 123 composed of AND logics 122A and 122B, a line counter 124 for selecting a subpixel line of the pixel, delay units 125A and 125B which subject vacant subpixel(s) to operate reversion, if there is a vacant subpixel(s) between the specific subpixel and the subpixel reversed by the boolean operation, a line counter for selecting the subpixel line, and an OR gate 127 and an AND gate 128 which subject the subpixel reversed to black to disjunction, or the subpixel reversed to white to conjunction, wherein the subpixel VDATA passed through the gates are sent serially to the LED head circuit. Referring to Fig. 14, the procedure of the image smoothing unit 120 will be described. Firstly, the N line shift register 113 detects the specific pixel A where there is a shift from white pixels (0) to a black pixel (1); secondly, following line counter signals i through iv, the selectors 121A and 121B select reference pixel(s) bordering the specific pixel among the pixels B through M on the preceding scanning line P-1 and succeeding scanning line P+1, or in other words, select the reference pixel(s) among the bordering pixels B through G during the line counter signals i and ii, and among the bordering pixels H through M during the line counter signals iii and iv. The specific and reference pixels are subjected to conjunction operation with the AND logics 122A and 122B to generate reversion signals as desired. If necessary as described above, with the reversion signal generated with the AND logics 122A and 122B, the delays 125A and 125B generate another reversion signal to form 2 bits sequential reversion signals to fill the vacant pixel. Thus arranged subpixel signals are sent to the OR gate 127 and the AND gate 128 through the OR units 129A and 129B, wherein the arranged subpixel signals together with the video signals of the specific subpixel from the N line shift register 113 are subjected to boolean operation, disjunction in the OR gate 127 or conjunction in the AND gate 128, to achieve the image smoothing procedure. The smoothed subpixel VDATA are transferred serially up to the shift register 11 of the LED head array 1 shown on Fig. 10. According to the embodiment of the invention, therefore, the procedure is so constituted to subject the specific and reference subpixel to conjunction operation that the image smoothing procedure can be done without a sophisticated circuit, with a simple processing, and with finer resolution. Figs. 15 through 17 are illustrations of circuit block diagrams for the image smoothing describing an embodiment of the present invention for the fourth image smoothing procedure shown on Figs. 6 through 9B; in which Fig. 15 illustrates the whole circuit block diagram; Fig. 16 illustrates a detailed circuit block diagram for the bordering data generator 30 shown on Fig. 15; and Fig. 17 illustrates detailed circuit block diagrams for the boolean logic unit 40 shown on Fig. 15. Referring to Fig. 15, VDATA unfolded in an image RAM (not illustrated) are transferred serially to a shift register 24 synchronized with a VCLK signal, the VDATA are converted into parallel data in the shift register 24, and the VDATA for a full line of the first scanning line are stored in the first bank of a SRAM memory 25. Repeating sequentially the same procedure, VDATA for the second line are stored in the second bank of the memory 25, VDATA for the third line are stored in the third bank of the memory 25, and so forth, wherein all of the VDATA for the N lines are stored in the 1st through (N)th banks of the memory 25. During the time for preparation of VDATA for the (N+1)th line to be stored in the first address of the (N+1)th bank of the memory 25, a memory control 23 reads sequentially the VDATA stored in the first address of the 1st through (N)th banks on receiving a latch signal, and stores the data in latches of latch & shift registers 26A, ..., 26N. Following signals from the memory control 23, the VDATA stored in the latches of the latch & shift registers 26A, ..., 26N are loaded into shift registers of the latch & shift registers 26A, ..., 26N. Repeating the same procedure, VDATA for the (N+1)th line are stored in the (N+1)th bank of the memory 25 synchronized with VCLK, and simultaneously to the above, VDATA for the 1st through (N)th lines stored in the memory 25 are transferred to latches of the latch & shift registers 26A, ..., 26N, and then, the VDATA in shift registers of the latch & shift registers 26A, ..., 26N are loaded sequentially in a bordering data generator 30 synchronized with VCLK. The bordering data generator 30, as shown on Fig. 16, consists of N pairs of bistable triggers or flip-flops F/F311, ..., F/F31N, F/F321, ..., F/F32N which shift the VDATA loaded from the latch & shift registers 26A, .., 26N following the VCLK, and X-OR gates 33 (331, ..., 33N) which subject the specific pixel and the bordering pixels to nonequivalence operation, or exclusive OR (E-OR in short) operation as shown on Fig. 6(a). The procedure of the generator 30 will be that the VDATA loaded in the F/F312 of the first row are transferred to the F/F322 of the second row upon receiving each signal of the VCLK signals. The VCLK signals act on F/F's simultaneously to transfer the VDATA