Method and apparatus for increasing image generation speed on raster displays

Abstract: An image data generation circuit for a conventional raster display comprises a graphics systems processor and a standard video dynamic random access memory (VRAM) interconnected by an address translator circuit. The VRAM is connected to the raster display. The graphics system processor is preferably an off-the-shelf graphics system processor capable of drawing horizontal lines very quickly. This graphics system processor is configured to transpose raw data to achieve the same horizontal drawing speed while drawing in the vertical direction and feeds the resulting image data to the address translator circuit. The address translator circuit reconverts the image data for storage in the VRAM so that the image data can be accessed in a conventional manner to modulate the electron beam of the raster display. In one example, this results in an eight-fold increase in the update or refresh rate of the corresponding image on the raster display.

Description: The present invention relates to an apparatus for displaying images on raster display means for displaying image data in the form of images in accordance with the preamble portion of claim 1 and to a method for generating images on raster display means in accordance with the preamble portion of claim 16. Generally, the invention relates to the creation of images on raster displays. These images can consist of textual and/or graphical information. Specifically, the invention is directed to a method and apparatus for digitally processing image data, which increase the speed or rate of generation of corresponding images on a raster display. One type of raster display is a cathode ray tube (CRT) on which images are displayed by a technique known as raster scanning. Raster scanning involves driving a deflection control circuit which directs an electron beam modulated by image information onto discrete areas of luminescent material on a display screen. The image information determines whether or not each discrete luminescent area is illuminated. Typically, raster scanning involves sweeping the electron beam from the upper left hand corner of the screen horizontally across the screen to the right to selectively illuminate a horizontal row of discrete luminescent areas and repeating the process for each row of the screen from top to bottom, selectively illuminating each discrete luminescent area in accordance with the corresponding image information which modulates the electron beam. The electron beam can be modulated in various ways depending on the manner in which the CRT is being used. One example is a television in which image information is transmitted through the atmosphere and detected by a television receiver which decodes the received image information and modulates the electron beam to display images on a screen. The deflection control circuit sweeps the electron beam to generate images on a television screen as many as 60 times a second. CRTs are also used as displays for other purposes. One such use is in computer terminals. Here, images are displayed by sweeping the electron beam in the same way as in a television. Unlike television, however, the image information is not generally transmitted through the atmosphere, but rather is input to the computer at a local or remote location and stored in a screen memory. A display control processor feeds the stored image data in the screen memory to the CRT for modulating the electron beam to generate an image corresponding to the stored image data. Another use of a CRT is in electronic instrumentation, such as an oscilloscope, spectrum analyzer, or network analyzer. These instruments measure characteristics of received signals transmitted through the atmosphere or responses of electronic devices connected to them. Typically, the measured information is processed and stored in a screen memory, similar to the way in which image data is stored in the screen memory for display on computer terminals. Unlike computer terminals in which data is entered and displayed at relatively low rates or speeds, instruments make measurements at significantly higher speeds. For example, data can be entered in computer terminals by a keyboard at typing speed, say at an average of 80 characters a minute, whereas sophisticated instruments make measurements at a rate of between 300 to 3,600 times a minute. In most instruments with CRTs, standard off-the-shelf graphics system processors are used to update the display, due to their relatively low cost (compared to custom graphics system processors or dedicated graphics engines). The resultant update rate is typically two to five times a second during normal measurement operation. Examples of such instruments include the Hewlett-Packard Company HP 4195A Network/Spectrum Analyzer, HP 54110 Color Digitizing Oscilloscope, and HP 70000 Modular Measurement System, as well as the Wiltron Company 360, Wiltron 561, and Wiltron 6409 network analyzers. Achieving a fast display update rate is very important in many instrument applications. If the display cannot be updated as fast as measurements are made, the data collection process must be slowed down, or else the user of the instrument will not see the data that has been collected. The measurement traces will be updated sluggishly, making the instrument less responsive to the user. In addition, if the display is not updated quickly, the instrument will not have a "real-time" feel; that is, the images will dance in steps to the final displayed values rather than appear to move smoothly and instantaneously to those final values as the display is updated with new measurement data that has been collected. A display update rate of at least 10 to 20 updates a second (10 to 20 Hz) is needed in order to achieve a "real-time" feel. One disadvantage of raster displays used in instruments is that pixels on the display screen must be written by the graphics system processor into a region of screen memory. The process of writing image data corresponding to a single line into the screen memory can require the graphics system processor to access hundreds of screen memory locations, consuming a significant amount of time. In fast instruments, which make measurements at 3,000 or so times a minute (as fast as 60 Hz), changes in the measured data are not faithfully displayed on the CRT quickly because of