The present invention relates to an image formation apparatus for a laser printer, and more particularly, to a method and apparatus for enhancing the resolution of a laser printer by using a laser beam modulation technique.
Generally, in a laser printer, a laser beam is driven according to an input image, a drum is electrically charged, and toner (being oppositely charged) is adhered to the drum. The drum-adhered toner is then transferred onto a sheet of paper by heating, to thus produce (print) an output image corresponding to the input image. Here, the signal used for driving the laser beam is in the form of an image bitmap signal.
To enhance resolution in such a laser printer, that is, to print an image with greater precision and fineness, one or more requirements must be met. That is, the capacity of a buffer memory should be increased, the powder of the toner should be very fine, and/or an electromagnetic process of the optical drum should be improved for smooth operation in high-resolution conditions, all of which, however, result in cost increases since an expensive high-resolution engine and large memory capacity are required.
An image formation apparatus which can increase resolution in the laser printer using a laser beam modulation technique without the need for a larger capacity memory or high-resolution engine, is disclosed in Japanese Patent Laid-open Publication No. 92-323959 (Matsushida), as shown in FIG. 1. Here, a laser beam modulation technique controls the quantity and position of an electrical charge on the drum, by varying the shape of a pulse of a laser beam driver according to an image pattern. Since the amount and position of the drum-adhered toner can be precisely determined by variation of the electrical charges, such a technique can ultimately increase the resolution of the laser printer.
Referring to FIG. 1, the conventional image formation apparatus is composed of a memory portion 110, an edge detector 120, a weight processor 130, a logic operator 140, a correction controller 150, a signal generator 160, and an edge data selector 170.
The FIG. 1 apparatus shows a technology for determining an image pattern from 7.times.7 window information (refer to FIG. 6) and processing the determined image pattern. To briefly explain an algorithm for determining an image pattern, a weight value is assigned to an edge in the weight value processor 130 when a specific portion in FIG. 6 is determined as an edge. Then, the assigned weight values are summed in the logic operator 140, the result being compared with a number representing a predetermined reference pattern to determine a modulation pattern which will ultimately be printed. Then, the correction controller 150 outputs a correction control signal of the image data and the signal generator 160 produces a modulation signal according to the correction control signal.
In more detail, the memory portion 110 produces a bitmap data window for processing a signal and is composed of a 7-row, 4,096-column bit memory circuit 111, a sample window generator 112 and a memory controller 113. The memory circuit 111 is a first-in-first-out (FIFO) memory composed of 7-row shift registers and 4,096-column flip-flops, and the sample window generator 112 produces sample window data of FIG. 6 using the data from the memory circuit 111. Meanwhile, the memory controller 113 controls the memory circuit 111 to enable an input video data to be sequentially stored therein and outputs a pixel clock for external circuitry synchronization. Here, the horizontal synchronization (Hsync) signal is also supplied from a laser printer controller (not shown).
In the operation of the memory portion 110, first, the input video data is sequentially stored in the memory circuit 111, per each bit, starting from the seventh row. Whenever one bit is stored in the memory portion 110, the value of a counter in memory controller 113 increases by one. When the counter value reaches the number of dots corresponding to the printed width of the paper, the data in the seventh row is shifted to the sixth row and the counter is reset to zero so that the input video data can again be stored in the seventh row. When the seventh row again becomes full, the data therein is shifted to the sixth row and the data in the sixth row is shifted to the fifth row. In this manner, the lastly input data is always stored in the seventh row, with the previously input data being sequentially stored in the other rows in the same manner on down to the first row. Thus, the sample window generator 112 produces a sample window as shown in FIG. 6, from the seven rows of memory circuit 111. Here, assuming that D4 (Dth row, fourth column) is the current position of a pixel to be processed in memory circuit 111, the sample window generator 112 reads out the third through fifth columns in the Ath and Gth rows, the second through sixth columns in the Bth and Fth rows, and the first through seventh columns in the Cth, Dth and Eth rows, to produce a 7.times.7 sample window.
FIGS. 2-5 show image patterns for right- left- bottom- and top-edge detection in the edge detector of FIG. 1, respectively. Here, each circle represents an edge and has an assigned weight value of 1, 2 or 4. The edge detector 120 detects an intermediate portion (edge) between pixels when the bitmap data of two adjacent pixels differ, i.e., detects whether a circle location shown in FIGS. 2-5 is an edge. Edge detection is classified as one of four different types (left-, right-, top- and bottom-edge) according to the relationship between the current pixel (i.e., D4) and its periphery pixels, and is controlled by the edge data selector 170 which selects one kind of edge detection, to thereby determine the direction, i.e., left (L), right (R), up (UP) or down (DN), of the edge.
Once a circled portion (pixel boundary) is detected as an edge by the edge detector 120, the weight value processor 130 assigns the weight values of each circle shown in FIGS. 2-5.
The logic operator 140 sums the edges to which the weight values are assigned, compares the summed result with a predetermined value, determines a modulation pattern, and outputs data to be corrected based on the modulation pattern.
The correction controller 150 outputs a correction control signal for controlling the correction of image data D4 according to the correction data output from the logic operator 140.
The signal generator 160 produces a modulation signal to be supplied to a laser driver (not shown) according to the correction control signal output from the correction controller 150.