in the F/F322 to the X-OR gate 332B, as well. The potentials, therefore, in front of the F/F312, between the F/F312 and the F/F322, and at rear of the F/F322 correspond to the signals or logic values of preceding, the specific, and the succeeding pixels, that is, represent the states of A2, B2 and C2 pixels shown in Fig. 6(a). The output signal XV1 of the X-OR gate 332V corresponds to the bordering data d1, a nonequivalence value with the pixel B1. The output signal XN1 of the X-OR gate 332N corresponds to the bordering data d0, a nonequivalence value with the pixel A2. Further, the output signal XB1 of the X-OR gate 332B corresponds to the bordering data d2, a nonequivalence value with the pixel C2. And lastly, the output signal XV2 of the X-OR gate 333V corresponds to the bordering data d3, a nonequivalence value with the pixel B3. Thus, the four X-OR gates 332N, 332V, 332B and 333V generate four bits of the bordering data d0, d1, d2 and d3, the nonequivalence values between the specific pixel and the four bordering pixels. Because N pairs of bistable triggers F/F311, .., F/F32N are constituted in the generator 30, the four bits of bordering data d0, d1, d2 and d3 are generated for N vertical pixel lines, and are transferred to the shift register 35 sequentially on each signal of VCLK. The four bits of bordering data stored in the shift register 35 are shifted to the logic unit 40 together with the information of the specific pixel from the latch & shift register 26C to be subjected to the image smoothing procedure. Fig. 17 shows detailed block diagrams of the logic unit 40, which includes a horizontal logic unit 41, a vertical logic unit 42 and a reversion shift register 43. The horizontal logic unit 41 receives any desired lines, 7 lines for example, of the bordering data d0, d1, d2, d3 stored in the shift register 35 for smoothing the stairstep types (1) to (4) shown on Fig. 7A, while the horizontal logic unit 42 receives the desired rows, 7 rows for example, of the bordering data for smoothing the stairstep types (5) to (8) shown on Fig. 7B. The reversion shift register 43 reverses the subpixels corresponding to the information generated by the logic units 41 and 42. The horizontal logic unit 41 further includes: a first stairstep detector 44 for detecting the stairsteps (1) and (2) shown in Fig. 7A; a second stairstep detector 45 for detecting the stairsteps (3) and (4); a reversion code detector 46 for measuring the length of level pixels with respect to the detected stairstep, and for assigning pixel(s) with its reversion code referring to the reversion code table; and a reversion decoder 47 for assigning subpixel(s) to be reversed referring to the reversion data table. The reversion decoder 47 has been installed with the reversion data table shown on Fig. 8(b) made of memories or random logics. The reversion decoder 47, hence, as previously described, is able to refer to the table with the type of stairstep ((1) or to (8)), and the reversion code (1 or 2) for assigning which of the subpixel(s) will be reversed. Similar to the horizontal logic unit 41, the vertical logic unit 42 consists of a first stairstep detector 44' for detecting stairsteps (5) and (6), a second stairstep detector 45' for detecting stairsteps (7) and (8), a reversion code detector 46', and a reversion decoder 47', for which no further detailed procedures are described, as it works similarly to the horizontal logic unit 41. From the latch & shift register 26C, the reversion shift register 43 receives serially the VDATA for one scanning line for four times repeatedly corresponding to the pixel division number of (four) subpixels. The VDATA stored in the reversion shift register 43 are reversed with the signals corresponding to the VDATA pixels from the logic units 41 and 42 for reversion as desired to be transferred to the LED head. Referring to Figs. 9A and 9B, the reversion procedure will be described as follow: The VDATA shown in Fig. 9A (A) are transferred from the latch & shift registers 26A, ..., 26N into the bordering data generator 30 which generates the bordering data d0, d1, d2, d3 to be stored in the shift register 35. The logic unit 40 receives the bordering data d0, d1, d2, d3 stored in the shift register 35, and the corresponding pixels to be modified from one of the latch & shift registers, 26C for example. The stairstep and its type are detected with the stairstep detector 44, 45 analyzing the bordering data d0, d1, d2, d3. In the event when the stairstep(s) is detected, a reversion code detector 46 measures the length of level pixels as shown on Fig. 9A (C), and then assigns the pixel(s) with the reversion code referring to the reversion code table with the type of stairstep and the length of level pixel as shown on 9B (A). On receiving the type of stairstep and the reversion code, the reversion decoder 47 assigns which part of the subpixel(s) will be reversed referring to the reversion data table, and then outputs the reversion signals to the reversion shift register 43 as shown on Fig. 9B (B). Lastly, the pixels in the reversion shift register 43 are subjected to reversion procedure conducted with the reversion signals from the reversion decoder 47 as shown on Fig 9B (C).

