Raster scan CRT displays form a principal communication link between computer users and their hardware/software systems. The basic display device for computer-generated raster graphics is the CRT monitor, which is closely related to the standard television receiver. In order for the full potential of raster graphics to be achieved, such displays require support systems which include large-scale random access memories and digital computational capabilities. As the result of recent developments of large-scale integrated circuits, the price of digital memories has been significantly reduced, and microcomputers are now available with the capability of controlling such displays at affordable prices. As a result, there has been a surge of development in raster graphics. Typically, each PIXEL in a rectangular array of a picture elements (PIXEL) of a CRT is assigned a unique address, comprising the X and Y coordinates of each PIXEL in the array. Information to control the display is stored in a random-access memory (RAM) at locations having addresses corresponding to those assigned to the PIXELS. The source of PIXEL control data stored in the RAM is typically a microcomputer located in a graphic controller which will write into the addressable memory locations of the RAM the information necessary to determine the type of display. This frequently includes an address in a color look-up memory, at which location there are stored binary color control signals to control the intensity of the color of each pixel of an array. The horizontal and vertical sweeps of the raster scan are digitized to produce addresses of the PIXELS, which addresses are applied to the memory in which the controller has previously written information determinative of the display; i.e., the color and intensity of the addressed PIXEL as it is scanned. As stated above, this information can be an address in a color look-up memory. The digital color control signals are read out of the addressed locations in the color look-up memory. The digital color control signals are converted to analog signals and applied to the three color guns of the typical CRT to control the intensity and color of each PIXEL as it is scanned.

A well-known technique for controlling the displays of such a system is to have a PIXEL memory containing a PIXEL image of the display in each addressable location of which is stored a background/foreground control bit and a corresponding behavior memory that describes the color, or behavior, of each PIXEL. The size of the memory required is equal to the number of PIXELS times the sum of the background/foreground bits plus the number of behavior bits, typically four per PIXEL.

There is a need in color graphic systems to reduce the memory requirements of such systems without a commensurate degradation in the versatility of colors displayed by each PIXEL.

It is, therefore, an object of this invention to provide an improved method for controlling the colors of a raster graphic system in which the memory requirements are decreased without decreasing the number of behaviors the system can display.

It is another object of this invention to provide an improved method for controlling the colors of a raster graphic system in which the background or foreground behaviors can be independently changed.

These objects are achieved by the invention as characterized in claim 1. Preferred improvements are described in the subclaims. In principle the new apparatus operates as follows: For controlling the colors displayed by a raster graphic system, which includes a color cathode ray tube having an orthogonal array of picture elements (PIXELS) each PIXEL has a unique binary address. An addressable memory is provided with memory locations, the addresses of which correspond to those of one of a set of PIXELS. The information stored at each addressable location includes a set of background/foreground control bits and a group of behavior bits. The background/ foreground control bits are read out of a memory during a memory cycle and stored in a shift register. At the same time, the behavior bits are read out of the memory and are applied to an escape-code detector and to a foreground behavior register and a background behavior register, each of which is capable of storing a group of behavior bits. The shift register shifts out one background/foreground control bit for each PIXEL clock pulse which determines the set of behavior bits stored in the background and foreground registers to be used in forming a color index. The color index, which includes a group of behavior bits and the background/foreground control bit, is then used as an address to a color look-up memory, and at which each addressable location are stored typically eight bits which determine the color. Color control signals read out of the color look-up memory are applied to D/A converters to produce analog signals to control the intensity of the red, green and blue guns of a cathode ray tube..One set of the behavior bits is defined as constituting an escape code. Whenever this particular net of behavior bits is read out of the behavior memory, that set is not stored in either the background or foreground behavior registers. When the escape code is detected, the next set of behavior bits read out of the behavior memory is stored into the background behavior :register. In the absence of an escape code being detected, the behavior bits are stored in the foreground behavior register. The bits in the background behavior register remain the same until the next escape code is detected, at which time the next set of behavior bits is stored into the background behavior register.

In a preferred embodiment the invention provides apparatus which includes a random-access memory in which is stored four control bits and four behavior bits at an addressable location corresponding to a pixel address of one of a set of four adjacent PIXELS. Addressing four adjacent PIXELS at a time takes advantage of the fact in most displays, text, bar charts, trends, etc., that four such PIXELS will have the same background or foreground color. Thus, the memory size required is two bits per PIXEL in a system utilizing a five-bit behavior signal compared to prior art system which requires five bits of memory per PIXEL.

The sweep signals are digitized to form PIXEL addresses which are used to address the random-access memory with the address of one of four of the PIXELS being used to address the memory. A PIXEL clock produces PIXEL clock pulses, one as each PIXEL is scanned. During each memory read cycle, the four background/foreground bits stored at the addressed location of the random-access memory are written into a shift register which will read out, or produce, one control bit for each pixel clock pulse produced by the PIXEL clock. Foreground and background register means are provided into which can be stored four behavior bits produced by the memory means during each read cycle. A detector for a unique set of behavior bits, an escape control set, or escape code, inhibits the escape code from being written into either the foreground or background behavior registers. Whenever the escape code detector detects that the four behavior bits stored at an addressed location form the escape code, the four behavior bits read out during the next memory read cycle are stored into the background behavior register. It should be noted that only the set of behavior bits immediately after the escape code has been detected is written into the background register. All other sets of behavior bits read out of the memory are written into and stored by the foreground register. The background/foreground control bits, as they are read out of the shift register, one control bit per PIXEL clock pulse, are applied to a 2:1 multiplexer, to which are also applied the signals stored in the foreground and background behavior registers. Depending on the value of the background/foreground control bit read out of the shift register and applied to the multiplexer, either the behavior bits stored in the foreground behavior register or the background behavior register and the control bit are applied to a five-bit latch to form the color index, or address. The color index is then applied to the color look-up memory and eight bits of color signals, for example, stored at the addressed location in the color look-up memory are applied to D/A converters to produce the red, green and blue color control signals which are applied to the color guns of the CRT of the system.

