GRAPHIC AND TEXTUAL IMAGE GENERATOR FOR A RASTER SCAN DISPLAY

Abstract: A raster scan display apparatus for generating both graphic and textual images. The display refresh memory (46) is broken into an information memory (79) and an attribute memory (120). The information memory has one location for each picture element in the display; the attribute memory is of lower resolution and has one memory location for a plurality of picture elements. The attribute memory provides foreground and background attributes which can be varied only in multiples of picture element locations. In response to the information memory contents for each location, either the foreground or the background attribute is provided to the display, from the attribute memory. The information and attribute memories operate in conjunction with a pattern generator (136, 160, 162) to establish the information to be displayed between pairs of points. The pattern generator provides a finite length, repeating pattern of 1's and 0's representing the selection of foreground or background attribute for each location intermediate first and second points on the display. The rate at which the pattern is shifted may be varied to alter character or object size on the display, thus permitting all sizes of a character to be represented in memory in a cell of small fixed size. For generating two-dimensional images, two pattern generators are employed; the second pattern generator provides the starting positions and patterns for the first pattern generator. This permits characters to be reoriented-i.e., tilted, italicized, etc., with simple commands which reset the parameters governing the operation of the pattern generators.

Description: GRAPHIC AND TEXTUAL IMAGE GENERATOR FOR A RASTER SCAN DISPLAY Field of the Invention The present invention has utility generally in the field of digital systems. More precisely, it addresses the problems of generating images for display on a video display device. Such a video display may be used as a computer output device. The invention is most clearly suitable to a raster-scan display. Objects and Intended Applications of the Invention The apparatus and methods disclosed herein are used for generating both graphic and textual images. Though of broad, general utility, they are not intended to be used for all display systems or for generating every conceivable type of two-dimensional image. Rather, the invention is intended to best cover a broad but specific range of image-generation applications which roughly require the following characteristics: 1. An ability to provide "presentation" level line- drawings. Such images include x/y data plots, bar graphs, pie charts, flow charts, block diagrams, room layouts and so forth. In short, these are the types of graphic images which a user might typically wish to use in a slide presentation, with a quantity of information content and complexity appropriate to such a presentation. 2. Fairly general text display capabilities. In particular, some or all of the following capabilities would be desirable for the presentation: character size variability; font selection from among a variety of alternate and user-definable fonts (foreign language alphabets, mathematical symbols, etc.); character spacing options providing variable uniform spacing of characters on a line, proportional spacing of characters, etc.; italic or other slanting of characters; writing of text at angles other than horizontal (such as to label the - AT axis of a graph) ; subscripting and superscripting; overstriking of characters (such as for certain computer languages and foreign language accents) ; arbitrary placement of text strings on the display screen; and arbitrary combinations of the above features. In addition to being useful for projecting presentations, these features also can be used to advantage in (1) work stations for typeset users, (2) terminals for computer aided instruction and (3) work stations for management pesonnel. 1. In a typesetting work station, the video display would be used during the document preparation phase, to present a rough draft of the final document. This type of applicaiton would be particularly useful in newspaper, textbook, journal report, conference paper and in-house report preparation environments. The combination of graphical and textual display ability permits the video display to be used to show the figures integrated with the text, the ability to prepare line art figures on the same terminal as the text portions, and the display of text in much the form as it will take (in size, font, etc.) on typesetting. In a manager's work station, the invention permits the display of reports, memos, mail messages and summaries of data analysis including PERT charts, GANTT charts, and so forth. Accordingly, it is an object of the invention to provide apparatus for generating both graphic and textual images on a raster scan display, suitable for the uses above-described. It is a further object of the invention to provide such capability wherein the speed of image generation is a good match to the average speed of transmission of characters on a transmission line from a host computer to the display device. «_. • ■ -1 j C'.PI Yet another object of the invention is to provide such capabilities in a device which is economical to construct and does not overburden or excessively burden a host computer to which it might be attached. These and other objects and advantages of the present invention will be apparent from the description which follows below and explains how the invention achieves such objectives. Background of the Invention To illustrate the intended range of applicability of the present invention, it is helpful to divide into several categories the types of images which video display terminals may be capable of presenting. These are listed below, generally in increasing order of complexity. The most basic type of image is typewriter text consisting entirely of characters found on typical typewriters. These characters are all of the same size and no provision is made for varying the spacing between them. Some very simple types of graphic images may be made available in such machines by building in certain special characters (such as line segments) which can be used to construct boxes, lines and so forth. Examples of such terminals are the models VT52 and VT100 terminals sold by Digital Equipment Corporation of Maynard, Massachusetts, U.S.A. The next level of image complexity is represented by line graphs. Such images consist of typewriter text plus the ability to present a fixed number of x/y data plots using a fixed number of data representations, such as point plots, bar graphs and line charts. Exemplary terminals with this level of capability are the models VT55 and VT105 terminals, also sold by Digital Equipment Corporation. A third level of image complexity is represented by typewriter text plus basic line art. Images of this type consist of typewriter text, as defined above, and also allow fairly general line art images such as flow charts, block diagrams, PERT charts, and so forth. The majority of graphics terminals on the market today have this level of capability. Examples are several of the terminals sold by Tektronix Company of Beavertown, Oregon, the HP 27XX Family of terminals sold by Hewlett-Packard Company and the IBM 3279 and many Ramtek.Inc. terminals. General line art images represent a next level of complexity. Such images consist of line art drawings and allow text to be written in a fairly general fashion. For example, text may be written in a variety of sizes and fonts (including foreign and scientific alphabets). In addition, characters may be displayed in a variety of italic slants, at arbitrary angles, with variable spacing (uniform expansion or contraction of characters on a line), and with proportional spacing (i.e., with spacing between characters in a word, varying according to character width) . Characters may also be written by arbitrary overstriking, in subscript and superscript positions, and with a wide variety of other textual features found in textbooks and other typeset documents. The present invention was developed for this level of complexity. Generally the most complex level of imaging is represented by photograph type images, which are defined as images of a type which any camera is capable of taking. This level of imaging capability is usually used for image processing (i.e., analysis of data taken by actual cameras) and for the development of realistic cartoons and animations. While various types of electronic display devices are available, the popularity of the raster-scan display has been increasing in recent years, particularly for computer terminal applications. The present invention relates principally to that type of display device. " BUR- ■ U OΓ-.-.ΓI In a raster-scan display, the screen is composed of a rectangular matrix, or raster, of dots. Each dot comprises a picture element, or pixel. The display hardware controls the intensity of each pixel, or its color if a color display device is employed. A raster scan image is generated not by tracing out a sequence of geometric items, such as lines and characters, but by plotting point-by-point the intensity value (or color) of each pixel in a two-dimensional raster or matrix. To display an image, the intensity or color of every pixel in the raster matrix must be determined. Complex images are constructed by creating appropriate patterns of pixel illumination. Strictly speaking the term "raster scan" also implies that the image is scanned onto the screen surface in a raster sequence - i.e., as a succession of equidistant scan lines, each scan line being made up of a series of pixels. Nevertheless, for purposes of this explanation, other types of displays are included under the heading raster scan, even though no such scanning out takes place, since the invention is not directed at any particular display refresh methodology. Various architectures are available for creating the pixel pattern. Indeed, the history and variety of architectures for presenting computer generated images at a user terminal is nearly as long and as extensive as for computers themselves. As has been the case for basic computer design, the evolution of hardware technology has played a major role in the development of imaging architectures. For purposes of comparing the present invention to the prior art, four types of architectures are relevant: fixed character architecture, variable character cell architecture, bit-map architecture, and combined character/bit map architecture. In fixed character machines (of which all current typewriter text level imaging