Claims: A method for smoothing an image composed of a matrix of pixels, said matrix having rows of pixels extending in a first direction and columns of pixels extending in a second direction perpendicular to the first direction, each pixel being divided into a plurality of subpixels, said subpixels being offset from one another in one direction and having the same dimension as the pixel in the other direction, wherein the image is formed under control of a data signal having a plurality of pixel signals, each pixel signal being associated with a respective pixel and having a first value indicating that the respective pixel is to be visible or a second value indicating that the respective pixel is to be left blank, said method comprising the steps of detecting a pixel (A) in a line of said image, where a series of pixels turn from being visible to being blank or vice versa, causing a stairstep in said image; determining at least a reference pixel (B, E, F) being next to said detected pixel (A) and located at a preceding or succeeding line of said image compared to the line on which the detected pixel is located; carrying out a boolean operation on said detected pixel (A) and said reference pixel (B, E, F); and converting at least a subpixel (*1,a1,a2,b1) in the neighbourhood of said detected pixel from being blank to being visible or vice versa on the basis of the result of said boolean operation so as to smooth said stairstep. A method according to claim 1, wherein the image is formed on a sheet by an LED printer having a printer head while the sheet advances in the one direction in which each pixel is divided into subpixels. The method according to claim 1 or 2, wherein a border exists between every two neighbouring pixels and the at least one subpixel being converted is located next to the border of said detected pixel. The method according to any of claims 1 to 3, wherein each pixel is divided into P subpixels and, when the pixel signal of a pixel has the first value indicating the pixel is to be visible, the first value is assigned to N selected subpixels of the pixel, where N < P. The method according to any of claims 1 to 4, wherein said boolean operation comprises at least one logical conjunction or disjunction operation between the value of the pixel signal of the detected pixel with the value of the pixel signal of said at least one reference pixel. A method for smoothing an image composed of a matrix of pixels, said matrix having rows of pixels extending in a first direction and columns of pixels extending in a second direction perpendicular to the first direction, each pixel being divided into a plurality of subpixels, said subpixels being offset from one another in one direction and having the same dimension as the pixel in the other direction, wherein the image is formed under control of a data signal having a plurality of pixel signals, each pixel signal being associated with a respective pixel and having a first value indicating that the respective pixel is to be visible or a second value indicating that the respective pixel is to be left blank, said method comprising the steps of determining the location and the type of stairstep in said image on the basis of boolean values (d0 - d3) produced by performing a boolean operation between a specific pixel (B2) and each of bordering pixels (A2, B1, C2, B3) having a common boundary with said specific pixel; counting the length of the level pixels before or after said determined stairstep; and converting at least a subpixel of pixels at the stairstep from being visible to being blank or vice versa in accordance with the counted length to smooth said stairstep. The method according to claim 6, wherein the boolean values of said specific pixel are composed of four data bits each obtained as an exclusive OR function of the value of the pixel signal of a specific pixel and the value of the pixel signal of a respective bordering pixel. The method according to claims 6 or 7, wherein the location of said stairstep is defined when, in a given direction, a plurality of successive pixel signals having the first value adjoin a plurality of successive pixel signals having the second value, and the type of stairstep is determined by whether the given direction is the first direction and by the order of the first and second pixel signals in the given direction. The method according to claim 8, wherein the location and the type of stairstep are determined as a function of binary data for each of said bordering pixels of said specific pixel. The method according to any of claims 6 to 9, wherein said subpixel converting step is performed on the basis of the pixel code provided in a pixel code table, each pixel code being associated with a respective number of successive pixels having one value in said stairstep and on the basis of the type of stairsteps.

Inventors: KOJIMA TAKETOSHI, NAGATA KATSUMI, TAMADA YASUTO, KOJIMA, TAKETOSHI, NAGATA, KATSUMI, TAMADA, YASUTO, KOJIMA, TAKETOSHI, KYOCERA CORPORATION, NAGATA, KATSUMI, KYOCERA CORPORATION, TAMADA, YASUTO, KYOCERA CORPORATION
Applicants: KYOCERA CORP, KYOCERA CORPORATION
Application year: 1991
Main classification: G06K15
IPC codes: G06K15