limitations of the graphics system processor which is not able to quickly fill video memory at a rate that can be accommodated by the 60 Hz maximum update capability of the deflection control circuit of conventioal CRTS. In view of the above prior art, the present invention is based on the object of providing an apparatus and a method of the above-mentioned kind having a respective increased rate or speed with which image data can be processed so that the rate or speed can be better matched to the display update capability of the deflection control circuit of the raster display so that changes in images can be more quickly updated on the raster display and perceived by the user. This object is achieved by an apparatus for displaying images on raster display means for displaying image data in the form of images in accordance with claim 1 and by a method for generating images on raster display means in accordance with claim 16. One embodiment of the invention increases the speed with which image data is written into a screen memory. Accordingly, the image data is stored in a manner which enables updated image data to be accessed and displayed more quickly on a raster display, such as a CRT, by the use of a conventional deflection control circuit and raster scanning technique. For example, the invention results in an eight-fold increase in the speed of updating the display of traces of measurement data in a network analyzer. This enables new measurement data to be more rapidly displayed to the user without the data trace appearing to dance on the screen. In accordance with one embodiment of the method and apparatus in accordance with the invention, a graphics system processor receives information which is to be displayed. At least a portion of this information is in the form of X,Y coordinate data. The graphics system processor transposes each received set of X and Y coordinates so that X,Y becomes Y,X. The graphics system processor then processes adjacent pairs of transposed coordinates to generate a line segment or segments by using the first set of transposed coordinates as the starting point and the second set of transposed coordinates as the end point. Next, the graphics system processor can compute a best-fit set of points between the starting and end points to interconnect them using a conventional technique, such as Bresenham's line-drawing algorithm. Preferably, the method in accordance with one embodiment of the invention optimizes the line segment drawing process in the graphics system processor. The graphics system processor is connected to a conventional screen or video memory which stores the image or video data produced by the graphics system processor. In order to write the image data into the video memory so that the image data is properly fed to modulate the electron beam of the CRT, an address translator circuit interfaces the graphics system processor to the video memory. The address translator circuit writes the image data in banks of vertically oriented image data, as compared to horizontally oriented banks of image data, so that image data is stored in the video memory in a conventional format for updating the display on the CRT. The address translator circuit writes into the video memory by reversing the address select lines to the video memory so that the image data is correctly stored for later access. By first transposing the coordinates with the graphics system processor and digitally processing in a conventional horizontal mode, changes in vertical distances between adjacent points of information are more quickly written into the video memory and hence more quickly reflected on the screen of the CRT as the electron beam is modulated in the conventional way. In other words, limitations on the speed of operation of the graphics system processor are removed by allowing the graphics system processor to operate in a pseudo-horizontal mode without affecting the appropriate image data storage needed to generate the CRT display. Accordingly, the graphics system processor is able to process image data characterized by vertical excursions as fast as conventional processing of image data, such as horizontal line drawing. The address translator then reconverts the image data to the appropriate form for conventional storage in the screen memory so that a conventional CRT can be used. Also, a pulse stretching circuit is preferably provided to replicate an adjacent pixel for each pixel of each line segment to provide a smooth, high resolution trace. Brief Description of the Drawings The above and other features of the invention and the concomitant advantages will be better understood and appreciated by persons skilled in the art in view of the detailed description given below in conjunction with the accompanying drawings. In the drawings: Fig. 1 is a block diagram of one embodiment of an image data generation circuit in accordance with the invention; Figs. 2A and 2B comprising 2A.1 to 2A.18 and 2B.1 to 2B.17, respectively, are a detailed schematic drawing of one implementation of the image data generation circuit shown in Fig. 1; Figs. 3A and 3B are a flow chart of one embodiment of the method in accordance with the invention for speeding generation of raster displays; and Fig. 4 illustrates an example of a trace generated from measurement data in accordance with one embodiment of the invention. Detailed Description of the Preferred Embodiments The following additional background information is intended to facilitate an understanding of the invention. Typically, the graphics system processor and its associated video memory are optimized to operate with image data being written into horizontally adjacent memory cells in the video memory. This constraint is imposed by a conventional deflection control circuit in a CRT, which raster scans horizontally, and the required interconnection of the video memory output shift register to the deflection control circuit for modulating the electron beam of the CRT. This constraint slows the speed of the graphics system processor in processing measurement data for display, since measurement data is typified by vertical excursions as opposed to horizontal ones. Therefore, more time is required to update the video memory when sequential measurements vary. Considered in more detail, in many instrument applications, the data display is a graph, with the controlled variable drawn along the