terminals are an example) , the visible viewing area of the display is broken up into a coarse grid of C horizontal character positions (or columns) and R vertical text lines (or rows). The values C = 80 and R = 24 are typical for many of the current machines used for computer programming and general interactive computing. Very simple terminals, such as home computers, have values of these parameters as low as C - 24 and R = 10, which is consistent with the resolution capabilities of the standard color television receivers which are used as the display devices for many of such machines. The image forming circuitry for these machines generally consists of two memories in a sort of hierarchy. The first refresh memory is a random access memory (RAM) which has one storage location for each of the major grid positions. The second or character memory is a read only memory (ROM) which contains the definition of the rectangular pixel arrays which are used to present the visual pattern which defines each of the possible characters available in the character set. These character patterns are generally referred to as "cells". Each character cell in the memory has CC x CR bits „ of storage, where CC is the number of bits that define the column width of the character image and CR is the number of bits required to define the vertical rows in each character pattern. Typical values of these parameters are 5 x 7 for a low resolution home computer device, 9 x 10 for a medium resolution computer terminal and 30 x 40 for a phototype setting machine. For a machine capable of displaying an alphabet of A characters, the cell memory would contain A x CC x CR bits of storage, although various design tricks may be employed to reduce this number as much as 50%. Typical values of A are 64 for an alphabet having only upper case characters, 96 for terminals which are capable of displaying the full ASCII character set and about 200 for machines capable of displaying these alphabets plus some ■ 0Λ number of special foreign alphabet characters and simple line graphic patterns. In these devices, the image is drawn by writing into each refresh memory location the unique code for the character pattern desired to be displayed at each visual location. For example, if the display is to be shown as a blank "sheet of paper" (i.e., "cleared"), then the code for the blank character is written into each of the refresh memory locations. During the refresh phase of machine operation, which of course takes place continuously, the contents of the refresh memory are read out in sequence from consecutive locations. For each code read out of the refresh memory, the visual pattern for the corresponding cell pattern is read out of the ROM and used to control the intensity of the CRT image by the simple mechanism of turning on the appropriate beam when a cell entry bit is a logical "1" and turning off the beam (black intensity level) when the cell entry bit is a logical "0". The reason behind the wide use of this architecture is that it has relatively small memory requirements. For example, for a machine having C = 24, R = 10, A = 64, CC = 6 and CR = 8, (a typical home computer system), the machine requires only 1440 bits of RAM and 3072 bits of ROM. Translated into current integrated circuit technology, such a device would require at most three integrated circuits for the memory portion of the design; the control functions, including the analog interfaces to the display device, are performed by one or two more integrated circuits. A typical computer terminal for which C = 80, R = 24, A = 128, CC = 9 and CR = 10 would require 13,440 bits of RAM and 11,520 bits of ROM, which is still less than a half dozen modern integrated circuits. In some of the machines implemented under this architecture, a limited fixed number of visual attributes -BU EA U OVFI are provided (e.g., blinking and intensity level selection) by adding one bit to each of the refresh RAM codes for each of the possible display locations. When these bits are read out of the memory during the image refresh process, special hardware circuits are used to modify the character pattern appropriately. For example, character blinking is implemented by using a timer which disables the character cell bits when the character location attribute bit is set and the timer is in its ON state. When the timer is in its OFF state or if the attribute bit is not set for a particular character position, the character pattern is displayed normally. The ability to show the characters in color is usually handled in a similar manner; that is, each refresh memory location will have a certain number of bits which indicate the color to be displayed for the character at that screen location when the cell pattern definition bits are logical "1". Because of the fixed coarse grid arrangement, this architecture limits the imaging level to the typewriter text level of capability, and the use of a fixed cell definition memory. Character spacing is restricted by the fixed horizontal positions, character size is constrained to the size of the fixed coarse grid size, the display of subscripted characters is prohibited and it is very difficult to display circles, and so forth. The second design approach, variable character cell architecture, involves an extension of the fixed character architecture which allows the presentation of simple line graphs and (in some cases) text plus line art type graphics. In this approach, the number of bits in each of the refresh memory locations is increased so that the hardware can access several possible cell pattern definition memories. Usually one of these cell definition memories is still a ROM, so that the typewriter text image capability is maintained. The / D - ' -A / O-'.PI other cell definition memories are RAM's; thus a fairly general image may be formed by first writing bit patterns into the redefinable character memories and then writing the codes for these new "character" patterns into the refresh memory at the locations where they are to be shown. For example, to draw a circle on these machines, the circumference of the circle is broken up into sections small enough so that each will fit into the definition of a cell (this may take as many as 100 cells); then the code for each of these cells is written into the refresh RAM at the location where its segment is to be displayed. Visually the pieces of the circle seem to all fit together, and the user is not aware that the image was pieced together. Variable character cell architecture has the advantage of requiring relatively little memory, but it still limits the presentation of text to the typewriter level. The principle disadvantages to this approach are that: (1) in the worst case, this type of machine may require more memory that the bit-map approach discussed next, since the complexity of the image it is capable of drawing is limited to the number of redefinable cells; and (2) the algorithms for drawing the images are generally quite complex. In the latter respect consider, for example, what happens when two straight lines cross. The definition of the cell must be read out of the memory then modified and written back into the memory. For this reason, the cells for these types of machines are either generated on a host computer or in the local controller of the machine, which is generally relatively slow. The third basic type of architecture is termed the bit-map approach. These machines take a fairly direct route to forming the image, by eliminating the cell definition memory and making the refresh memory large enough so that each possible pixel location has a corresponding location in the memory. Therefore, a machine of this type which can display a black and white image of the same complexity as a fixed character machine having C = 80, R = 24, CC = 9 and CR = 10 requires 172,800 bits of memory, with one bit per memory location. For machines of this type that allow intensity variation or color, additional bits per pixel (or, as it is sometimes described, additional "planes" of memory) must beadded; thus, a typical machine capable of presenting a photographic type image with a quality comparable to that of broadcast television would require around a million bits of memory. An image for this type of device is drawn by writing a logical "1" into those memory positions for which the image is to be visible or writing the color code into that location in the case of a photograph type image. This architecture can be used to provide the general text writing features described in relation to the present invention, since characters can be written at any location and at any size. However, the image forming process is relatively slow for normal text writing and considerably more memory and other hardware is required. Because the display of a photograph type image does not normally require as many horizontal pixels as are needed for displaying 80 columns of text, these machines are often limited to about 512 horizontal pixel locations (about 64 horizontal character locations). Also, the drawing of line art type images is usually faster in these machines than in variable character machines, since the images do not have to be broken up into cell sections. The fourth of the above-listed approaches involves a combination of the character and bit-map techniques. Such machines attempt to get the advantages of both the relatively low cost and speed of the fixed character architecture and the general capability and display versatility of the bit-map approach; they do this by simply combining the two machines into one and "or-ing" the image produced by each part into one visible image. This approach is typical of many of the more advanced computer terminals which are currently available. The relative disadvantages are that it becomes difficult to maintain registration between images written using the bit-map section and text characters whose locations are constrained- by the character grid. Further, the text displayed is limited to typewriter text. Summary of the Invention The present invention involves an architectural design for a raster scan display apparatus capable of presenting both graphic and textual images. The architecture described in greater detail below is a modification of bit-map architecture. It uses the bit-map approach to gain generality, but breaks up the refresh memory into two components, one called the "information memory", or IM, and the other called the "attribute memory", or AM. The information memory is made to have sufficiently high resolution so that general text (i.e., with variable spacing, character sizes, orientations and so forth) can be written. The attribute memory, however, is