Vertical filtering method for raster scanner display

Abstract: The present invention relates to the field of raster-scanned video display for a computer where data for the display is stored in a frame buffer (10), and where the data is organized in the frame buffer by scan lines with data being stored for each pixel along each scan line. According to the invention, a method for generating filtered pixel data for a first display line is characterized in that it comprises the steps of : (a) storing a first copy and a second copy of the pixel data in the frame buffer, said buffer comprising a plurality of video RAMs; (b) reading from said first copy the video data for a first pixel of each scan line in the following scan line order so as to provide four-way interleaved groups: 0, 1, 2, 3; 4, 5, 6, 7; 8, 9, 10, 11; . . . , and reading from said second copy the video data for a first pixel of each scan line in the following scan line order so as to provide four-way interleaved groups: 2, 3, 4, 5; 6, 7, 8, 9; 10, 11, 12, 13; . . . (c) convolving the pixel data for said first pixels to provide the filtered pixel data for said first pixel of said first display line where either of said first or fourfh scan line of pixel data in each of said four-way interleaved groups is not used for said convolving; (d) repeating steps (b) and (c) for each subsequent pixel on said first display line.

Description: BACKGROUND OF THE INVENTION: 1. Field of the Invention. The invention relates to the field of filtering of video signals for a raster scanned display, particularly one employing computer generated pixel data. 2. Prior Art. Most cathode ray tube (CRT) computer video displays are formed with a raster scan. Many of the standards used with these displays can be traced to television standards. For example, two interlaced fields are frequently used to form a frame. Many early personal computers provided compatible NTSC signals to permit a user to use low cost television receivers. In other instances, computers generate signals such as overlays which are used in conjunction with NTSC signals. Thus, personal computers often generate pixel data for use on interlaced, raster-scanned displays. Computer generated data has some characteristics which make it less desirable for an interlaced, raster-scanned display than video signals originating in a video camera. For example, pixel data can exhibit changes (e.g., amplitude) over its entire range from pixel-to-pixel. That is, virtually any change in pixel data can occur from one pixel to the next. In contrast, video data from a traditional video camera uses a beam spot which encompasses more than a single pixel area. The data interpreted for a single pixel in this case takes into account to some extent the intensity and color of the surrounding area. Therefore, there is a softening, even a blurring, that occurs as the beam scans the image in a camera. The human visual system is an edge-detection system. The eyes are very good at finding contours that delineate shapes. To give an example, when displaying a sequence of adjacent gray bars of increasing density on a computer display, the edges between the bars seem emphasized. Perceptually the gray bars do not look like solid colors, but rather they look like they have been shaded between their edges. In other words, the border between the gray bars appear enhanced by the edge-detection mechanisms of the eye. When a typical real world scene is displayed on an interlaced display, there are no abrupt transitions from one scan line to the next. Objects generally do not have very hard edges, and those that do usually do not have edges lined up with a scan line. The result is the eye cannot find an edge from one scan line to the next.If the eye cannot find an edge between one scan line and the next, it cannot distinguish between lines. In an interlaced display a complete frame is drawn each 1/30th of a second, however, because of the interlacing each 1/60th ot a second, either a given scan line or the next scan line is flashed. The eye perceives these multiple scan lines as thick single lines flashing at a 60 frame/second rate even though they are in fact flashing at 30 frames/second. By this model, close viewing of an interlaced display should result in perception of flicker at 30 frames/second. This is in fact what happens; if one is close enough to view individual scan lines on a NTSC television, interlace flicker (i.e., 30 frame/second flashing) is seen, even with a real world image. In the case of a computer generated image such as a MACINTOSH computer image on a interlace display, virtually every place where there is other than solid white or solid black there are abrupt transitions in the vertical dimension. (Macintosh is a registered trademark of Apple Computer, Inc.) In the case of the "racing stripes" (alternately black and white horizontal lines) on the top of a typical Macintosh window, there is the most abrupt transition possible, black to white, stretched across the length of the window and repeated for several lines. Here, it is easy for the human eye to detect the edge from one scan line to the next, so it considers the scan lines as individuals, flashing at 30 