X-axis, and the independent variable drawn along the Y-axis. (See, for example, Fig. 4.) Such graphs tend to exhibit a more rapid data variation in the Y direction than in the X direction. As a result, the graph is predominantly composed of vertically oriented lines and contains far fewer horizontally oriented lines. There are various significant disadvantages of off-the-shelf graphics systems processors. Unfortunately, presently available off-the-shelf graphics system processors are typically designed in a way that optimizes their drawing speed in the horizontal direction. This results in much lower performance when drawing in the vertical direction. Generally, horizontal lines can be drawn two to 16 times faster than vertical lines. In order to achieve a rapid display update rate, the graphics system processor must be able to draw vertical lines very quickly. For example, one conventional graphics system processor has a 16-bit data bus, allowing it to write four 4-bit pixels to the video memory in one cycle. The graphics system processor is designed such that the four pixels it writes during a memory cycle are horizontally adjacent. Hence, the standard technique of interfacing the graphics system processor to the video memory is to have it access four 64K x 4 video dynamic random access memories (VRAMs) in parallel. The result is that the adjacent pixels on a horizontal raster scan line are interleaved among the four VRAMs. When the pixels are to be shifted out of the VRAMs to the CRT, the four banks are all shifted simultaneously, and the interleaved pixels are multiplexed onto a single video bus. Typically, the number of bits per pixel, n, is 1, 2, or 4. If, for example, 16 colors are desired on a CRT, four bits per pixel are needed to specify one of the colors. Since the graphics system processor accesses 16 bits of memory per cycle, it is able to write 16, 8, or 4 pixels to memory per cycle, respectively. Thus, m, the number of graphics system processor data outputs (16-bit data bus) divided by the number of bits per pixel, n, is 4 for a 16-color CRT. The pixels that are written together in a single memory cycle are pixels that are horizontally adjacent in video memory rather than vertically adjacent for the following reason. The video memory is configured in such a way that the horizontally adjacent memory cells (which each contain a pixel) have adjacent addresses. That is, incrementing the video memory address by one results in the selection of the pixel immediately to the right of the current pixel. Accordingly, graphics system processors are typically designed to work with standard video memories. This requires that they be architected in such a way that they convert the X,Y position of a pixel into a video memory address in which Y selects the most significant portion of the address and X selects the least significant portion of the address. Since the graphics system processor accesses several horizontally adjacent pixels in a single memory cycle, it is able to generate horizontal lines significantly faster than vertical lines. EP-A-0149188 discloses a display control system comprising a master processing unit and a video processing unit which in turn is connected with address registers, address counters, comparator circuits and an address composing circuit. The video processing unit, the registers, comparators, counters and the address composing circuit together form a graphic processor. The latter registers, comparators and counters merely receive data specifying a starting point and a vertical and horizontal extension of a straight line to be generated by the graphic system from the video processing unit to thereinafter sequentially generate the physical addresses of the straight line making use of a predetermined algorithm. Thus, the video processing unit is freed from the burden of executing the straight line forming processings. The registers, counters, comparators and the address composing circuit perform the generation of the sequential physical address under said algorithm only if the gradient of the straight line exceeds a certain threshold. Otherwise, horizontal and vertical components of the algorithm are interchanged before conducting the generation of the sequential physical addresses of the straight line. The sequentially generated dots of the straight line are stored into a video memory under the respective sequential vertical and horizontal addresses generated by the address composing circuit of the graphic system. In other words, color code data defining the color of the respective dots of the straight line are stored into the video memory under the addresses as generated above. The method and apparatus in accordance with the invention alter the graphics system processor to operate as though it is drawing horizontal lines, when it is actually drawing vertical lines. This is achieved by exchanging the X and Y coordinates of each line segment endpoint and computing vertical line segments in a pseudo-horizontal mode. Then, the X and Y halves of the memory address are exchanged by means of an address translator circuit so that image data is written into the video memory in the appropriate format for modulating the electron beam. A preferred embodiment of the image data generation circuit in accordance with the invention, generally indicated by the numeral 10, is shown in Fig. 1. The image data generation circuit 10 comprises a graphics system processor 12. The graphics system processor 12 is preferably a conventional graphics systems processor integrated circuit, for example, a Texas Instruments TMS34010 Graphics System Processor (GSP). The operation and programming instructions for this processor are described in "Texas Instruments TMS34010 User's Guide" published in 1988 by Texas Instruments. The graphics system processor 12 is programmed in a conventional manner to read raw data from a display list memory 14 on an I/O data bus 16 which interconnects the graphics system processor and the display list memory. At least a portion of the raw data is stored in the display list memory 14 in X,Y coordinate form. In accordance with the invention, the first cycle of the graphics system processor 12 after reading the raw data in X,Y coordinate form is to transpose this raw data to Y,X coordinate form. The graphics system processor 12 then commences a line drawing operation which translates the raw data to a pictorial