Apparatus and method for specifying windows with priority ordered rectangles in a computer video graphics system

• US 5,216,413 A
• Filed: 12/04/1991
• Issued: 06/01/1993
• Est. Priority Date: 06/13/1988
• Status: Expired due to Term

• Current Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Original Assignee: Digital Equipment Corporation (HP Inc.)
• Inventors: Seiler, Larry D., Rose, Robert C., Pappas, James L.
• Primary Examiner(s): Brier, Jeffery A.
• Application Number:
  US07/803,706
• Time in Patent Office: 545 Days
• Field of Search: 340/721, 340/734, 340/720, 340/723, 340/729, 340/747, 340/701, 364/521, 364/522, 395/157, 395/158, 395/164
• US Class Current: 715/797
• CPC Class Codes: G09G 5/06 using colour palettes, e.g. look-up tables
• CPC Class Codes: G09G 5/08 Cursor circuits
• CPC Class Codes: G09G 5/14 Display of multiple viewports

Abstract

A method and a device for distinguishing which pixels are to be displayed in a set of overlapping, rectangular windows in video graphics display are provided. Windows are specified as a priority ordered list of rectangles or other shapes, so that each window is computed as being visible at those pixels that are inside its rectangle but outside the rectangles of all higher priority windows. The number of rectangles required is equal to the number of rectangular windows, irrespective of the degree of overlap.

Claims

1. A clocked computer video graphics system for displaying on a monitor pixels belonging to overlapping windows, comprising:

a. a frame buffer for sequentially and continuously providing pixel values to the system from the first memory location in the frame buffer to the last memory location in the frame buffer to be interpreted for display;

b. means for assigning a priority to each window to be displayed;

c. window definition registers for specifying the pixels within each window;

d. a set of counters coupled to the window definition registers for tracking the x and y values of the pixel being displayed at any time;

e. a comparator for determining if the pixel specified by said counters is within each window for each clock pulse of the system; and

f. an arbitrator for selecting the highest priority window of those which the pixel is within.

2. The graphics system according to claim 1 and including a monitor and means for displaying the pixels on the monitor.

3. The graphics system according to claim 1 wherein said highest priority window is used to clip drawing operations to the frame buffer.

4. The graphics system of claim 2 further comprising a means for assigning a default window number to each pixel value for which no bit was produced by the comparator.

5. The video graphics system of claim 1, wherein the arbitrator comprises a priority tree for determining which of the windows containing each pixel has highest priority.

6. The video graphics system of claim 1, wherein the frame buffer stores pixel values as overlapping windows.

7. In a computer video graphics system having a pixel processing unit and a frame buffer with memory locations and which is capable of displaying pixels sequentially and continuously from the first memory location in the frame buffer to the last memory location in the frame buffer in a plurality of windows of selectable priority, a method of determining a window number for each said pixel comprising the steps of:

a. sequentially and continuously obtaining pixel values from the first memory location in the frame buffer to the last memory location in the frame buffer;

(b) providing respective window numbers for each said pixel value;

(c) specifying the pixels within each window;

(d) comparing for each window each pixel position to determine if the pixel is or is not within each window;

(e) performing a priority operation that determines, of the windows that contain the pixel, the window number having the highest priority; and

(f) providing said determined window number to the pixel processing unit for controlling operations performed on the pixel value.

8. The method of claim 7 wherein a pixel that is not contained in any window is assigned a specified default window number.

9. A computer video graphics system for displaying on a monitor pixels belonging to overlapping windows, comprising:

a. a pixel value memory containing pixel values to be displayed on the monitor;

b. means for sequentially and continuously reading the pixel value memory in the order in which pixel values are contained within the pixel value memory;

c. means for assigning a priority to each window to be displayed;

d. a set of registers for storing the locations of the edges of each window;

e. a set of counters for tracking the pixel being displayed at any time;

f. comparators for comparing the contents of the counters with the contents of the registers and producing a bit for each pixel that is within a window; and

g. an arbitrator for selecting the highest priority window of those for which a bit was produced by the comparator, whereby said pixel belongs to the highest priority window.

10. In a computer video graphics system having a pixel processing unit and a frame buffer with memory locations and which is capable of displaying pixels sequentially and continuously from the first memory location in the frame buffer to the last memory location in the frame buffer in a plurality of windows of selectable priority, a method of determining a window number for each pixel comprising the steps of:

a. sequentially and continuously obtaining pixel values from the first memory location in a frame buffer to the last memory location in the frame buffer;

(b) providing respective window numbers for each said pixel value;

(c) providing in registers the minimum and maximum extents of each window;

(d) performing a comparison operation in parallel for each window to determine if a pixel is or is not in that window;

(e) performing a priority operation that determines, of the windows that contain the pixel, the window number having the highest priority; and

(f) outputting that number to the pixel processing unit for controlling operations performed on the pixel value.

11. An apparatus for specifying windows in priority order in a video graphics system, the windows comprising pixels and the video graphics system having a refresh process, the apparatus comprising:

a. a frame buffer with memory locations and which is capable of displaying pixels sequentially and continuously from the first memory location in the frame buffer to the last memory location in the frame buffer in a plurality of windows of selectable priority;

b. an X counter and a Y counter for calculating the X,Y position of refresh logic in the video graphics system;

c. a set of window definition registers coupled to the X and Y counters for storing the locations of the edges of the windows defined in the system;

d. a set of window detectors for receiving a window definition for each X,Y position from the X and Y counters contained within a window;

e. a priority tree for determining which of the windows containing each pixel has highest priority, the priority tree outputting a window number for each X,Y position within a window in the refresh process and for each X,Y position that is not within a window, outputting a default window number;

f. a mapping memory for receiving each window number from the priority tree; and

g. means for receiving pixel values from the frame buffer in the order in which the pixel values are stored in the frame buffer, each pixel value received having a corresponding window number received by the mapping memory.

12. A method of specifying windows in priority order in a video graphics system having a frame buffer for storing pixel values, the windows comprising pixels and the video graphics system having a refresh process, comprising the steps of:

a. calculating the X,Y position of refresh logic in the video graphics system in a set of X and Y counters;

b. storing the locations of the edges of the windows defined in the system in a set of window definition registers coupled to the X and Y counters;

c. receiving a window definition in a set of window detectors for each X,Y position in the X and Y counters contained within a window;

d. determining in a priority tree which of the windows containing each pixel has highest priority, the priority tree outputting a window number for each X,Y position within a window in the refresh process and for each X,Y position that is not within a window, outputting a default window number;

e. receiving in a mapping memory each window number from the priority tree; and

f. receiving pixel values sequentially and continuously from the first pixel value in the frame buffer to the last pixel value in the frame buffer in the order in which the pixel values are stored in the frame buffer, each pixel value received having a corresponding window number received by the mapping memory.