made to have a lower resolution in general, to take advantage of the fact that in most cases the color (or blinking state or intensity) in general text and line art imaging does not have to change as often as in the case of photograph type imaging. This significantly reduces the cost of display generation as compared to the general bit-map approach. Thus, the intended class of images can be displayed with as much as 80% less memory than required by the general bit-map approach. Further, text can be written at speeds comparable to those provided by the fixed character approach. In contrast to the character/bit-map combination, the present invention utilizes simpler image-forming circuitry, since there is only one image-forming circuit, instead of two. Also, both text and line art images are written by the same mechanism, so there is no problem in keeping these parts of the display registered in the proper relative positions; they are not spliced together. Digressing briefly, it should be understood that each of the foregoing approaches to display generation is the best choice for certain types of applications and certain levels of cost. In this regard, the present invention fills an important gap between the limited character-oriented machines and the general bit-map machines for many applications where very general text and graphics capabilities can be used to advantage. Such applications include, but are not limited to, the preparation of reports which require the integration of text and drawings, charts, and the making of formal presentations wherein the more general text capabilities provided by the invention can be used to enhance visual interest. Among other things, mathematical formulas and the like which are difficult to generate with the simpler architectures are easily displayed with this architecture. In the present invention, although the refresh memory is separated into two parts, both the AM and IM respond to the same address bus, although each has its own data input/output format and resolution. This permits the memory address and data format parameters to be varied and general images to be presented at lower cost than would result f om * the prior art bit-map approach. The invention is further characterized in having the refresh memory for the imaging process distributed, or separated, from the main memory of the machine controller. That is, the architecture inherently has two separate address and data buses: one for normal controller memory usage and the communication of instructions to the image generating hardware and another devoted to allowing both the image generating hardware and the display refresh hardware to access the refresh memory. According to the invention, images are formed in memory entirely by the execution of sequences of "pattern modulated" line segments drawn from a current position to a given relative end point position based upon the contents of definable registers in the image generating hardware. One of these registers, called the pattern (PAT) register, identifies the data to be written into the successive IM pixel locations during the line segment writing process. Other registers in the image generating hardware, during a single invocation of a line segment operation, identify the attribute values to be written into the AM for each of the locations written in IM. The architecture also allows the writing process to be reversed, so that data may be read from the refresh memory into the PAT register; the processor can thereby access arbitrary data from the memory. By a particular sequence of pattern modulated vector operations, character patterns may be constructed in the refresh memory. The direction, repetition, and location of these sequences may be varied dynamically, creating the ability to vary the size, direction and spacing of characters, as well as the fonts, plus providing a number of other text enhancements. Text characters are formed in the same manner as graphics features. No additional apparatus is necessary for text imaging; of course, one could be included if the net system has more desirable characteristics. BRIEF DESCRIPTION OF THE DRAWING These and other features objects and advantages of the present invention will be better understood from the following detailed desciption read in conjunction with the accompanying drawing in which: Fig. 1 is a high level block diagram of a system in which the architecture of the present invention is used in a terminal for remote hook-up; Fig. 2 is a high level block diagram of a system utilizing the architecture and hardware of the present invention in a stand-alone terminal; Fig. 3 is a more detailed block diagram illustrating the flow of . operation in apparatus based on architecture of the present invention; Fig. 4A is a timing diagram illustrating the sharing of the memory between the refresh and image generation processors; Fig. 5 illustrates a block diagram describing the relationship between the IM memory the refresh processor and the image generation processor, including the multiplexing of the IM memory between the two processors; Fig. 6 is a block diagram illustrating the counters needed to generate the horizontal and vertical sync and the RX, RY coordinates; Fig. 7 illustrates in block diagram form the basic registers used for single pixel writing and their relationship to each other; Fig. 8 is a diagram showing the eight pixel vector directions; Fig. 9 is a listing in the PASCAL computer language describing the process for drawing a straight line horizontally to the right with length DU number of pixels; Fig. 10 illustrates a procedure for causing the coordinates to be changed by one pixel in the major compass direction defined by the parameter DIR; Fig. 11 is a listing of a procedure for drawing a straight line in one of the eight major directions of Fig. 8; Fig. 12 is a diagram of the eight octant areas between the eight major pixel vector directions; BUREΛTT C"PI Fig. 13 is a listing of a procedure for drawing an arbitrary angle vector using the rate multiplier approach of the present invention; Fig. 14 is an illustration of an example of the execution of the procedure of Fig. 13; Fig. 15 is an illustration of character drawing according to the present invention, showing how the letter "A" may be formed in italics or written at a slanted direction, for example; Fig. 16 lists a procedure for constructing character patterns; Fig. 17 is a diagram illustrating the writing attributed according to the present invention, showing the Boolean equations and giving examples of the types of images which result after using each of the modes; Fig. 18A is a block diagram of the simplest implementation of an attribute memory/information memory arrrangement according to the present invention; Fig. 18B is a block diagram of a slightly more complex embodiment wherein the information memory has a resolution of 256 x 256 picture elements and the attribute memory has a lower resolution of 64 x 64 entries; Fig. 19 is a block diagram illustrating the minimum register configuration for the display generation hardware of the present invention; Fig. 20 is an illustration of the basic timing and memory sharing for the process of drawing successive pixels into the memories of the present inventipn, showing also one possible sequence of register transfers for BREAK circuit computations; Fig. 21 illustrates the minimum instructions according to the general image generation instruction set for register read and mode operations; and Fig. 22 lists the minimum instructions according to the general image generation instruction set for pixel sequence operations. DETAILED DESCRIPTION From the viewpoint of system structure, the present invention is intended primarily for use in terminals which serve as input/output devices for computer systems. Most frequently, such terminals use serial interface lines in communicating with the computer. The present invention is well suited to such systems, since the speed of image generation it provides is a good match to the average speed of transmission of characters on the transmission line, assuming the use of a high level graphics language protocol. This apparatus also may be used in stand-alone terminals in which the graphics and extended text capabilities are accessed either by a higher level graphics language or by direct sub-routine reference to a lower level, but higher speed instruction set discussed below. Fig. 1 illustrates a high-level block diagram of a system in which the architecture of the present invention is used in a terminal for remote hook-up. The system as a whole consists of a host computer system 10 and a remote graphics device 12 incorporating the invention. The host computer system 10 provides a stream of high level commands which are sent to the remote terminal 12 over low to medium speed serial interface 14 (including telephone data transmission lines). Within the remote graphics device 12, the high level commands are interpreted and converted by a local processor 16 into lower level commands which are understood by and directly executed by the display generation hardware 18 (alternately referred to herein as the "image generator"). Other entry devices, such as keyboards and digitizers may be connected as at 22, to local processor 16, as a means for preparing and presenting visual images, as well. Fig. 2 illustrates a high level block diagram of a system utilizing the architecture and hardware of the present invention in a stand-alone terminal system. The principal difference from the system shown in Fig. 1 is that the local processor 16 in the stand-alone system must execute the higher level software as well as control the display generation hardware. For that reason, the image generation process may be slower on the average than for the case shown in Fig. 1. This deficiency may be overcome to achieve a system of even greater performance than the remote case, by the direct use of the lower level instructions. From an architectural point of view, such an application could also use an intermediate level, macro- instruction set instead of the low-level instructions sent directly to the display generation hardware 18. The macro-instruction set would be provided by application programs 24 and converted into the lower level instruction set by imaging software 26. The advantage in using the macro-instruction set is the elimination of the time-consuming overhead of interpreting the higher level language character streams, while allowing the user to take advantage of a high level language format (i.e., commands, etc.). This is preferable to using the lower level instruction set directly, since that instruction set has to be