frames/second. The visual perception of the human observer is that where there are abrupt transitions on the display, the NTSC image flickers noticeably enough to be distracting. One additional subtlety is worth mentioning. The human eye will see flicker display wherever there are transitions (i.e.,edges) in the vertical dimension. But, the degree of flicker is not uniform for each type of graphic pattern. The worst pattern is the racing stripes across the top of a window, mentioned above. Text and other random patterns flicker as well, but not nearly as severely. This is accounted for by the fact that it is easier to discern vertical edges where there is a high horizontal correlation to the pattern (as in the case of the racing stripes), but harder to find the edges when there is a low horizontal correlation (as in the case of text). (As will be seen, since the present invention provides adaptive filtering for the subtlety.) Numerous prior art techniques are known including those employing anti-aliasing filters for removing this flicker. In some cases, fiiters duplicate the softening effects of the camera beam, that is, pixel data for a cluster or spot of pixels is "averaged" or "convolved" to produce filtered pixel data. In general, these techniques require considerable computational overhead. US-A-4 215 414 teaches the use of both vertical and horizontal filtering of pixel data to provide blurring, a desirable characteristic inherent in a TV camera, and in particular a two-line convolution for phosphorous tubes. Alternate embodiments are also described, induding the filtering of a 3x3 array of pixels or of a 3x2 array. However, this reference recommends to store an entire line in a delay line, which is costly and therefore disadvantageous. Further, to the extent that it does discuss vertical filtering without delay lines, the reference provides the teaching of multiplexed reading of pixel data and then demultiplexing for presentation to an adder. However, there is no suggestion in the description of this reference on how to obtain a practical system for vertical filtering without delay lines. For instance, it is far from obvious to implement the multiplexing and the demultiplexing with ordinary DRAMs, typically used in a frame buffer. This would require reading data from the memory at a rate too fast to enable providing a real time quality display. For example, if one assumes a dot clock rate of 12 MHz and 3 accesses per pixel (for a pixel in three different lines), this requires an access time of approximately 28 nanoseconds. This access time is not achievable with ordinary DRAMs. If one were to select VRAMs with their greater bandwith, then the multiplexing and demultiplexing would not work since one could not arbitrarily pick the needed information from the bit streams. Therefore, US-A-4 215 414 simply does not provide any teaching for vertical filtering which can be practically implemented. The present invention seeks to provide a method for providing filtered pixel data (in the vertical direction only) by means of a convolution technique which is capable of being performed "on the fly", by having computational demands which are substantially less than that required by the prior art systems. SUMMARY OF THE INVENTION The present invention provides a method as claimed in claim 1. Preferred aspects of the invention are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a general block diagram illustrating the general placement of the present invention in a video system. Figure 2 is a diagram used to illustrate a method used by the present invention to read data from a frame buffer. Figure 3 is a diagram used to illustrate an alternate method used by the present invention to read data from a frame buffer. Figure 4 is a block diagram illustrating an embodiment of a convolver used in the present invention Figure 5 is a block diagram illustrating another embodiment of a convolver used in the present invention. Figure 6 is a block diagram illustrating another method for obtaining convolved data particularly useful where not many bits are stored for each pixel. Figure 7A is a block diagram of a general prescaler which can be used with the convolver of the present invention. Figure 7B is a block diagram of another prescaler which can be used with the convolver of the present invention. Figure 8 is a block diagram of a circuit illustrating another hardware embodiment of the present invention, this one employing four-way interleaving. Figure 9 is a diagram illustrating pixel data flow used to describe the operation of the circuit of Figure 8. Figure 10 is a diagram illustrating the relationship between CPU and VRAM addresses for the four-way interleaved frame buffer of Figure 8. DETAILED DESCRIPTION OF THE PRESENT INVENTION A method for providing filtering data, in a raster-scanned video apparatus is described. The invention provides filtering in the vertical direction (perpendicular to the direction of the scan lines). In the following description numerous specific details are set forth in order to provide a better understanding of the present invention. It will be obvious, however, to one skilled in the art that the present invention may be practiced