Browse Prior Art Database

Polygon Filling in Graphics Display System

IP.com Disclosure Number: IPCOM000052431D
Original Publication Date: 1981-Jun-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 3 page(s) / 22K

Publishing Venue

IBM

Related People

Simmons, J: AUTHOR [+2]

Abstract

This idea relates to a graphics display system having the ability to draw vectors forming polygons enclosing selected areas of the image displayed. It is sometimes convenient to fill in the space defined by a polygon with a special character in order to make the space opaque or more clearly recognizable and thereby form a mask. This idea provides an arrangement implemented in hardware and microcode for filling in the space defined by a polygon.

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Polygon Filling in Graphics Display System

This idea relates to a graphics display system having the ability to draw vectors forming polygons enclosing selected areas of the image displayed. It is sometimes convenient to fill in the space defined by a polygon with a special character in order to make the space opaque or more clearly recognizable and thereby form a mask. This idea provides an arrangement implemented in hardware and microcode for filling in the space defined by a polygon.

The arrangement utilizes a mask buffer, microcode logic for loading the mask buffer and hardware logic for generating a mask from the contents of the mask buffer. The mask buffer contains picture element positions corresponding with the picture element positions on the display. Each picture element position in the mask buffer can contain a 2-bit mask code. The code is used by the mask generation logic to determine if a particular picture element is within a polygon. Before any polygons are drawn, the mask buffer is cleared to zero. When a vector is to be drawn on the display, a code for the vector is selected in accordance with the direction of the vector. As the vector is drawn, the picture element positions in the display used to display the vector are calculated and the corresponding picture element positions in the mask buffer are loaded with the logical OR combination of the code corresponding to the vector using the mask buffer loading logic and any code already loaded in that portion and relating to a preceding vector. When the first vector is completed, the code for the next vector is selected and, as the next vector is drawn, this selected code is used, as described above, for the first vector.

As a polygon is drawn, the code for the first point of one vector is combined with the code of the last point of the previously drawn vector. This combination is required to ensure that the corners of the polygon are correctly coded. This process is repeated until all the vectors have been drawn and the polygon is complete.

The code representing a vector is a two bit code in accordance with the following table: Y2 > Y1 Code 01 (1) Y2 = Y1 Code 10 (2) Y2 < Y1 Code 10 (2) Where (X1, Y1) and (X2, Y2) represent respectively the coordinates of the position from which and to which a vector is to be...