optimized at the software/hardware interface to a degree that does not support the direct generation of single characters or an arbitrary end point line image. A more detailed diagram illustrating the flow of operation in apparatus based on the architecture of the present invention is shown in Fig. 3. This illustration is applicable to both the remote terminal and stand alone terminal situations. As illustrated therein, the general flow of operation (which is similar to that in many other imaging architectures) is as follows: First, a stream of instructions in a high (i.e., user) level graphics instruction language is received on line 28 from a host computer keyboard or other source. That instruction stream is received by a real-time interpreter 30 which includes processor 16 and associated commonly-understood hardware and software. The output of the real-time interpreter 30, on line 32, comprises a stream of instructions in a general image generation instruction protocol or language understood by the display generation hardware 18. The interpreter 30, it should be observed, is part of the system even for a stand-alone device. This permits the graphics image to be stored and transmitted in the same higher level graphics instruction set used for communications on line 28. A syntax translator 34 takes the user level instructions received on line 28 and converts them to macro-instructions provided on line 36. In turn, a semantics generator 38 receives the macro-instructions and converts them to the low level instructions recognized by the display generation hardware and provided to it on line 32. This low level instruction set is referred to below as the "general image generator" instruction set. The macro-instructions on line 36 are also provided to a macro high-level converter 42, so that the graphic images in this system can be based on a single higher level language facility. The protocol generated by the macro high-level converter 42 must be consistent with the protocol provided on line 28; the requirement of standardization on a single higher level language restricts the level of the instruction set on line 28 to one compatible with the ability of converter 42 to construct the same syntax and semantics. " In Fig. 3, the display generation hardware is shown broken down into a general image generator 44, refresh memory 46 and display refresh processor 48, which feeds a display device, such as CRT 50. Semantics generators 38 perform such functions as generating a sequence of "pattern modulated vector" operations (described below) to write characters and sequences of line segments for drawing curves and filling areas. It is therefore generally not possible to reconstruct the syntax of the higher level definition of an image from its definitions at the general image generator instruction set level. Therefore, the general image generator instructions are not meant to be used for the transmission or storage of a graphic image; this usually is not a problem, since it is generally less efficient to use the general image generator level for image definition. Although a local application program generally has access to the general image generator level of instructions, the use of this facility is limited to "algorithmic" based imaging applications, such as on-line picture generation and animation. In display generation hardware 18, the general image generator 44 takes the instructions received on line 32 and creates in refresh memory 46 the appropriate dot patterns based upon the values of parameters established in registers contained in the general image generator. (Such registers will be explained below.) For the purpose of providing hard copy output of the current image to a hard copy screen dump device, or for detailed modification of the refresh memory contents by the local application program, the general image generator circuit 44 is capable of reading the contents of the refresh memory 46 back to the controlling processor, which communication is indicated generally at line 52. This facility generally allows higher quality hard copy output than normally is obtained by devices BU R £A OXPI " which use a terminal's video output line as the basis for the hard copy output. It is a characteristic of this architecture that once an image has been written into the refresh memory, it is not possible in general to reconstruct the original syntax which produced it. However, since the image can be stored by storing the high level instructions which produced it, it then may be exactly duplicated from that definition. On a continuous basis, a display refresh processor 48 reads the refresh memory 46 in the proper order, to maintain the visual impression of the currently defined image. In the present invention, the general image generator 44 and the display refresh processor 48 have equal accessiblity to the refresh memory on a time-shared basis. The display refresh processor 48 must always read the image from the refresh memory in a fixed order, but the general image generator 44 may access the memory in any sequence. The display refresh processor 48 generates a serial stream of information in either digital or analog form, to drive a variety of display devices capable of displaying an image in raster form. For the purposes of the discussion which follows, certain conventions are adopted with respect to that raster. It shall be assumed hereinafter that the raster image is displayed by a sequence of horizontal lines starting from the top of the display. Further, each line is displayed from left to right. Each possible pixel position is given a unique physical x,y position on the display device and has a corresponding logical x,y position in the refresh memory. The operation of the display refresh processor under the raster scan convention used herein starts reading data from the refresh memory at location row (i.e., y) = 0 and column (i.e., x) = 0. The y value is maintained at this value during the display of the first line, and the x value is incremented until all the defined x values for that line have been read out of memory. The y value is then incremented and the process is repeated starting with a value of x = 0. In turn, the line readout process is repeated until all of the defined lines have been readout, at which point the y value is returned to 0 and the process is repeated. By contrast, the general image generator circuit is capable of modifying randomly the information and attribute value of pixels at locations having arbitrary x,y values. When a pixel value is changed, its new value is displayed the next time the display refresh processor reads that location. Thus, the image appears to change visually at the rate by which the general image generator hardware modifies the image, which may be very rapidly (as in the case for clearing the screen, for example) or more slowly, depending upon the complexity of the image or the use of image animation techniques in which it is the desire of the user to have the system illustrate the change and make it apparent to the user. It will be understood now that the architecture of the present invention is very flexible and that a wide variety of devices can be used for display device 50 simply by changing the parameters of the memory size and display refresh process. Thus, such devices would include low resolution binary level black and white displays, full tone black and white monitor displays, high resolution black and white monitors, binary level RGB color or television displays (using only the primary and complimentary colors to generate the display) , and full tone color images on high resolution color devices. The cost of increased quality and quantity of information display is proportional to that increased capability, by way of gradually increasing the number of memory devices used in the refresh memory. As used herein, the term "architecture" is intended to indicate the specification of the overall structure of the system at the processor memory switch (PMS) level; the levels of language in the system and the instruction set processor (ISP) definition at each level; and the set of registers and register transfers available. The invention, broadly defined at the system level, is characterized by the use of software programs written in standard computer languages which generate a sequence of high level graphics instructions which are transmitted to (or stored for later transmission) to an interpreter which generates sequences of macro-instructions; a macro- instruction interpreter implemented either in software or hardware generates detailed hardware recognizable instructions which are supplied to display generation hardware which generates a sequence of pixel level images in a refresh memory which are continously read for display by a display refresh processor, which generates a raster scan format serial signal for a raster scan display device. The Set of Macro-Instructions As described earlier, the invention utilizes a set of macro-instructions designated as a general image macro-instruction set. This set of instructions comprises a group of application program-callable routines which are semantically equivalent to the high level instructions normally supplied by the user or host computer, but provides some speed-up in operation since the overhead of syntax translation from the high level instructions to the macro-instructions is avoided. The specific form of macro-instructions is to some degree a function of the application language used. For purposes of illustration, therefore, the following function definitions are offered; these are typical of what such subroutines would look like for use with languages such as FORTRAN, BASIC and PASCAL. The instruction CLEAR (XL, YU, XR, YB) clears the screen and sets the screen coordinates to be (XL, YU) at the upper left hand corner B'υι<t- of the screen and (XR, YB) at the lower right hand corner of the screen. If the parameters XL, YU, XR and YB parameters are all zero, then the screen coordinates are not changed. The command BCOLOR(c) sets the background color to intensity C. The instruction POSITION (XREL, X, YREL, Y) sets the current writing position to the X, Y values defined by the command parameters. The XREL and YREL parameters are integers which indicate whether the X and/or Y position arguments are relative or absolute; a non-zero value for XREL indicates relative positioning and a zero value indicates absolute positioning. The sign of XREL for a non-zero value indicates the relative direction with respect to the current screen coordinate definition. If XREL is positive and X = 0, then the X position component is not changed. The instruction BEGINP sets a marked