without these details. In other instances, well-known circuits and computer operations have been shown in block diagram form, in order not to obscure the present invention in unnecessary detail. OVERVIEW OF THE PRESENT INVENTION Referring first to Figure 1 a frame buffer 10 is illustrated which in the presently preferred embodiment may be an ordinary frame buffer, for example, one fabricated from dynamic random-access memories (DRAMs) or video random access memories (VRAMs). Most often, data is organized in the frame buffer by scan lines with data being stored for each pixel along each scan line. In some cases, the pixel data is organized in planes such that the pixel data for a given pixel is stored with each bit on a different one of the planes. When data is organized in in this arrangement, a scanning address causes a bit from each plane to be read from the memory, the bits are assembled to form a pixel, and hence, the data for a given pixel is read from the memory for a video display. (Often when writing data to a memory organized by planes, an 8 or 16 bit word is written to each plane; this is particularly useful for a black and white, or two-color display, where only a single bit is stored per pixel and hence data is written into only a single plane). For some embodiments of the present invention, the data is stored in an ordinary manner as described above. An address generator 11 is used to address the data in the buffer to provide an output signal for a video display. As will be seen with the present invention the order in which the data is scanned from the buffer is different than that used in the prior art and hence, the address generator 11 provides this unique addressing order. (This is referred to as "kernel-scanned" in Figure 1.) The specific order employed will become apparent, particularly from the discussion below for Figures 2 and 3. Ordinary circuits may be used to implement the generator 11 and provide the order described in Figures 2 and 3. As in the case with prior art generators, the address generator 11 is generally synchronized with a dot clock. The output from the buffer 10 is convolved by convolver 12. The output of the convolver 12 is pixel data which can be used in an ordinary manner for a video display. The convolver 12 is described in conjunction with Figures 7A and 7B. In the currently preferred embodiment the output to the convolver 12 is gamma corrected. Such gamma correction is well-known in the art and used to compensate for the non-linear light intensity curve of CRT displays. The digital information on line 14 is converted to an analog form for coupling to a display. In the following description it is assumed that the buffer 10 stores pixel data. It will be appreciated that the buffer may store pointers to another memory such as a color hookup table. In this event, the output of the buffer 10 is coupled to a color hookup table and the output of the color hookup table is coupled to the convolver 12. In Figure 2 it is assumed that each of the blocks in the illustrated grid represents a pixel in a bit mapped buffer. In the horizontal direction the pixels are numbered from 0 through 9; it will be appreciated that in a typical memory, many more pixels are used in the display. In the vertical direction the rows of pixel data are numbered by scan line beginning with line 0, and ending at line 5. Again, it will be appreciated that in a typical display many more scan lines are used. Figure 2 thus represents the data organization that is found in a typical frame buffer. For the present invention the data for a given pixel (e.g., pixel 0) is read from the memory for several lines (e.g., lines 1, 2 and 3) before the pixel data for pixel 1 is read from the memory. The pixel data for the several lines of a given pixel is convolved to provide the pixel data used by the display. More specifically, in Figure 2 the data at locations 16, 17 and 18 is read from the memory before the data for pixel 19 is read from the memory. The pixel data from locations 16, 17 and 18 is then convolved to provide pixel data for pixel 0 of a display line. Next, the pixel data at locations 19, 20 and 21 is read from the memory and convolved to provide pixel data for pixel 1 of the display line. This process continues for each of the pixels 0 through 9 for scan lines 1-3 to provide pixel data for a given display line. For the illustrated embodiment three lines of data are used in the convolution process. Any number of lines may in fact be used, for example, the data from lines n, n+1, n+2 . . . N+n may be first used to provide pixel data for a first display line. Following this, the data from lines n+1, n+2, n+3 . . . n+N+1 is used to provide the pixel data for a second display line. However, the data is used from the buffer such that all the pixel data for, for instance, pixel M is read for all the scan lines being used in the convolution before the pixel data for pixel M+1 is read from the buffer. In some cases, the addressing and mapping scheme used for a frame buffer provides more than the data for a single pixel for each address. As illustrated in Figure 3 assume that a single address provides the pixel data for pixel 0 and pixel 1 of line 