Further objects, features and advantages of the invention will be readily apparent from the following description of a preferred embodiment thereof, taken in conjunction with the accompanying drawing.

The sole figure is a schematic block diagram of the apparatus for controlling the colors displayed by a raster graphic system.

In the sole figure, there is illustrated apparatus for controlling images displayed by a computer-generated, or controlled, raster graphic system. Graphic controller 10 has the capability of writing into random-access PIXEL memory 12 and behavior memory 14, as well as color look-up memory 16, binary digital information that is used to control the intensity and color of each PIXEL of a conventional color cathode ray tube 17. Raster scan logic 18 of tube 17 includes conventional digitizing circuits to digitize the horizontal and vertical sweep signals of the raster scan of tube 17 so that for each PIXEL on the face of tube 17 there is a number, or address. To uniquely identify each of the 640 pixels in a horizontal line and in the 480 vertical lines of a standard cathode ray tube raster requires a 19-bit address, with the X component comprising 10 bits and the Y component 9 bits. The X address corresponds to the ordinate and the Y address to the abscissa of PIXELS of the substantially rectangular raster. While in the sole figure, PIXEL memory 12 and behavior memory 14, as well as the color look-up memory 16, are indicated as being separate, they may be combined, or located, in a single conventional random-access memory. PIXEL clock 20 produces a clock pulse each time that a PIXEL in the raster is scanned. ' The output of the PIXEL clock 20 is applied to memories 12, 14 and 16, as well as to the control circuitry to synchronize their operation. To minimize the size of the memory subsystem and to permit the use of slower memories, a single address for a set of adjacent PIXELS, such as four PIXELS lying in a horizontal scan line, is used as a memory address, or, stated another way, by addressing four PIXELS at a time the two lower order bits of the address of each individual PIXEL are ignored, or, more accurately, they are deemed to be logical zeros. PIXEL memory 12 will store at each of its addressable memory locations four background/foreground control bits which determine whether the color of the corresponding PIXEL will be a background or a foreground color, as will be described below. The four behavior bits stored at each addressable memory location of memory 14 form part of the address of a memory location of color look-up memory 16. Stored at each addressable memory location of color look-up memory 16 are eight bits, or a byte, of binary color control signals. During each read cycle of the random-access memory subsystem, and particularly of behavior memory 14, four behavior bits are read out of behavior memory 14 and are applied to escape code detector 22, which checks to see if the four behavior bits applied to it have a predetermined value or comprise a predetermined set of behavior bits, such as, for example, all four behavior bits are logical ones, sometimes hereafter referred to as the escape code. If the bits stored in the addressed location of behavior memory 14 are not the escape code, then detector 2₂ produces a register enable signal that enables register 24 to store the behavior bits read out of behavior memory 14. If the four behavior bits read out of behavior memory 14 constitute the escape code, then the escape code detector will prevent, or inhibit, either register 24 or background behavior register 26 from storing that particular set of behavior bits, the escape code, but will produce a register enable control signal which enables background behavior register 26 to store the set of behavior bits read out of behavior memory 14 during the next read cycle of memory 14. During each read cycle of memories 12 and 14, the background/foreground control bits are stored into shift register 28. The control bits loaded into shift register 28 are shifted out of shift register 28 at the rate of one for each pixel clock pulse, and each control bit when produced is applied to multiplexer 30. If the background/foreground control bit is a logical one, for example, multiplexer 30 will apply the four bits stored in foreground behavior register 24 to color index latch 32. If the background/foreground control bit is a zero, multiplexer 30 will apply the four signals stored in the background behavior register 26 to color index latch 32. The background/foreground control bit read out of shift register 28 during each clock period is combined with, or concatenated with, the four behavior bits from multiplexer 30 to form a five-bit color index, or address. These addresses are stored in latch 32 and then applied to the address logic of color look-up memory 16. Typically, at each addressable location of color look-up memory 16, there are stored eight bits, color control signals, which when read out of memory 16 are applied to conventional D/A converters 34. The color control signals are converted by _D/A converter 34 into analog signals for controlling the intensity of the red, green and blue color guns of conventional CRT 17. In synchronism with the scanning of each PIXEL of the array, or raster, color look-up memory 16 produces an eight-bit byte of color control signals for the PIXEL being scanned, which byte is applied to D/A converter 34. D/A converter 34 converts six of the eight bits of the color control signals for that PIXEL into three analog signals which control the intensity of the red, green and blue electron beam guns of color cathode ray tube 17. In the preferred embodiment, two bits of each color control signal are applied to a fourth D/A converter, which converts these two bits into a monochrome analog signal that can be used to produce a permanent record of the raster display using conventional equipment, as is well known in the art.

During normal raster scanning, the background/foreground control bit produced by shift register 28 determines if the PIXEL being scanned is to have a foreground or a background color. Multiplexer 30, to which the background/foreground bits are applied, determines which set of behavior bits will be applied to latch 32. The five bits from latch 32 are used as the address of a memory location in color look-up memory 16 in which the color control signals for each PIXEL are stored, and which determine the color each PIXEL displays as it is scanned by the electron beams of CRT 17.

From the foregoing, it is obvious that the apparatus of this invention significantly reduces the memory requirements of color graphic systems without reducing the number of colors displayed, other variables remaining substantially the same. In addition, this invention enables the background and foreground colors to be changed independently of each other.