position to be returned to later. The instruction ENDP sets the current writing position to the X,Y values defined by the command parameters. The instruction PIXELP(PV) performs a pixel vector move in the direction PV. Similarly, macro-instructions may be provided for drawing a vector (i.e., an approximation of a straight line from the current writing position to the position defined by the command parameters) . Similarly, macro-instructions may be provided for numerous additional functions, such as drawing curves, changing the writing options (i.e., mode of writing, as those modes are defined in Fig. 17), drawing circles, drawing characters from one of the available alphabets, defining text writing options, creating character sets, etc. All of these instructions are executed by various sequences of the general image generation instruction set. A Minimum Embodiment For purposes of discussing a basic machine constructed according to the present invention, it will be assumed that the size of the screen matrix is 256 horizontal pixels by 256 vertical pixels. To further simplify matters, it will be assumed that the device can display only binary black and white; thus no separate attribute memory is required. The information memory (IM) 79 contains 256 x 256 = 65,536 storage locations which are addressed by a 16-bit number. Each of the unique 16-bit addresses corresponds to a visible pixel point on the display and the address is formed by combining an 8-bit X address with an 8-bit Y address. By convention, the address X = 0 and Y = 0 represents the upper left hand corner of the display (i.e., the "home" position) , X = 255 represents the right hand margin and Y = 255 represents the bottom margin. It will further be assumed for this basic machine that the refresh display processor is required to supply successive serial values from the IM at a rate of one pixel every Tp seconds. (Tp typically is on the order 10-200 nanoseconds.) By design, the invention requires the use of memories which allow two accesses for each pixel cycle Tp seconds. One of these accesses is allocated for use by the refresh processor to read the pixel needed at the current -position on the screen. The other memory access during each cycle is used by the display generation hardware to either read the memory location, write into a memory location or perform a combined read and write operation on a specific memory location. Let (RX, RY) represent the address which the refresh processor uses to access the successive locations in the IM. Letting the display generation hardware address the memory at coordinate (GX, GY) , a typical timing sequence for the use of the memory is illustrated in Figs. 4A and 4B. These figures show the basic sharing of the memory between the refresh and image generation processors, and the possible variations of this time-sharing within the context of the invention's architecture. In Fig._ 4A, the display generation hardware performs an equal number of write operations 62A, 62B for the equivalent number of refresh read operations 64B, 64C. In Fig. 4B, by contrast, the display generation hardware performs one read operation 65 and one write operation 66 for each two successive refresh read cycles 67, 68 of Tp seconds. In the more complex machine discussed below as an alternative embodiment, the display generation hardware generally needs to read the memory and rewrite it after a delay of one Tp cycle. This time-sharing of the memory is accomplished by a multiplexer circuit which, in its simplest form, is shown in Fig. 5 along with the IM data output to the display device. The basic timing of the machine is provided by an oscillator 72 which generates one pulse every Tp/2 seconds. The signal from oscillator 72 is counted down in a counter 74 which provides timing signals to define four states to the memory sharing time cycle. In the first state (count = 0), the refresh bit at (RY, RX) is read and displayed. In the second state (i.e. , count = 1) the display generation hardware reads the bit at (GY, GX) . In the third state, (i.e. , count = 2), the refresh bit at (RY, RX+1) is read and displayed. Finally, in the fourth state, (i.e., count = 3), the display generation hardware writes the bit at (GY, GX) to the IM 79. This basic cycle continues indefinitely. In the Fig. 5, the two bits of output from counter 74 needed to define the four basic machine states are labelled DOT and DOT2, provided on lines 74A and 74B, respectively. These counter states are used by both the refresh and image generation processors (76 and 78, respectively) to determine when to generate the proper addresses. The DOT2 signal (i.e., the basic clock divided by two) directly defines the address to the multiplexer 77. The refresh processor uses the memory exactly half of the time and the image generation hardware processor uses the memory the other half of the time. The refresh processor also generates vertical sync (VSYNC) and horizontal sync (HSYNC) signals. Those signals are used by the display device to determine when to start a new line of the display and when to move the visible position to the home position. Note that the refresh processor only reads data, whereas the image generation processor generally both reads and writes data. For a very simple machine, such as discussed in this section, the refresh processor may be implemented by using a commercially available integrated circuit such as the Motorola 6845 VATG (Video Address and Timing Generator) . Fig. 6 illustrates a simplified circuit which accomplishes the basic operation of any such refresh processor. The circuit consists of four counters, 82-88. The RX and RY counters 82 and 84, respectively, generate the respective coordinates of successive refresh addresses, while counters HSYNC and VSYNC (86 and 88, respectively) generate the delays needed by the physical display devices to actually move the current display position to either the beginning of a new line (HSYNC) or to the home position of the display (VSYNC) . The basic operation is a continuous cycle beginning with the RX and RY counters 82 and 84 cleared (i.e., RX = 0 and RY = 0 ) and the HSYNC and VSYNC counters 86 and 88 also cleared. The RX counter 82 is incremented on each occurrence of the DOT clock signal, causing successive horizontal pixels to be read from the refresh memory and shown on the display. This cycle continues until all of the horizontal positions on the current RY-valued line have been displayed, at which time the RX counter generates an overflow signal, RXV, which causes the HSYNC counter 86 to begin counting. The RX counter 82 is disabled during this time and the HSYNC signal is generated for the duration of the HSYNC counter cycle. As soon as the appropriate number of horizontal sync time pulses have been counted by the HSYNC counter, the HSYNC signal is disabled and the RX counter 82 is enabled to count again, beginning at a count of 0 and continuing until the RY counter 84 is incremented by 1. This cycle continues until all of the lines have been displayed, at which time the RY counter 84 generates an overflow signal, RYV causing the VSYNC signal to be generated and causing the VSYNC counter 88 to begin counting additional line counts. After the appropriate number of delay counts (the multiple of the time required for the RX cycle to complete), the VSYNC counter overflows, the VSYNC signal is dropped and the process begins again with all the counters having a value of 0. The horizontal and vertical delay times have no effect on the operation of the display generation hardware. The display generation hardware continues to perform its operations on the memory independent of the operation of the refresh process, except for the time multiplexing of the memory which continues whether the data output is used for a display or not. The refresh process read operations which occur during the HSYNC and VSYNC time operations are not used for display purposes, but may be used for more complex refresh process operations as described below. - 13R H A7J Pixel Writing With this background, the basic registers of the display generation processor will now be defined and the basic timing for writing a single pixel will be developed. As shown in Fig. 7, the writing of a single pixel at a location (GX, GY) in the memory requires the use of four registers, called GX, GY, PAT and GMODE, elements 92,.94, 96 and 98, respectively. GX and GY registers 92 and 94 define the location in the memory to be written or read; the PAT register 96 defines the value of the data to be written into the memory or the value of data read from the memory; and the GMODE register 98 defines whether data is to be read from or written into the memory. Fig. 7 also illustrates the basic interface to the controller processor. The exact nature of this interface varies depending upon the exact requirements of the processor used, but generally consists of the following signal lines: (1) a set of controller-generated addresses, indicated at 102; (2) a set of data input lines 104; (3) a read/not write signal (RD/-WR) , supplied on line 106; and (4) a device enable signal line (DE) 108. A subset of all possible addresses presented on line 102 are uniquely defined for use by the display generation hardware processor and identify which of the instructions is to be executed. Data input lines 104 may be bidirectional to allow both data output and data input and are used to supply data to the display generation hardware processor. The RD/-WR signal on line 106 tells the display generation processor that data is to be either read (value = 1) or that an instruction is to be executed (value = 0). The device enable signal (DE) on line 108 tells the display generation processor that it is the one processor amongst all processors which is to respond to the instruction read or write operation. Based upon the foregoing four registers, five of the basic low level image generation language instructions can be defined. The first instruction, LDMODE, loads the GMODE register from the controller input data. The second instruction, LDX, causes the GX register to be loaded with -the data from the controller. The third instruction, LDY, similarly causes the GY register to be loaded with the data from the controller. Likewise, the fourth instruction, LDPAT, loads the pattern register from the controller input. Fifth, an instruction termed EXDOT causes the data value in the PAT register to be written into the refresh memory at the location specified by the GX and GY registers at the fourth state (when the display generation hardware writes to the IM) of the next basic refresh memory cycle, provided that the GMODE bit is one; otherwise, the