1, this is shown by the enclosing line 23. With the present invention, the data associated with line 23 is first read from the memory, followed by the convolution of data associated with lines 24 and 25. Then convolution is performed on the data for pixel 0 of lines 1, 2 and 3 for pixel 0, followed by the data for pixel 1, for lines 1, 2, 3. Now the data associated with lines 26, 27 and 28 is read from the memory, and so on. In a specially organized frame buffer such as that described in a presently preferred embodiment, a single address provides the data for several lines in the buffer. For example, a single address may provide the data associated with lines 23, 24 and 25. In this event, the first data for pixel 0 is convolved, then that of pixel 1 is convolved. After that the data associated with lines 26, 27 and 28 is read from the memory and the data for pixel 2, and then 3 is convolved. This process continues for all the data along the line for scan lines 1, 2 and 3. Thus, in general, the data for a first pixel for scan lines n, n+1, n+2 . . . n+N is read from the buffer before the pixel data for subsequent pixels on these scan lines is read from the buffer. This data is then convolved to provide pixel data for a single pixel. This process is repeated for each of the pixels along the scan lines n, n+1, n+2 . . . n+N. Following this the data for a first pixel along lines n+1, n+2. . . n+N+1 is read from the buffer, again before the pixel data for subsequent pixels along these lines. This data is then convolved to provide the filtered pixel data for the first pixel of the next display line. This process is repeated until vertical filtered data is provided for the entire display. EMBODIMENTS OF THE CONVOLVER As mentioned above, the pixel data trom "N+1" lines of data may be convolved. In the currently preferred embodiment N=2. (There is a discussion of convolution for other Kernels later in this application.) In this case, N implements the equation : aP 1 +bP 2 +aP 3 2a + b where P 1 is the pixel data for the first pixel of the nth scan line, P2 the pixel data for the first pixel of the nth+1 line, and pixel 3 the pixel data for the n+2 scan line. "a" and "b" are constants with "b" usually being greater than "a". In a typical application a=1 and b=2. In Figure 4, the convolver (corresponding to kernel convolver 12 in Figure 1) includes a prescaler 32 which receives the input pixel data from the buffer. The amount of prescaling performed by prescaler 32 is controlled by the output of the coefficient table 33. The output of table 33 is controlled by the current cycle number which will be discussed. The output of the prescaler 32 provides one input to an adder 34. The other input to the adder 34, in effect is the output of the adder 34 after being coupled through the latch 35 and the multiplexer 31. The multiplexer 31 either provides as an input to the adder 34 the output of the latch 35 or the value 0. As will be seen at "cycle 0", the 0 input is provided to the adder 34, otherwise the contents of the latch 35 is the input to the adder 34. The contents of the latch 35 is normalized by a normalizer 36, the amount of normalization, typically a constant, is shown as normalization value 37. The output of the normalizer 36 is latched by latch 38, and the contents of this latch provide the pixel data for a pixel along a display line. In practice, the prescaler is simply a digital shifter that provides digital multiplication by a factor of 1 or 2 and the normalizer 36 is another digital shifter which performs division by shifting the digital data by, for example, 2 places for division by four. Assume first that a=1 and b=2 in the equation discussed above. Further assume that data is being scanned from a buffer in the manner illustrated and described in conjunction with Figure 2. The convolver can be seen to operate in a clock cycle sequence. During a cycle 0 the data associated with circle 16 is coupled to the prescaler 32. Cycle number 0 when applied to the coefficient table 33 causes the prescaler 32 to multiply this data by one, hence, the data is directly coupled to the adder 34. The cycle 0 coupled to the multiplexer 31 selects the zero input to the adder; therefore, 0 is added to the data associated with circle 16. This data is simply latched within latch 35 under control of the pixel clock. Next the data associated with circle 17 is coupled to the prescaler 32 on cycle 1. The cycle 1 input to the table 33 causes the prescaler to multiply this data by 2 (a left-shift of one) before coupling it to the adder 34. At the same time the output of the latch 35 is coupled through the multiplexer 31 and is added to the output of the prescaler 32. Hence, the sum P1 +2P2 is formed and coupled to the latch 35. Following this on cycle 2 the data associated with circle 18 is coupled to the prescaler 32. The cycle number "2" coupled to table 33 causes this data to be directly coupled to the adder 34. The adder 34 adds this data to the data contained within latch 35 forming the sum P1 +2P2+P3. This sum is latched within latch 35 and then normalized by normalizer 36. For the described embodiment, normalizer 36 divides the data by a factor of 4 (a right-shift by 