representation in the form of image data. Considered in more detail, the graphics system processor 12 reads one X,Y coordinate from the display list memory 14 and then reads the adjacent X,Y coordinate from the display list memory. The graphics system processor 12 then transposes the X and Y coordinates for these points. Next, the graphics system processor 12 determines the horizontal separation between the points. Preferably, if the horizontal spacing is less than a predetermined distance, for example, less than two pixels, the vertical spacing is determined. If the vertical spacing is greater than the horizontal separation, i.e., the slope is greater than 45 degrees, then the line is broken into a set of vertical line segments offset horizontally from one another by one pixel. Next, if the cumulative offset between X coordinates is one, the line is broken into two segments of equal length, ignoring round-off. If, on the other hand, the cumulative offset is two or more, the number of segments is computed to be Delta X plus 1, where Delta X equals the number of pixels separating the adjacent X coordinates. The length of each vertical segment is then determined by computing the vertical spacing so as to determine the number of pixels between the Y coordinates of the adjacent points, inclusive of the end points, and dividing the result by Delta X, ignoring round-off. Finally, the first and last segments are preferably half the length (number of pixels) of the remaining segments. This last feature is so that the broken line connects well with the preceding and/or subsequent lines, if any. Interestingly, this produces the same result as Bresenham's line-drawing algorithm, but the graphics system processor 12 performs the modified line drawing procedure in accordance with the invention considerably faster, on the average of ten times faster using the TMS34010 GSP. To perform the actual line drawing, the graphics system processor 12 executes a routine which examines the now horizontally oriented line that has been broken into a series of individual horizontal line segments. Since these line segments are horizontal, and not just horizontally oriented, it is now possible to use the fill rectangle command ("FILL") of the graphics system processor 12, which is very fast at drawing horizontal lines. As the graphics system processor 12 generates the horizontal line segments of each line, it takes each group of m horizontally adjacent pixels and attempts to write them to video memory in a single cycle. (m can be calculated as the width of the data bus of the graphics system processor 12 divided by the number of bits per pixel, and, typically, m = 4, 8, or 16. In an exemplary application of the invention in which 16 colors are available, m = 16 / 4 = 4.) This technique of writing multiple horizontally adjacent pixels in each memory cycle is what makes horizontal line drawing fast. Table I below is a listing of the source code for a Texas Instruments TMS34010 GSP, which performs this line drawing operation. The image data generated by the graphics system processor 12 appears on the I/O data bus 16 in Y,X coordinate form with the Y,X coordinate address information appearing on output address lines 18 and 20 of the graphics system processor. Conventionally, when the graphics system processor 12 converts the X,Y pixel location into a video memory address, it uses the X value as the least significant portion of the video memory address and the Y value as the most significant portion of the video memory address. But since the X and Y values have been exchanged, the video memory address that is generated will have the least significant portion generated from the Y value, and the most significant portion generated from the X value. In order to write the pixels into the proper, conventional locations in a video memory 28, it is now necessary to exchange the upper and lower portions of the address bus before presenting the address to the video memory. The Y coordinate address line 18 and X coordinate address line 20 are therefore connected to an address translator circuit 22 included in the image data generation circuit 10. The address translator circuit 22 converts the address information on the Y and X coordinate address lines 18 and 20 to X and Y coordinate information on address lines 24 and 26, respectively. Thus, when the graphics system processor 12 outputs the video memory address, the address translator circuit 22 reverses it again. The result is that the Y-half (low order bits now) determine the row address in the video memory 28, and the X-half of the address (higher order bits now) determine the column address in the video memory. The X and Y coordinate address lines 24 and 26 are connected to the video memory 28 which also receives image data on the data I/O bus 16. The video memory 28 stores image data in memory locations identified by the address information on the X and Y coordinate address lines 24 and 26 to write the image data into the appropriate memory locations for generating a raster display. In accordance with the invention, the video memory 28 preferably comprises 16 VRAMs, arranged as four banks of four. For example, the video memory 28 can comprise conventional 256K-bit video memories arranged as 64K x 4 VRAMs available from any one of a number of integrated circuit manufacturers. The video memory 28 also comprises a video memory shift register 28A into which image data for drawing each individual row of the raster display is sequentially written and subsequently fed on a video data bus 32 to a CRT 30 to modulate the electron beam. For example, the CRT 30 can be a Sony Part Number CHM-7501-00 color monitor. Accordingly, the graphics system processor 12 accesses the video memory 28 in groups of either vertically adjacent memory cells or horizontally adjacent memory cells. During drawing, the video memory 28 is accessed vertically, resulting in the writing of four vertically adjacent pixels a memory cycle. During screen update or refresh, the video memory shift register 28A is accessed horizontally, shifting out four horizontally adjacent pixels per shift cycle to the CRT 30. The effect of this dual access is that it allows faster writing of vertical lines, yet it still allows conventional transfers of pixels to the CRT 30 for screen update or refresh. Fig. 2, comprising Figs. 2A and 2B, shows a detailed implementation of the image