Abstract:
A video switch having at least two inputs from separate frame buffers and being controlled by a select control signal as to which buffer signal will control the electron guns in the cathode ray tube. The determination of which frame buffer signal is applied on a pixel-by-pixel basis is determined by priority bits from priority buffers. In case of a tie in priority, a tie bit is provided.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to computer graphics terminals and displays, and more particularly to circuitry for overlaying and underlaying video images to form a single video graphics display image. 
     BACKGROUND OF THE INVENTION 
     It is often necessary to display both graphics and video on a screen such as a cathode ray tube (CRT). This is especially true when the display is employed as part of a graphics workstation. 
     Prior art devices for displaying multiple graphic processes provided a fixed, that is, not changeable (without redrawing), combinations of text and graphics on a screen. Once the manner of presentation was chosen, it always appeared in that way. For example, if it was determined that text should always overlay the graphics then it would always appear that way. There were no priorities to be determined because the manner of presentation was fixed. 
     One reason for this manner of presentation is that all of the data, including graphics and text, would pass through a single frame buffer so it would simply provide the information on the screen in accordance with the data inputs. Often in such cases there would be a central processing unit (CPU) for graphics and a separate CPU for text but the outputs of both of these CPU&#39;s would be connected to the single frame buffer. In order for this frame buffer to handle the data, the CPU&#39;s would have start and stop. The frame buffer would tend to be somewhat overloaded and the speed of operation would be reduced. In some instances where it was determined to have only text where there had been both, the graphics would be deleted but the rest of the screen would be blank rather than have the text fill in the unused space. 
     SUMMARY OF THE INVENTION 
     Broadly speaking, this invention is employed as part of the computer graphic display presenting computer graphic multiple video images from different frame buffers, overlaying or underlaying each other on a pixel-by-pixel basis to form a single video graphics display output. These video images are often referred to as windows. 
     In systems with multiple graphic processes or one processor with multiple needs, for example, graphics and text, each process or need requires full access of a frame buffer and color look-up table to produce the desire images without competing for the frame buffer or destroying any of the contents of the frame buffer. In order to have the desired display, the multiple graphic processes or graphics and text should not both be put through the same frame buffer. To do so decreases the speed of operation of the buffer and provides a less than optimal display on the CRT. 
     The invention incorporates separate frame buffers for each graphic process or other needs such as text which are addressed by a separate or individual dedicated CPU. In addition, each frame buffer has an associated priority buffer which puts out a priority code bit for each pixel being transmitted through the system. Further, the CPU has a previously determined tie bit for resolving ties of priority from the priority buffers. The outputs of the priority buffers and the tie bits are all addressed to a priority decoder, the output of which is termed a select control signal. This select control signal is addressed to an analog video switch. The other inputs to the analog video switch are the outputs of the frame buffers, as modified by the color look-up table, and having been converted from digital to analog form. This system now controls on a pixel-by-pixel basis the output of the analog video switch as it controls the blue, green, and red video guns of the CRT. 
     By the means of the system of this invention, the full CRT may have a graphic display with two or more levels of images thereon and it may also have text at predetermined locations on the CRT. Each set of graphics information is operated upon by a dedicated frame buffer which addresses the video screen. If, for example, there is a composite video from the two frame buffers so that certain video graphics overlays or underlays other video graphics, the entire screen may be occupied in proper video presentation, that is, with certain segments in front or behind other segments. If it is determined that text has a higher priority then that text that passes through its frame buffer will be presented as windows where determined on the screen. It will not destroy but merely overlay and supplant the information which is beneath it. If the text input to its frame buffer is turned off, the entire graphics display would then cover the screen just as if the text had never been there. In other words, it would not leave blank spaces where the text was to have been as is the case with some of the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The objects, advantages and features of the invention will be more readily perceived from the following detailed description when read in conjunction with the accompanying drawing, in which: 
     FIG. 1 is an exemplary view of a graphics workstation showing a screen with multiple text data thereon; and 
     FIG. 2 is a block diagram of the system of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference now to the drawing and more particularly to FIG. 1 thereof, there is shown a typical workstation having graphics display or CRT 11, keyboard 12, graphics tablet 13, mouse 14, track ball 15, and eight-knob valuator 16, all connected to or on desk top 17. Central processing unit (CPU) 18 is positioned adjacent the desk. 
     Screen 21 shows graphics in central area 22 and various different elements of text in areas 23, 24, 25, 26 and 28. Note also a graph, which may be considered text or graphics, in area 27. It may be observed that the text elements clearly overlay the graphics portion to the extent that it blanks out a portion of the graphics in certain areas, for example, text areas 26 and 28. The graphics portion overlays text areas 23, 24, 25 and 27. 
     The screen of FIG. 1 is made possible by the structure of FIG. 2. This figure shows CPU&#39;s 31 and 32. For purposes of illustration, it is assumed that CPU 31 processes graphics information and CPU 32 has text information. It could just as easily be that they both have graphics information and there may be more than two CPU&#39;s involved. The outputs of CPU 31 are data lines 33 and address lines 34, both sets of lines being inputs to frame buffer 35. Priority buffer 36 receives an input from data lines 33 and from frame buffer 35 over line 37. The output of this frame buffer is provided on line 41 to color lookup table 42, the output of which is provided to digital-to-analog converter (DAC) 43 over line 44. The output of DAC 43 is the blue/green/red control signal on line 40 which is intended to control the color guns of the CRT. 
     Similarly, CPU 32 has data and address outputs 45 and 46, respectively, as inputs to frame buffer 47. The data line is also connected to priority buffer 51 as is line 52 from frame buffer 47. The output of frame buffer 47 is applied to color lookup table 53 over line 54 and the output of the color lookup table is applied through line 55 to DAC 56 to provide the blue/green/red signal on line 57. Each priority buffer has an output which is applied over respective lines 61 and 62 to priority decoder 63. There is also a tie bit from CPU 31 data line 33 which is connected through line 64 to the priority decoder. The signal from the priority decoder is provided to analog video switch 65 over line 66. This is a select control signal which determines on a pixel-by-pixel basis which signal from the DAC&#39;s will energize the blue/green/red guns of the CRT. 
     The priority buffers determine the output of priority decoder 63 based upon the priority bit assigned to each pixel. In cases of priority ties, the output of CPU 31 over line 64 determines which frame buffer information is displayed on the CRT as that tie bit determines, through the priority decoder, the operation of the analog video switch. 
     To put it concisely, the purpose of this invention is to allow two or more frame buffers output video displays to be combined, while each maintains its own integrity. This leaves the frame buffer&#39;s output control and access separated from each other. This separation of frame buffers is needed to display the desired images and to allow maximum speed of each process to or from the process&#39;s frame buffer. This allows multiple overlapping images to be displayed, selectable by the user on a pixel-by-pixel basis. 
     With two processes and two frame buffers, or one process with multiple needs using two frame buffers, one for text and one for graphics, each process image is drawn into its frame buffer and a plane of importance or priority attribute bit is written with the object. This priority is compared with the priority bit of the other process frame buffer to enable the video switch, under control of signals from the priority decoder, to select the higher priority frame buffer&#39;s video to be displayed. In addition to the priority bits being sent to the comparator or priority decoder, a user settable tie bit is also sent and is used to select the outcome when the priority bits from two frame buffer have the same priority. Each process can draw multiple images at different priorities depending on the image&#39;s importance within its frame buffers and it will overlay or underlay the other process&#39;s images in accordance with the outcome of the priority decoder and tie bit, displaying them as a single image on a CRT screen. 
     In view of the above description, it is likely that modifications and improvements will occur to those skilled in the art which are within the scope of the appended claims.