data at the location (GX, GY) is read into the PAT register at the second state (i.e., count = 1) of the next refresh memory cycle. Since all of these basic operations are generally executed in a time which is short compared to the time that the controller processor executes one instruction, an arbitrary sequence of such instructions may be executed to draw an arbitrary image on the screen. Further, since the GX, GY and PAT registers are loaded by separate, instructions, any register which does not change value between successive EXDOT operations does not have to be reloaded. These basic instructions are sufficient to generate all of the images possible in the basic machine. However, the primitive nature of these instructions is such that the software in the controller would spend far too much time performing trivial tasks. By contrast, the display generation hardware is busy doing operations only a small percentage of the time relative to the controller operations, thus creating a situation in which the rate at which a new image may be drawn on the screen is much less than the hardware allows. Under the timing scheme disclosed above, the machine is capable of changing the display at a rate of l/2Tp pixels per second. In practice, the average rate of screen modification using only these instructions is on the order of 1% to 5% of this maximum value. It is therefore a desire to provide a structure which allows the screen to be updated at much more efficient rates by optimally increasing the amount of hardware utilized in executing more complex operations and, therefore, decreasing the amount of work that the processor must perform by executing software. The goal is to have the hardware busy performing useful image generation functions during the period that the processor is working to figure out what operation is to be performed next in the hardware. In practice, it has been found that the hardware structure disclosed below achieves 50-80% efficiency with minimal addition of hardware. One approach to implementing the more capable hardware is just to implement the macro-instruction set as defined earlier. While this accomplishes what is desired, it turns out that in practice the processor software spends a lot of time waiting for the hardware to complete its operation, thus creating another less than optimal situation. A better approach, therefore, is to implement an intermediate level of capability through the use of a low-level general image generation instruction set, which allows low cost with maximum performance. In summary then, it is the approach of the architecture of the present invention to implement the translation from macro-instruction to general image generation instruction set in software, while implementing the transformation from general image generation instructions to primitive dot writing operations in hardware. The architecture of O.V. the present invention allows this translation to be performed in hardware for high performance systems where it is desired that time in software be usable for more activities than controlling the display. Such may occur in the context of the stand-alone configuration of terminal discussed above in contrast to the remote terminal case, in which there is little for the controller to do anyway so that the software translation approach provides a lower cost machine. Vector Writing The simplest type of line to draw is one in which the cursor of the display is moved in one if the eight major directions of the compass. These eight directions are referred to as pixel vectors and are illustrated in Fig. 8. The simplest type of vectors consist of solid straight lines drawn in one of the foregoing eight major directions. Using an abbreviated form of the PASCAL computer language as a mechanism to describe hardware operation. Fig. 9 describes the process for drawing a straight line horizontally to the right with length DU number of pixels. The vector (the term "vector" will be used to refer to a finite segment of a line) starts at location (X,Y) and the functions LDX(n), LDY(n) and so forth all refer to the primitive instructions defined above. The variable REP is a loop control variable. Similar procedures may be defined for writing vectors of any length in the other seven major directions, but it is more efficient to define one procedure which draws in any one of these directions based upon the desired directions. To do this, a procedure called PVMOVE(DIR) is defined, to cause the values of X and/or Y to be changed by one pixel depending upon the value of DIR, which is a number having value 0- 7, corresponding to the major compass directions defined in Fig. 8. This procedure is listed in Fig. 10. B ' u*<£-At7 Using this new function, one procedure may be formed for drawing a straight line in any of the eight major directions based upon the parameter (DIR) . This procedure assumes that the vector is to be drawn from the current position (GX, GY) using the current value of PAT and with a length DU. The procedure is listed in Fig. 11 (note that this procedure does not draw the first pixel at the initial location (GX, GY) ) . The convention adopted herein is that the initial pixel of a vector is not drawn. If the first pixel is to be drawn, then the EXDOT instruction must precede the procedure of Fig. 11. The use of the PVDRAW procedure of Fig. 11 is sufficient to draw a wide variety of box type figures, but lacks the generality necessary to draw angularly disposed vectors. To draw such line 'segments, the present invention utilizes a variation of what has been called in this field a "rate multipler." Of course, any image constructed on a device which uses a rectangular grid of pixels cannot draw an exact straight line. The rate multiplier approach provides an approximation to a straight line which is the best that can be accomplished using that grid. If the device has a sufficiently high resolution, then the line will be of high quality. The procedure is simplified by taking advantage of the fact that the sequence of pixel vectors needed for drawing for an arbitrary vector between the current position (GX, GY) and the desired end position (XE, YE) is similar for vectors drawn in different "octants" if the absolute values of (XE-GX) and (YE-GY) are the same. These octants are illustrated in Fig. 12 and generally refer to the areas between the eight major pixel vector directions. One way of viewing the general angular vector process is to consider the approximation as just being the drawing of the vector in the direction of the nearest major axis (horizontal or vertical) with "occassional" modifications to cause the pixel vector sequence to steer toward a diagonal line. The more that the sequence is modified by a diagonal pixel vector, the greater the angle of the line. For a specific vector, it turns out that the total pixel vector sequence is comprised of pixel vectors in only two directions: one pixel vector along one of the horizontal or vertical directions (which will be called a "MOVE" pixel vector) and a pixel vector along an adjacent diagonal which will be called the "BREAK" pixel vector direction. There are two major parts to drawing an arbitrary angle vector using the rate multiplier approach. In the first part, the values of five variables are determined. These are (1) DU, the total number of pixel vectors to be written; (2) DV, the number of times that the "BREAK" pixel vector is to be drawn, which is never larger than DU; (3) MOVE, the direction of the MOVE pixel vector; (4) BREAK, the direction of the BREAK pixel vector; and (5) VER, the vector error which accummulates over the sequence and determines when the BREAK pixel vector is to be drawn rather than the MOVE pixel vector. The second part of the procedure uses those parameters to generate the actual sequence of pixel vectors needed to approximate the line segment. The full procedure is listed in Fig. 13; an example of the execution of this procedure is illustrated in Fig. 14, for the construction of an arbitrary line segment. The procedure shown in Fig. 13 is a variation of the so-called Bresenham's algorithm which is explained, for example, in W. Newman and R. Sproull., Principles of Interactive Computer Graphics (2d ed.) at 25-27. Line Patterns The procedure of Fig. 13 is sufficient to construct the majority of line art type drawings. It is desirable however in using line-art patterns, to be able to distinguish different types of lines on the same figure. This is accomplished in the present invention by BΌREΛ requiring the PAT register to be a shift register of MAXPAT bits in length. Into this register a pattern of binary l's and 0's is loaded initially, the binary pattern representing a dash/space sequence, with the l's indicating dashes and the O's indicating spaces. Each time or multiple of times that the EXDOT operation is performed, this register is shifted, so that successive binary values are presented for writing into the refresh memory. By shifting the output data back into the input of the shift register, the shifting pattern is maintained independent of the length of the vector sequence. And by not changing the value of the PAT register between successive arbitrary vector operations, the line pattern is maintained, even around corners of the vector sequence being written. Character Drawing The approach to drawing characters in the present invention is simply to break the overall two dimensional character pattern into a set of one-dimensional patterns. An illustration is provided in Fig. 15 using the letter "A" as an example. Each row of the character pattern is drawn by executing the general vector procedure. Thus rows of the characters may be drawn at any angle. In doing so, DU is set equal to the width of the character; DV and VER are set appropriately to generate the proper angle; and the MOVE and BREAK parameters are selected to identify the octant in which the row is to be written. The VDRAW procedure of Fig. 13 is then executed with the PAT register set appropriately for each row. Between the drawing of each row, the (X,Y) location is changed so that the appropriate position is maintained for the beginning of each successive row. If the size of the PAT register is eight bits and it desired to draw a character in its normal horizontal orientation without italic slanting, and the size of the character cell is 9 x 10, then the procedure illustrated in Fig. 16 is sufficient REA to construct the character pattern. Of course, once the vector parameters are set up, only the second part of the general Fig. 13 procedure is used. Successive characters are drawn on a reference line by setting the GX and GY registers to the appropriate