data generation circuit 10 shown in Fig. 1. The correspondence between the elements of the block diagram in Fig. 1 and the corresponding implementation shown in Fig. 2 is indicated by labeled boxes in Figs. 2A and 2B. As shown in Fig. 2A, measurement data is preferably entered through the graphics system processor 12 to the display list memory 14 shown in Fig. 2B, rather than directly to the display list memory. During this operation, the graphics system processor 12 serves as a slave processor, and this operation does not form any part of the image data generation method in accordance with the invention. Referring to Fig. 2A, the high address information (X coordinate information) on address lines 20 appears at pins LAD 2 through LAD 11. The low address (Y coordinate information) appears on pins LAD 0 through LAD 9. Image data appears on pins LAD 0 through LAD 15. This occurs during three sequential output periods of the graphics system processor 12. Generally, the circuitry which performs the video memory interface is shown on the right half of Fig 2A. The VRAMs of the video memory 28 are shown on the left half of Fig. 2B. The hardware is all standard off-the-shelf components. The component types are indicated in Figs. 2A and 2B. The address translator circuit 22 is preferably implemented by two PALs U49 and U50. This logic controls accesses to the video memory 28, choosing the proper VRAMs for a given memory cycle. This hardware latches the row address from the graphics system processor 12, using LRAS. Next, it sends the column address from the graphics system processor 12 to the VRAMs of the video memory 28, followed immediately by a LRAS signal. (This LRAS signal will come from the LCAS signal of the graphics system processor 12.) Thus, the VRAMs of the video memory 28 will use the column address as their row address. Next, the hardware waits 24 nS, determined using the LCLK1 and LCLK2 outputs from the graphics system processor 12. Finally, it sends the row address (latched in the first step above) to the VRAMs of the video memory 28 followed immediately by a LCAS signal. (This LCAS signal will come from the LCAS signal of the graphics system processor 12, delayed 24 nS.) Thus, the VRAMs of the video memory 28 will use the row address from the graphics system processor 12 as their column address. The equations for the logic implemented in the two PALs U49 and U50 which implement the address translator circuit 22 are shown in Table II below. As shown in the right hand portion of the block 22 in Fig. 2A, the address information which appears on the address lines 24 and 26 is preferably multiplexed to the video memory 28 (Fig. 2B) on lines A0 through A7. A DMUX U46 and latch U47, shown below block 22 in Fig. 2A, interconnect the graphics system processor 12 and the video memory 28 (Fig. 2B) to control loading and shifting of the video memory shift register 28A whose outputs appear on lines SD0 through SD15 from the video memory, which form the video data bus 32. The four D flip-flops U30A, U30B, U41A, and U41B, shown below the block 34 in Fig. 2B, provide the required timing on a shift control bus 38B that controls the shifting of image data from the shift register 28A. In summary, in the graphics system processor 12, the X-half of an X,Y register constitutes the lowest order bits of the video memory address, and the Y-half of the X,Y register constitutes the higher order bits. In order to draw vertical lines quickly, the X and Y halves of the X,Y register need to be arranged exactly the opposite. The solution is to reverse the X and Y positions in the registers of the graphics system processor 12, and then to reverse the row and column address lines going to the VRAMs of the video memory 28. Accordingly, the X and Y pixel coordinates are reversed in software, defining the lower half of each X,Y register as the Y half and the upper half as the X half. Transposing the X and Y addresses requires additional hardware to reverse the row and column addresses supplied by the graphics system processor 12 before they are presented to the VRAMs of the video memory 28. Accordingly, the address translator circuit 22 reverses the X and Y pixel address coordinates during drawing cycles of the graphics system processor 12, but maintains standard addressing for screen update or refresh and memory update or refresh. Doing so, the pixels end up being stored in VRAM with their X and Y values oriented in a standard manner. It is now possible to shift these pixels out to the CRT 30 in a standard fashion. As shown in Fig. 1, the image data generation circuit 10 also preferably includes a pixel processing circuit 34 connected between the video memory shift register 28A and the CRT 30. The pixel processing circuit 34 replicates pixels based on the image data that appears on the video data bus 32 to double the width of the trace displayed on the CRT 30. Pixel stretching is a method of doubling pixel positionability in the horizontal direction, while not actually doubling the resolution requirements of the CRT 30. To implement pixel stretching, the horizontal resolution of the raster display is doubled, resulting in much smoother appearing, near-vertical lines. In doubling the horizontal resolution alone, a problem occurs. Vertically oriented lines are now twice as thin as horizontal lines, and so they appear much dimmer. To compensate for this dimming, each pixel is stretched to twice its width in the horizontal direction. The result is vertically oriented lines of proper brightness and with improved smoothness. A resolution of 512 x 400 (identical to a Series 300 Bobcat computer with medium resolution graphics and 35741 display) is initially chosen. In accordance with pixel stretching, the resolution is doubled to 1024 x 400. In spite of the doubled horizontal resolution and video rate, the same 512 x 400 CRT can be used. The reason is that the video input signal to the CRT is still composed of standard 1/512 width pixels (1/1024 * 2). The only difference is that the pixels are sometimes offset half a pixel width, due to 1/1024 positionability. Note that since each pixel is still 1/512th of a line wide, and not 1/1024th, the bandwidth requirements of the CRT are not increased. Table III below illustrates an example of how a vertically oriented line would appear using various display techniques. Each "X" represents 1/1024th of the screen width. Since the