2) forming the final equation P 1 +2P 2 +P 3 4 . The resultant pixel data is latched in latch 38. On cycle 0 this data may be read from the latch 38 while new data for the next pixel is being coupled to the prescaler 32. A fourth cycle may be used (i.e., cycle 3), in which event cycle 3 can control latch 38 with no data being shifted into the prescaler 32 during cycle 3. This can be used if 3 cycle timing is inconvenient. An alternate convolver is illustrated in Figure 5. In this embodiment, an adder 40 receives as a first input, the output of the prescaler 43. Once again, the prescaler 43 receives the pixel data from the buffer. The amount of prescaling of prescaler 43 is controlled by the coefficient table 44. The output of table 44 is controlled by the cycle number coupled to the table. The other input terminal of the adder 40 receives the output of the latch 42. The input to the latch is the output of the multiplexer 41. Multiplexer 41 selects either the output of the prescaler 43 or the output of the adder 40. The multiplexer 41 is controlled by the cycle 0 signal; for cycle 0 multiplexer 41 selects the output of the prescaler 43, otherwise it selects the output of the adder. The output of the latch 42 is coupled to a normalizer 46. The amount of normalization is controlled by the values shown as "normalization value 45". The output of the normalizer 45 is coupled to a latch 47. The output of the latch 47 provides the filtered pixel data. The circuit of Figure 5 performs the same convolution as the circuit of Figure 4. Assume that the data for line n for pixel 0 is coupled to the prescaler 43. During the cycle 0 the multiplexer 41 selects the output of the prescaler 43 and couples the data into the latch 42. The prescaler 43 does not scale the data, because a=1 in the equation discussed above. The data for pixel 0 of line n+1 is prescaled by 2 and this data is then added to the contents of the latch with the sum being coupled to the multiplexer 41 and latched in latch 42. The process continues until the sum aP1 +2P 2 +aP 3 is formed, computed and stored in latch 42. The normalizer 46 divides this sum by a factor of 4 and the resultant normalized value is coupled into the latch 47. Again, on cycle 0 (the start of new data into the prescaler 43 for the next pixel) the data is clocked from the latch thereby providing the filtered pixel data for the display. Once again, a four cycle scheme may be used with the fourth cycle (cycle 3) controlling latch 47. In Figure 7A a general prescaler is shown comprising a multiplier 50. The input pixel data is coupled to the multiplier, the output of the multiplier provides the scaled pixel data. The amount of multiplication is controlled by the output of the coefficient lookup table 51. This output is determined by the cycle number. The cycle number (e.g., 1,2, 3 . . . ) selects the amount of multiplication required for the convolution being used and thereby controls the amount of multiplication performed by the multiplier 50. Figure 7B illustrates a prescaler which may be used when the multiplication used by the convolution step involves multiplication by one or by two. In this case a multiplexer 53 receives the input pixel data at one terminal and the input pixel data multiplied by two (i.e., left-shifted by 1 with a zero filling in on the right) at its other terminal. The cycle number requiring the "x2" pixel data is used to select the "0" input to the multiplexer 53 and thus provides the needed scaled input pixel data. The convolvers discussed above are particularly good for a serial kernel data stream. Figure 6 illustrates a convolver implemented in a table 71 which can be used for a parallel data stream. It is particularly useful when a limited number of bits are used; for example, in a 1 bit/pixel display where the 1-2-1 kernel is used. The results of the convolution arithmetic are precomputed and placed in the table. This is used as will be seen for software embodiments of the invention where the color hookup table is preloaded for use as a convolution lookup table. A CURRENTLY PREFERRED EMBODIMENT OF THE INVENTION EMPLOYING FOUR WAY INTERLEAVED VRAM The results of the invention can be obtained from a single bank of memory where four-way interleaving is used. This embodiment has the disadvantage that additional memory capacity is required since two copies of the data are needed for the four-way interleaving, as will be described . Figure 8 illustrates a block diagram for the four-way interleave embodiment which again employs a dual port VRAM memory. The RAM port of the memory is connected through a latch 73 to the CPU to permit data to be read into and read from the memory. The shift register port of the memory is coupled to three latches such as latch 74 with each latch being coupled to a shift register, such as shift register 75. The multiplexer 76 selects one of the shift registers and couples its output to a CLUT 77. A digital color output from the CLUT, after convolving by convolver 78, provides RGB outputs. The latches, shift registers, multiplexer, CLUT and convolver for this embodiment operate in the same manner as for a two-way interleaved VRAM embodiment except as discussed below. The addresses