values which represent the upper left hand corners of each new character position. A variety of text writing features are made possible simply by changing the various parameters and extending the foregoing procedure. For example, character width may be varied. Width may be set to an amount WIDTH times the normal, by setting DU to WIDTH times the normal number of pixels in the width of a character and by only shifting the PAT register after every WIDTH number of EXDOT operations. Character heights may be varied by repeating the execution of the VDRAW step in the Fig. 16 procedure a HEIGHT number of times, without changing the value of the PAT register. (The PAT register may need to be reloaded between each repetition.) Character rows may be drawn at any angle by the appropriate selection of the DV, VER, MOVE and BREAK parameters. An interesting property of the general vector procedure is that the value of the VER variable is the same at the completion of drawing a vector as it was at the beginning, so its value need be set only once for the complete drawing of the character or a sequence of characters all of which have the same angle. Italic slanting is accomplished by using a second rate multiplier procedure. The parameters appropriate for drawing define the successive changes in X,Y position at the beginning of each row of the character, to cause the angle of the columns of the drawn matrix to lie on a line with the appropriate angle. A variety of spacing features such as proportional spacing, subscripting and so forth are accomplished similarly by computing the successive character start '.' point positions by different procedures, depending on the features desired. Basic Curve Drawing The preferred approach to curve drawing in the present invention is to use short line segments to approximate portions of curves. This provides a mechanism for drawing curves at reasonable speeds without requiring additional hardware. Further, this approach is capable of the same level of approximation quality as many of the more specialized curve-drawing techniques. Area Filling The present invention allows a variety of area filling procedures to be used by first reading the memory, making the appropriate modifications, and writing the data back into the memory. This is generally a slow process. A high percentage of the time the area to be filled represents geometric images bounded by rectangles, triangles and circles and general convex polygons and curves. For these cases, the architecture of the present invention provides a relatively fast mechanism for filling areas, a technique called "reference shading." In this technique a reference Y position is selected; this position is called YSHADE. All subsequent vector operations (including vectors used to approximate curves, thus allowing curve images to be filled also) are carried out in a two dimensional manner very similar to what was described for character pattern drawing. By contrast with character drawing, the "rows" are always drawn vertically and with a length which is the difference between the current GY value on the vector and the YSHADE value. The vertical direction is chosen for the simple reason that in many images, such as data plots, the shape is naturally more convex in this direction than in the horizontal direction. By using successive row patterns from a character pattern definition, and repeating this pattern when necessary, the area may be filled with an arbitrary two-dimensional pattern. Naturally, it should be understood that apparatus constructed according to the present invention may provide horizontal, as well as vertical, drawing and filling. Writing Attributes The term "writing attributes" refers to those parameters which control the manner in which successive pixels are written into the refresh memory. The manner of writing described so far is called "REPLACE" mode writing, in that the current contents of the refresh memory are simply replaced by the contents of the PAT register bit. Three other modes of writing are readily available: OVERLAY, COMPLEMENT and ERASE. These modes are mutually exclusive, in that a change to one mode causes the previous mode to be forgotten. The ERASE mode simply writes "0" values into the memory without regard to its current contents or the contents of the PAT register. The OVERLAY mode causes only data with a logical 1 value to be written into memory; otherwise, the value in memory is left unchanged In the COMPLEMENT mode, the pixel values in the refresh memory are inverted if the current PAT bit has a logic value of 1; otherwise, memory contents are left unchanged. It is an interesting property of the COMPLEMENT mode that when the same vector is drawn into the memory two successive times, starting at the same position, then no matter how many other vectors are crossed by that vector, the visual image is returned to its prior value. This is particularly useful for interactive editing since it allows a sub-image to be selectively erased, and for drawing cursors and other sub-images which appear to move on the screen without affecting the basic image. \_ P. Ε. C.VFI All of the writing modes may be modified while using the negative writing option. This option essentially inverts the value of the PAT register bits and reverses the data of the ERASE operation. Fig. 17 summarizes the Boolean equations and gives examples of the types of images which result after the use of the various modes. Reading Attributes In addition to binary black and white writing, the invention permits, through the use of the AM (attribute memory) certain visual transformations to the IM data. These transformations include, but are not limited to, blinking, intensity variation and color display. Intensity variation on a gray scale is no different implementation-wise from color display; therefore, only color intensity display will be discussed from this point forward, it being understood that gray scale variations may be accomplished in like fashion. In fact, the output mapping mechanism may allow the use of the same attribute codes to be interpreted as either colors, by a color display device, or as grey scale, for a full tone black and white display device. The preference is to use the standard NTSC color-to-black and white luminance standard for this mapping; this is the approach used in television receivers to enable a color signal to be displayed as black and white image. The general model of read attribute handling and the architecture of the present invention is based upon a foreground/background concept. As illustrated in Fig. 18A, the simplest form of attribute memory 120 consists of two registers 122, 124. One register 122 contains the color attribute for the background and the other register 124 contains the color attribute of the foreground. The output of the IM 79 is used to select a current foreground attribute if the IM data is a logical 1, and a background attribute if the IM data is a logical 0. The types of images which can be displayed using this structure consists of those images which one can construct by using a specific color of crayon on a specific color of background paper. Although useful and very inexpensive, this approach does not take full advantage of the relatively expensive color CRT tubes. Some further complexity is therefore desirable. The next step up in capability and complexity is to use a foreground attribute memory which has one location for each vertical line position. With this structure, one may construct images using several different colored crayons on one color of background paper, so long as only a single color of crayon is used within the extent of an arbitrarily large vertical interval. By adding a background AM of similar resolution, it is possible to change the background color again on arbitrarily large vertical boundaries. Further capability is provided by allowing the foreground AM to have a certain number of horizontal positions for which the foreground color may change. For example, in the case of the IM having 256 x 256 positions, an AM of say 64 horizontal positions by 64 vertical positions may be used. This example is shown in Fig. 18B. Thus, the foreground color may change every four successive IM pixels in either the horizontal direction or the vertical direction. This works out well in practice since line drawings rarely change colors even that often across the extent of the image; the occassional pathological case of a line of one color crossing a line of another color and thus changing the color of a small portion of the previous line may be accepted, considering the reduction in cost obtained by the diminishing memory needs relative to the multi-plane bit-map approach. In the limit of the AM having the same resolution as the IM, the architecture of the present invention has the same capability as the general bit-map approach. The blinking attribute is implemented by inverting the value of the IM bits at a position where the blinking attribute is enabled. The inversion has an alternating duration based upon a blink rate clock. The blinking attribute is neither background nor foreground, but rather, requires a third type of attribute memory. The visual effect to the user is that a sub-image written with the blinking attribute enabled appears to alternate between the foreground and background intensities; that is the reason that the blinking attribute is also called the "alternate" attribute. Optionally, the attribute memories may have varying levels of attribute planes of varying resolutions. For example, a particular implementation may employ a "major color" AM having half the resolution of the IM and, hence, one-fourth the memory requirements. Four bits per location in such a case allows the selection of one of sixteen possible colors for this area. A second, "minor color" AM of half again of much resolution (i.e., needing one-sixteenth as much memory as the IM) may provide another four bits of color for the fine adjustment of color in an area. The net result is a machine which in some ways has the same capabilities as a full bit-map multi-plane system having nine bits per pixel. The amount of memory used for the present invention, however, is N*N bits for the basic embodiment of the invention, plus for the major color AM 4*N*N/4 bits and for the minor color IM, one-fourth that number of bits; a machine of N by N IM pixels thus needs a total of 2^N*N bits, or 2^ bits per pixel. Independent of the exact nature of how the AM resolution is selected, a specific IM/AM architecture will be referred to herein as an "L" plane machine, where L is computed by adding up all the memory in all of the IM and AM memories and dividing this total by