CRT has a horizontal resolution of only 512 pixels, the question arises whether or not 1024 pixels can be mapped onto it. Won't some pixels land between the colored phosphors? The answer is that the screen of a color RGB monitor can be thought of as a continuous field of RGB phosphor and not discrete colored phosphor trios. This conceptualization is valid because the electron beam which strikes the phosphor has a Gaussian distribution about its center. This distribution causes approximately 2.67 phosphor trios on the face of the CRT to glow. (A portion of this wide distribution is caused by the inability to turn the electron beam on or off instantaneously.) When viewed by the human eye, the brain quantifies the spot, making it appear to be emanating from a single point, not 2.67 individual phosphor trios. Thus, the exact point of electron beam landing, be it centered on a trio or in between two trios, has little effect on the resultant image. Pixel stretching is preferably implemented in hardware. As shown in Fig. 2B, the pixel processing circuit 34 comprises two multiplexers U15 and U16 for a 16-to-8-bit data reduction connected to a latch U29 and a PAL U14. Each adjacent pair of pixels enters the pixel stretching PAL U14. This PAL uses the following stretching algorithm. IF the current pixel is a background color (0000), THEN output the previous value of the pixel (stretch it). IF the current pixel is NOT a background color (0001...1111), THEN output the current pixel (don't stretch the previous pixel). An example of pixel stretching follows. input: ......R...G.B....... (R,G,B = colors) result: ......RR..GGBB...... (. = background) It is possible for a group of non-background color pixels to be packed so closely together that some of them cannot be stretched. For example, let's suppose that there were three adjacent red, green, and blue pixels. input: .......RGB.......... result: .......RGBB......... Note that the red and green pixels cannot be stretched to their full width, but they are still displayed at their unstretched width (limited somewhat by CRT bandwidth). On a 512-pixel wide screen, however, only two pixels could have been displayed in the same case, due to its lower resolution. Hence, pixel stretching is still advantageous. The PAL equations for the PAL U14 appear in Table IV below. As shown in the right hand portion of Fig. 2B, a video palette U1 is connected to the outputs of the pixel processing circuit 34 for converting the digital image data to analog signals which are input to the CRT 30. For the sake of simplification, the detailed circuit of CRT 30 is omitted from Fig. 2, since it forms no part of this invention. Operation of the image data generation circuit 10 is summarized in the flow chart shown in Fig. 3. As shown in Fig. 3, the graphics system processor 12 initially reads X,Y values from the display list memory 14, as indicated by the numeral 100. In accordance with the method of the invention, the graphics system processor 12 next reverses X and Y values to Y and X values, as indicated by the numeral 102. Then, the graphics system processor 12 examines the direction of the reversed line joining adjacent Y,X values, as indicated by the numeral 104. On the one hand, if the slope of the reversed line is nearly horizontal, as indicated by the numeral 105, the graphics system processor 12 breaks the reversed line into a series of horizontal line segments, as indicated by the numeral 106. Then, the graphics system processor 12 draws line segments using the "FILL" command, as indicated by the numeral 108. After the graphics system processor 12 executes the "FILL" command to draw line segments in groups of four horizontally adjacent pixels, as indicated by the numeral 110, the address translator circuit 22 reverses the upper and lower halves of the address bus, essentially reversing X and Y values, as indicated by the numeral 112. This selects the video memory 28 to allow writing of pixel data into four vertically adjacent memory cells (locations), as indicated by the numeral 114. The pixel data is thus written into the video memory 28 in conventional format, as indicated by the numeral 116. On the other hand, if the slope of the reversed line is not nearly horizontal, as determined by the step 105, the graphics system processor 12 draws a line using a conventional "LINE" command, as indicated by the numeral 118. Accordingly, the graphics system processor 12 executes the "LINE" command to write one pixel at a time into the video memory 28, as indicated by the numeral 120. This is relatively slow, due to a read/modify/write process described in more detail below, and added computation. Then, the address translator circuit 22 reverses the upper and lower halves of the address bus, essentially reversing X and Y values, as indicated by the numeral 122. In accordance with the read/modify/write process, the graphics system processor 12 next reads four vertically adjacent pixels from the video memory 28, three are masked, and one is modified, and then the resultant pixel data is written into four vertically adjacent memory cells in the video memory, as indicated by the numeral 124. The pixel data is thus written into the video memory 28 in conventional format, as indicated by the numeral 116. Next, the pixel data is read into the video memory shift register 28A and shifted out as indicated by the numeral 126. Preferably, pixels are stretched, as indicated by the numeral 128. Finally, the image is displayed by the CRT 30, as indicated by the numeral 130. In accordance with the invention, the conventional line drawing process of the graphics system processor 12 is improved to smoothly and consistently track vertical transitions in traces. This is particularly useful in displaying measurement data traces of instruments, such as network analyzers. An exemplary trace appears in Fig. 4. Table V below compares the drawing speed of the TMS34010 GSP as it was designed to be used versus being incorporated into the image data generation circuit 10 when drawing vertical lines. Pixel Size Normal Operation Transposed Operation 1 640 nS/pixel 20 nS/pixel 2 640 nS/pixel 40 nS/pixel 4 640 nS/pixel 80 nS/pixel 8 640 nS/pixel 160 nS/pixel 16 640 nS/pixel 320 nS/pixel In the example of 4-bit pixels, there is an eight-fold increase in the rate of updating the video memory 28 and a corresponding increase in the speed of updating the CRT 30.