to the VRAM are selected by the multiplexer 79 either from pixel address counter 80 or from the CPU after the CPU addresses have been coupled to the parallel load counter and latch 81 (as will be described). The most significant bits of the address are coupled to the counter 81, the least significant bits are simply passed through a latch onto the multiplexer 79. As mentioned, a single bank of VRAM is used with four-way interleaving and by storing two complete copies of the pixel data in the VRAM 72. This is necessary to obtain pixel data from three lines (sequentially) and to provide different groupings of lines as will become apparent. The four-way interleaving causes the CPU to write words into VRAM in a 4-line "zig-zag" order, however, from the CPU's standpoint this is transparent. In VRAM each of the two copies of the pixel data is stored with its interleaving offset by two lines from the other copy. That is, the word order of the first line of the even copy is: line 0/word 0, line 1/word 0, line 2/word 0, line 3/word 0, line 0/word 1, line 1/word 1, line 2/word 1, line 3/word 1, etc. And, the first line of the odd copy of the frame buffer is line-1/word 0, line-2/word 0, line 0/word 0, line 1/word 0, line-1/word 1, line-2/word 1, line 0/word 1, line 1/word 1, etc. (Lines -1 and lines -2 denote black lines above the first line on the display. They are used to avoid flicker at the perimeter of the display. They are initially loaded with data representing the color black and need not be accessed by the CPU, once loaded.) Even though two complete copies of the pixel data are stored in the memory, the CPU only has access to the even copy. When the CPU writes data into the memory, the addresses and data are latched and the write operation is repeated to write the data into the odd copy. Moreover, only the even copy is read by the CPU. This additional writing for the odd copy is done by ordinary circuitry in the frame buffer. The even and odd copies are distinguished by the most significant bit in the VRAM, thereby placing the even frame copy in the lower half of the memory and the odd copy in the upper half. The four-way interleaving for the even copy is obtained by taking the address bits from the CPU which select between odd and even scan lines and connecting them to bit 0 ot the VRAM. The address bit from the CPU which selects between even and odd scan line pairs is connected to bit 1 of the VRAM and then all other bits from the CPU are shifted "upward" as shown in Figure 10 in the righthand column. In Figure 10 the conventional interconnect is shown in the lefthand column. The effect of this bit remapping for the 1024 bytes per scan line display is that the odd and even scan line "bit" (e.g., bit 10) selects between odd and even words, and the odd and even scan line pair bit (e.g., bit 11 ) selects between odd and even word pairs. From the CPU's viewpoint, once again, the memory appears as a contiguous block. The four-way interleaving for the odd frame buffer copy is obtained in the same manner as the even copy, except that before the bit remapping an adder adds the length of two scan lines to the address generated by the CPU (for example, for 1024 bytes per scan line, the number 2048 is added to the CPU address prior to the bit remapping). The effect of this addition is to offset the contents of the odd copy in the buffer by two scan lines (i.e., after remapping by 2 words). (It is assumed the scan line length is a power of 2, otherwise addressing is more complex.) The two scan line adder is easily implemented since the only time the odd bank copy is addressed is after the even copy has been addressed. This addition is obtained from the counter 81 of Figure 8 by incrementing the counter. Assume that there are 1024 bytes per scan line, then the CPU address bits A0-A10 are latched in the latch of counter and latch 81, and the address bits 11 and higher are coupled to the counter. To write the even copy the address bits are simply latched by the counter and latch 81. To write the odd copy, the counter is incremented and the data which has been latched is written into the memory with a two scan line offset. Another effect of the address manipulation is to cause the output of the memory (shift register port) to be in scan lines of "line quads". The even line quads are 0/1/2/3,4/5/6/7, 8/9/10/11, etc. The odd line quads are -2/-1/0/1, 2/3/4/5, 6/7/8/9, etc. Each convolution triplet needed by the convolver is then available in one of these line quads. The line quads are sequenced in the same order for both fields: odd line quad -2/- 1/0/1, even line quad 0/1/2/3, odd line quad 2/3/4/5, even line quad 4/5/6/7, etc. The particular field displayed is determined by which three lines are convolved from each line quad. For example, in field 1 for lines -1, 1, 3, and 5 the triples -2/-1/0, 0/1/2, 2/3/4, and 4/5/6 are needed. Then in field 2 for lines 0, 2, 4, and 6 the triples -1/0/1, 1/2/3,3/4/5, and 5/6/7 7 are needed resulting in the same line quad sequence. The output of the memory is latched and then the shift registers are used in the same manner as for a two-way interleaved VRAM embodiment, except that the line order and the line quads is