The edge data selector 170 determines one edge detection pattern among the edge detection patterns shown in FIGS. 2-5, according to the relationship between a pixel currently being processed, that is, D4 of FIG. 6 and the periphery pixels, as described above. The edge pattern of D4 is classified with a left-central edge, a right-central edge, an upper-central edge and a lower-central edge. With respect to the following cases, edge detection patterns are selected as shown in FIGS. 2-5. That is, when the D4bitmap! is not equal to the D5bitmap!, the right-central edge pattern is selected; when the D4bitmap! is not equal to the D3bitmap!, the left-central edge pattern is selected; when the D4bitmap! is not equal to the E4bitmap!, the lower-central edge pattern is selected; and when the D4bitmap! is not equal to the C4bitmap!, the upper-central edge pattern is selected.
As described above, the respective forms of edge detection designate a pixel boundary as an edge when the bitmap values of either of the pixel differ from the other. For example, the circle (pixel boundary) between C3 and C4 in FIG. 2 is designated as an edge when C3 and C4 have different bitmap data values, and thus, C34 is an edge when the C3bitmap! is not equal to the C4bitmap!. Further, each detected edge has a weight value (the numerical figure within the circle) according to each edge detection pattern, which is processed in the weight value processor 130. For example, in FIG. 2, if C34 is an edge, a weight value (in this case, "4") is placed in the circle C34, and a "0" is placed therein if it is not an edge.
The logic operator 140 sums the weight values which have been assigned by the weight value processor 130, compares the summed result with predetermined values and determines a modulation pattern. The comparison is accomplished with various predetermined values, upon the result of which one modulation pattern is determined among the eight patterns of FIGS. 7A-7H, wherein a modulation pattern may be classified as a black-pixel type (FIGS. 7A-7D) or a white-pixel type (FIGS. 7E-7H). Thus, there are eight such modulation patterns: (FIG. 7A) when a laser drive signal is given for a certain pixel; (FIG. 7B) when a laser drive signal is given for the left-side two-thirds portion of a certain pixel; (FIG. 7C) when a laser drive signal is given for the right-side two-thirds portion of a certain pixel; (FIG. 7D) when a laser drive signal is given for the central two-thirds portion of a certain pixel; (FIG. 7E) when a laser drive signal is not given for a certain pixel; (FIG. 7F) when a laser drive signal is given for the left-side one-third portion of a certain pixel; (FIG. 7G) when a laser drive signal is given for the right-side one-third portion of a certain pixel; and (FIG. 7H) when a laser drive signal is given for the central one-third portion of a certain pixel. As shown in FIGS. 7A-7H, there are ten actual dot patterns owing to the up-direction and down-direction pixel arrangements, since a pixel modulation pattern has both up-direction and down-direction modulation patterns when the laser drive signal is given for the central portion of the pixel.
The correction controller 150 performs a correction operation according to the modulation pattern determined in the logic operator 140. Then, the signal generator 160 sends a modulation signal to a laser beam printer engine 860 of FIG. 8. The signal generator 160 has the eight modulation patterns of FIGS. 7A-7H, i.e., six modulation patterns and the 100% and 0% conditions. Either FIG. 7D or 7H represents the position of the output pixel when a modulation result of the pixel is determined as an up or down form. When a current laser printer represents the result of the up or down form, an electrostatic phenomenon of the toner is used, wherein the toner is automatically moved up when there is an upper dot and moved down when there is a lower dot, by an electrostatic force. In FIG. 7D, the modulation pattern (UP2, DN2) is made by just one modulation signal. Here, if a pixel which is located above the current pixel is a 100% black pixel, the modulation pattern becomes an up-direction modulation pattern UP2, while if a pixel which is located below the current pixel is a 100% black pixel, the modulation pattern becomes a down-direction modulation pattern DN2. The modulation pattern of FIG. 7H (UP1, DN1) is created in the same manner.
In the above-described conventional art, the seven 4,096-bit lines use a total of 28,672 bits for A4-sized paper, with a larger memory capacity being needed when A3-sized paper is used. Accordingly, when a corresponding memory is realized in a single chip, the total size of the whole chip becomes large due to the size of the memory. Also, besides the 100% and 0% conditions, the conventional technology is limited to just six possible modulation methods. Thus, various kinds of dot patterns cannot be adapted to accomplish a desired resolution increase.
In U.S. Pat. No. 4,933,689 issued on Jun. 12, 1990 as another form of the conventional art, the number of dots capable of being represented per inch is not increased, but a technology of bringing a high-resolution effect by minutely adjusting the thickness and position of a dot is disclosed. Here, a clock pulse is modulated to adjust an interval and intensity of a laser beam and to then obtain a particular dot pattern. The dot pattern is then matched against a prestored set of data on a real-time basis, to adjust the center position of a dot in the horizontal direction by thirds and fix the dot size in the vertical direction. Accordingly, a curved surface and linear component of the character can be smoothened to reduce a step phenomenon. However, only the curved surface can be smoothened and such problems as position error and shape distortion cannot be solved.
A double scanning line method as yet another form of the conventional art suppresses the need to increase memory size by using a data compression technique for enhancing horizontal and vertical resolution by two times or more. However, this method has a drawback in that linear lines differ from each other in thickness and vertical resolution cannot be improved by no more than two times.