Claims: Apparatus for generating images on raster display means for displaying image data in the form of images, comprising: a graphics system processor (12) for receiving information to be displayed from a display list memory (14), at least a portion of the information being in the form of X,Y coordinate data, said graphics system processor (12) having a capability of generating lines in a X direction faster than generating lines in a Y direction, and a video memory (28) for storing image data, the video memory being connected to the graphics system processor (12) and connectable to the raster display means (30), the video memory being adapted to store image data in the form of X,Y coordinate data; characterized in that the graphics system processor (12) is adapted for transposing each received set of X and Y coordinates so that X,Y becomes Y,X and then processing at least one adjacent pair of transposed coordinates to generate at least one line segment by using a first set of transposed coordinates as the starting point and an adjacent second set of transposed coordinates as the end point to generate image data and addresses for storage of the image data, said adjacent coordinates corresponding to sequentially addressed memory cells of the display list memory (14), said memory cells respectively containing untransposed raw data in X,Y coordinate form for said starting and end points for said at least one line segment; and that the apparatus comprises an address translator circuit (22) connected by first address lines (18, 20) to the graphics system processor (12) and by second address lines (24, 26) to the video memory (28), wherein the address translator circuit (22) retransposes each received set of Y and X coordinate data from the graphics system processor (12) so that Y,X is restored to X,Y, thereby enabling the writing of image data into the video memory (28) so that the stored image data is properly fed under control of the graphics system processor (12) to modulate the electron beam of the raster display means (30). Apparatus of claim 1 characterized in that the address translator circuit (22) writes into the video memory (28) by reversing address select lines to the video memory (28) so that the image data is correctly stored for later access. Apparatus of claim 2 characterized in that the address translator circuit (22) enables the image data to be written into the video memory (28) in banks of image data oriented in the Y direction, as compared to banks of image data oriented in the X direction, so that the image data is stored in the video memory (28) in a conventional format for updating images on the raster display means (30). Apparatus of one of the claims 1 to 3 characterized in that the raster display means (30) is a CRT. Apparatus of one of the claims 1 to 4 characterized in that the graphics system processor (12) computes a best-fit set of points between the starting and end points to interconnect them using Bresenham's line-drawing algorithm. Apparatus of one of the claims 1 to 5 characterized by a pixel processing circuit (34) connected between the video memory (28) and the raster display means (30) for replicating an adjacent pixel for each pixel of each line segment to provide a smooth, high resolution trace. Apparatus of one of the claims 1 to 6 characterized in that said information to be displayed is stored in the display list memory (14); that the graphics system processor (12) is connected to the display list memory (14) and programmed for reading said information to be displayed from the display list memory (14) on an I/0 data bus (16) which interconnects the graphics system processor (12) and the display list memory (14), the graphics system processor after reading the information to be displayed in X,Y coordinate form for transposing the information to be displayed to Y,X coordinate form commencing a line-drawing operation which translates the information to be displayed to a pictorial representation in the form of image data, the image data generated by the graphics system processor (12) appearing on the I/O data bus (16) in Y,X coordinate form with the Y,X coordinate address information appearing on the first and second output address lines (18,20) of the graphics system processor (12), such that a video memory address that is generated will have the least significant portion generated from the Y value and the most significant portion generated from the X value; that the address translator circuit (22) converts the address information on the first and second output address lines (18,20) of the graphics system processor (12) to X and Y coordinate information appearing on the first and second output address lines (24,26) of the address translator circuit (22), respectively, to exchange the upper and lower portions of the address bus before presenting the address to the video memory (28) so that the Y-half determines the row address in the video memory (28) and the X-half of the address determines the column address in the video memory (28); and that the video memory (28) is connected to the first and second output address lines (24,26) of the address translator circuit (22) and the I/O data bus (16) for receiving image data, the video memory (28) storing image data in memory locations identified by the address information on the first and second output address lines (24,26) of the address translator circuit (22) to write the image data into memory locations for generating images on the raster display means (30). Apparatus of claim 7 characterized in that the video memory (28) comprises 16 VRAMs arranged as four banks of four. Apparatus of claim 8 characterized in that the video memory (28) comprises conventional 256K-bit video memories arranged as 64K x 4 VRAMs. Apparatus of one of the claims 7 to 9 characterized in that the video memory (28) comprises a video memory shift register (28A) into which image data for drawing each individual row of the raster display means (30) is sequentially written and subsequently fed on a video data bus (32) to the raster display means (30). Apparatus of claim 10 characterized in that the raster display means is a CRT (30). Apparatus of claim 11 characterized in that the CRT (30) is a color monitor. Apparatus of one of the claims 7 to 12 characterized in that the graphics system processor (12) executes a fill rectangle command for performing actual line drawing. Apparatus of one of the claims 10 to 