different, this difference being shown in Figure 9. OTHER CONVOLUTION KERNELS In the previous section, most of the emphasis has been on the 1-2-1 kernel. Experiments have shown that neither a 3-line convolution nor on-off-on-off reduction of 50% gray is essential in all situations for effective interlace flicker reduction . It the constraint that on-off-on-off horizontal line patterns are reduced to a 50% gray is maintained and other kernel sizes are tried other than 1 x 3, for each kernel size there is one set of coefficients to meet the on-off-on-off constraint. These coefficients match Pasqual's triangle (i.e.,1; 1, 1; 1, 2, 1; 1, 3, 3, 1; 1, 4, 6, 4, 1; etc.). ADAPTIVE CONVOLUTION As mentioned above, the worst flicker patterns are the ones which have high horizontal coherence (i.e., repeat horizontally). Horizontal solid lines, horizontal dashed lines, and gray dither patterns are examples of patterns with high horizontal coherence. Text is an example of patterns without such coherence. The convolution discussed above may be adaptive, that is, it may be varied depending on the type of patterns being displayed. First, it is determined whether a repeating pattern is occurring in a local horizontal group ot kernels, for example, 8 pixels across. If there is a pattern in the kernels, then all of the kernels are convolved for example, with the 1-2-1 coefficients. If there is no such pattern, then the 8 pixels are convolved with coefficients making a sharper filter (e.g., 1-3-1 or 1-4-1). The test to determine whether a pattern is repeating must be applied continuously in a moving horizontal window, kernel by kernel. Since the test windows overlap, some kernels may be part of a pattern in one test window but not in another. For these kernels, the 1-2-1 convolution is used, since they are at the edge of the pattern. Different tests may be used for determining whether a pattern is being repeated, for example, the left four Kernels may be compared with the right four kernels within the window.

Claims: A method for generating filtered pixel data for a first display line in a raster-scanned video display for a computer where data for the display is stored in a frame buffer (10), and where the data is organized in the frame buffer by scan lines with data being stored for each pixel along each scan line, said method being characterized in that it comprises the steps of: (a) storing a first copy and a second copy of the pixel data in the frame buffer, said buffer comprising video RAM; (b) reading from said first copy the video data for a first pixel of each scan line in the following scan line order so as to provide four-way interleaved groups: 0, 1, 2, 3; 4, 5, 6, 7; 8, 9, 10, 11; ..., and reading from said second copy the video data for a first pixel of each scan line in the following scan line order so as to provide -2, -1, 0, 1; 2, 3, 4, 5; 6, 7, 8, 9; . . . as four-way interleaved groups; (c) convolving the pixel data for said first pixels to provide the filtered pixel data for said first pixel of said first display line where either of said first or fourth scan line of pixel data in each of said four-way interleaved groups is not used for said convolving; (d) repeating steps (b) and (c) for each subsequent pixel on said first display line. The method defined by Claim 1 wherein steps (b) through (d) are repeated for each display line in said display. The method defined by Claim 1 wherein said convolving step comprises the step of performing the following computer implemented computation: aP1 + bP2 + aP3 2a + b where P1 is the pixel data for a first pixel of a first line of a given triplet, P2 is the pixel data for the first pixel of a second line of the given triplet, and P3 is the pixel data for the first pixel of a third line of the given triplet, and wherein a and b are constants. The method defined by Claim 1 further comprising the step of passing the pixel data through a color look up table (CLUT) in advance of the step of convolving. The method as defined by Claim 1 where said step of reading comprises a step of passing the pixel data through a latch circuit. The method as defined by Claim 5 where the step of reading further comprises a step of passing the pixel data from the latch circuit to a shift register. The method as defined by Claim 1 wherein the video RAM is dual ported. The method as defined by Claim 1 wherein the step of convolving comprises the further step of generating an RGB output signal based on the pixel data. The method as defined by Claim 8 further comprising the step of applying the RGB output signal to a color monitor to display a representation of the pixel data. The method as defined by Claim 1 wherein the step of storing first a first copy and a second copy of the pixel data in the frame buffer comprises the further steps of: storing the first copy in an upper half of the frame buffer; and storing the second copy in a lower half of the frame buffer.

Inventors: LUNDBLAD JAMES A, OKUN JEROME M, PALEVICH JOHN H, PERLMAN STEPHEN G, LUNDBLAD, JAMES A., OKUN, JEROME M., PALEVICH, JOHN H., PERLMAN, STEPHEN G.
Applicants: APPLE COMPUTER, APPLE COMPUTER, INC.
Application year: 1989
Main classification: G09G1
IPC codes: G06T5, G09G5