the number of bits in the IM plane. For example, the above configuration will be referred to as a "two and one quarter" plane scheme. This approach to successively refining the color attributes is very useful, since a large number of images of even a photographic type of complexity are effectively displayed with little loss of quality. Indeed, the mechanism is similar to that which permits color television to work despite the constraint that color may change only about 20% as fast .as the effective black and white intensity may vary. As a further option, a display system according to the present invention may employ more than one IM plane. Then, if there are, for example, two IM planes, the value of a two-bit number per pixel is used to indicate how to mix the foreground and background colors in proportions. That is, for the four possible combinations, possible arrangements are to use the background only, to use the foreground only, to mix the colors of two-thirds background and one-third foreground and vice versa. The two bits from the IM would be used for entry to a lookup table with the lower resolution AM bits to determine the final color. In terms of performance relative to costs, such a three-plane system according to the present invention can nearly match an eight plane general bit-map system for many applications. Registers Based on the desire to provide a minimum hardware system but with reasonable performance, the minimum hardware register set necessary to implement the above-described primitive functions will now be described, with reference to Fig. 19. The GX and GY register counters 132 and 134 are, of course, necessary to supply successive addresses to the IM and AM to write the pixel information defined by the low level image generation instructions. -BUREA OA'.P The PAT shift register 136 is also necessary. It must be writable and, for some optional capabilities readable. The PAT register is used to modulate the data written into the IM during a sequence of successive pixel vector operations. A register 138 termed the WOPS register, controls the mode in which data is written into the memories (i.e., negative, replace, etc.). Current foreground attribute (CFA) and current background attribute (CBA) registers 142A,B are used to determine the data written into the attribute memory during a pixel write operation. These registers permit execution of the general EXDOT operation, which is used as the basis for all imaging in the invention. The hardware also supports a direction (DIR) register 144 which identifies the direction the counters are to move for a pixel vector operation. A repetition (REP) counter register 146 determines the number of times that a basic pixel vector move and EXDOT operation is to be performed. With the addition of this facility, the hardware is capable of executing the arbitrary length vector drawing operation in the major directions. A BREAK generation circuit 148 which includes the registers DU, (152) DVM (154) and VER (156) and an adder circuit (158A, B) together are used to accomplish the second part of the general vector drawing procedure, illustrated in Fig. 13. The direction register 144 in this case is used to identify the octant within which the vector is to be drawn and controls the counting of the GX and GY registers 132, 134 as a function of the BREAK signal generated by the circuit. That is, the DIR register 144 causes the MOVE direction (major direction of the octant) to be executed in the absence of the BREAK signal, and causes the BREAK direction (diagonal direction of the octant) to be executed for a pixel vector execution in which the BREAK signal is present. A pattern multiplier 160 ( "PMUL") and pattern counter 162 ( "PCTR") comprise a register pair which determines how often the pattern register 136 is to be shifted relative to the pixel vector writing sequence. This supports the drawing of arbitrarily sized line patterns and character rows. The hardware also contains a register 164 which can identify whether data is to be written into the memories from the PAT register, or read from the IM memory into the PAT register. This register further provides a bit to enable the recognition of the BREAK signal during the sequence of vector writing and control bits which determine whether the IM or AM or both memories are to be written together or separately. Basic Timing The basic timing and memory sharing for the process of drawing successive pixels into the memories is illustrated in Fig. 20. Also shown there is one possible sequence of register transfers for the BREAK circuit computations. General Image Generation Instruction Set There are basically two types of low level image generation instructions which the display generation hardware can execute: (1) register read and load operations and (2) pixel sequence operations. Control of whether a register is to be read or loaded is based upon the processor bus signals which identify the operation. Fig. 21 lists the minimum instructions in this category. The parameter "D" in all cases represents the data supplied by the controlling processor as part of the instruction. It will be observed that there is no instruction for loading the REP counter 146. In all instructions which use the REP counter, that counter is loaded initially with the current value in the DU -" ΛJRE. OMPI register 152. Further, some of these register load operations may be combined; typically, the LDCFA, LDCBA, LDWOPS, and LDMODE operations are combined and the LDPCTR instruction is combined with the LDPMUL instruction. In the illustrations, it is assumed that all register load instructions are executed in unit time relative to the controlling processor. That is, the controlling -processor may arbitrarily issue a sequence of these instructions without checking a BUSY flag (i.e., status bit) which indicaters when the display generation hardware is actively executing an instruction.. The second category of instructions, pixel sequence operations, are also referred to as the EXECUTE instruction category. For this category, data sent with the instruction normally is not used. The minimum instruction set for the executed instruction category is indicated in Fig. 22. For the case of the EXER and EXVEC instructions, the BUSY flag is set during execution of the instruction. The controlling processor must generally monitor this flag to ensure that the display generation hardware has completed its operations before continuing to issue additional instructions. Reduction in Memory Requirements In many applications of a graphic device it is not necessary for one dimension of the display to have the same resolution (in terms of pixels per unit distance) as the other dimension. For example, in applications involving mostly textual characters, the vertical resolution requirement is generally half the horizontal resolution requirement. It is a principle of the current invention to take advantage of this lessened resolution requirement by generating the successive pixels of a vector in such a manner that only half of the usual amount of memory is needed. This is accomplished, for example to reduce the memory in the vertical direction by one half, as follows. " 47 2. In an apparatus for generating information for display on a raster scan display device, the improvement comprising: means for generating a pattern modulated vector from a first point on the display to a second point on the display, comprising: vector generator means for establishing the locations intermediate the first and second points where the vector is to be displayed; and pattern generator means for determining the information to be displayed at each of those intermediate locations, such pattern generator means including a shift register and means for loading the shift register with a pattern of l's and O's representing the selection of foreground or background attributes to be displayed at each location. 3. The apparatus of claim 2 wherein the pattern generator means further includes means for shifting the shift register contents by one position responsive to the writing of a predetermined number of display locations. 4. In apparatus for displaying a two-dimensional image on a raster scan display device, the improvement comprising: first and second means for generating pattern modulated vectors; the first pattern modulated vector means generating a succession of picture elements in a first dimension; and the second pattern modulated vector means generating the starting position and patterns for the first pattern modulated vector means. ■" BU EΛT OMP

Claims: AMENDED CLAIMS (received by the International Bureau on 11 February 1982 (11.02.82)) 1. (Amended) In apparatus for generating graphic and textual information on a raster scan display device which includes a memory means for storing a representation of an image -to be displayed, the improvement comprising: the memory means having two components, an information memory and an attribute memory; the information memory having a memory location for each position on'the display defined as a picture element; for each picture element location, the attribute memory providing a foreground attribute and a background attribute; the attribute memory having a lower resolution than the information memory, such that foreground and background attributes can be varied only in multiples of picture element locations; said attribute memory being adapted to permit both foreground and background attributes to vary as a function of picture element location; and means for providing to the display device, at each picture element location, in response to the information memory contents for such location, either the foreground attribute or the background attribute provided by the attribute memory. OMPI 45 If each vertical line is numbered, then any pixel which is to be written on an odd numbered line is simply written into the next lower even numbered line. The result is vectors (i.e.,lines) which still appear visually to be properly connected. A special feature of the vector generator is the ability to carry out this operation properly for all cases of writing modes. Naturally, various improvements, modifications and alterations of the methods and apparatus disclosed herein will readily occur to those skilled in the art. Accordingly, this disclosure is intended to be exemplary, not limiting, and the invention is intended to encompass all such obvious improvements, modifications and alterations; the invention is thus limited only as defined by the following claims: B KcΛt / - _0."_PI

Inventors: RUPP CHARLE R
Applicants: DIGITAL EQUIPMENT CORP
Application year: 1981
Main classification: G09G1
IPC codes: G06F3, G09G5