13, characterized in that it further comprises a pixel processing circuit (34) connected between the video memory shift register (28A) and the raster display means (30) for replicating pixels based on the image data that a4pears on the video data bus (32) to double the width of a trace displayed on the raster display means (30). Apparatus of claim 14, characterized by a video palette connected between the pixel processing circuit (34) and the raster display means (30) for converting the image data to analog signals which are input to the raster display means (30). Method for generating images on raster display means, comprising the steps of: receiving information to be displayed from a display list memory (14), at least a portion of the information being in the form of X,Y coordinate data, by means of a graphics system processor (12) having a capability of generating lines in a X direction faster than generating lines in a Y direction, storing image data in the form of X,Y coordinate data in a video memory (28), the video memory being connected to the graphics system processor (12) and connectable to the raster display means (30) the video memory being adapted for storing image data in the form of X,Y coordinate data; the method being characterized by the steps of: transposing each received set of X and Y coordinates by means of the graphics system processor (12) so that X,Y becomes Y,X and then processing at least one adjacent pair of transposed coordinates to generate at least one line segment by using a first set of transposed coordinates as the starting point and an adjacent second set of transposed coordinates as the end point to generate image data and addresses for storage of the image data, said adjacent coordinates corresponding to sequentially addressed memory cells of the display list memory (14), said memory cells respectively containing untransposed raw data in X,Y coordinate form for said starting and end points for said at least one line segment; and exchanging X and Y halves of video memory addresses computed by the graphics system processor (12) by means of an address translator circuit (22) connected by first address lines (18, 20) to the graphics system processor (12) and by second address lines (24, 26) to the video memory (28), wherein the address translator circuit (22) retransposes each received set of Y and X coordinate data from the graphics system processor (12) so that Y,X is restored to X,Y, thereby enabling the writing of image data into the video memory (28) so that the stored image data is properly fed under control of the graphics system processor (12) to modulate the electron beam of the raster display means (30). Method of claim 16 characterized in that the step of receiving information comprises the step of: reading raw data in X,Y coordinate form (100) by means of the graphics system processor (12), that the steps of transposing and processing information comprise the steps of: transposing the raw data to Y,X coordinate form (102) by means of the graphics system processor (12); determining the X separation between adjacent points of the transposed data (104) by means of the graphics system processor (12); determining the Y spacing between adjacent points by means of the graphics system processor (12) if the X spacing is less than a predetermined distance; breaking the Y spacing into a set of Y line segments offset in the X direction from one another by one pixel by means of the graphics system processor (12) if the Y spacing is greater than the X separation or the slope is greater than 45 degrees; breaking the Y spacing into two Y line segments of equal length, ignoring round-off, by means of the graphics system processor (12), if the cumulative offset between X coordinates is one; and if the cumulative offset is two or more, computing the number of Y line segments to be Delta X plus 1, where Delta X equals the number of pixels separating the adjacent X coordinates, determining a length for each Y line segment by computing the Y spacing so as to determine the number of pixels between the Y coordinates of the adjacent points, inclusive of the end points, and dividing the result by Delta X, ignoring round-off, and, finally, setting the first and last Y line segments to be half the length of the remaining Y line segments by means of the graphics system processor (12). Method of claim 17, characterized by the step of pixel stretching (128) so that if the current pixel is a background color, then the previous value of the pixel is output, and if the current pixel is not a background color, then the current pixel value is output. Method of claim 16 or 17 characterized in that the steps of transposing and processing information comprise the steps of: initially reading X,Y values from a display list memory (100); reversing X and Y values to Y and X values (102); examining the direction of the reversed line joining adjacent Y,X values (104); breaking the reversed line into a series of X line segments (106), if the slope of the reversed line is nearly the X direction (105); and drawing line segments using a "FILL" command (108) to draw line segments in groups of pixels (110) adjacent in the X direction; wherein the step of exchanging X and Y halves of video memory addresses comprises reversing the upper and lower halves of an address bus (112), thereby reversing X and Y values to select a video memory to allow writing of pixel data into memory cells (114) adjacent in the Y direction so that the pixel data is thus written into video memory in conventional format (116). Method of claim 19, characterized by the steps of: drawing a line using a conventional "LINE" command (118) to write one pixel at a time into the video memory (120), if the slope of the reversed line is not nearly the X direction (105); reversing the upper and lower halves of the address bus, thereby reversing X and Y values (122); and performing a read/modify/write process by reading a predetermined number of pixels adjacent in the Y direction from the video memory, masking all but one, and modifying one, and then writing the resultant pixel data into the predetermined memory cells adjacent in the Y direction in the video memory (124) so that the pixel data is thus written into the video memory (124) in conventional format.

Inventors: PETERSEN ROGER J, PETERSEN, ROGER J.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1989
Main classification: G09G1
IPC codes: G06F3, G09G5, G06T11