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Graphics Executed on a PS Display

IP.com Disclosure Number: IPCOM000047126D
Original Publication Date: 1983-Sep-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 6 page(s) / 67K

Publishing Venue

IBM

Related People

Cottrell, RLA: AUTHOR [+2]

Abstract

Mechanisms are described for operating interactive or non-interactive graphics applications on a display provided with a programmable symbol (PS) cell structured screen bit buffer which has insufficient total bits for all the displayable screen pels. The IBM 8775 display terminal includes such cell structured refresh buffers having a two-byte entry for each PS cell position on the screen. These two bytes are known as the attribute (ATT) and element (EL) bytes. Each refresh buffer entry associates a screen area position with the PS cell to be displayed at that position. The IBM 8775 utilizes a number of sets of PS buffers, each set containing 192 cell buffers. A set is selectable using a 3-bit address field (bits 5-7) in the ATT byte. In the implementation described, bits 2-4 of the attribute byte are used for control flags.

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Graphics Executed on a PS Display

Mechanisms are described for operating interactive or non-interactive graphics applications on a display provided with a programmable symbol (PS) cell structured screen bit buffer which has insufficient total bits for all the displayable screen pels. The IBM 8775 display terminal includes such cell structured refresh buffers having a two-byte entry for each PS cell position on the screen.

These two bytes are known as the attribute (ATT) and element (EL) bytes. Each refresh buffer entry associates a screen area position with the PS cell to be displayed at that position. The IBM 8775 utilizes a number of sets of PS buffers, each set containing 192 cell buffers. A set is selectable using a 3-bit address field (bits 5-7) in the ATT byte. In the implementation described, bits 2-4 of the attribute byte are used for control flags. The second of the refresh buffer bytes (EL) is used to select the member of a set. Interactive graphic applications require the ability to draw vectors on the display between any two screen coordinates and to subsequently erase the vectors if the drawing requires change or deletion. Interactive operations such as vector drawing, vector erasure or the movement of lines or line groups can be specified by the user via a cursor which can be drawn on the display screen with its origin located at any screen pel. This cursor can be erased and redrawn to any new screen address usually under control of key strokes interpreted from the application code. To determine the correlation between the cursor and a graphic line feature requires a screen coordinate comparison facility. Graphics applications which allow the user to interact with the currently displayed drawing and generated modification, correction or selection produce a constantly changing pattern of screen pel usage. It is necessary that the display mechanism allocate the PS cells as economically as possible during the execution of the various graphic operations. In addition, due to the limited amount of screen buffering provided in low-cost display terminals, it is necessary that the screen buffering provided remains efficiently utilized while the pattern of usage alters as a result of operator or application activity. In particular, vector erasure either by cursor movement or by modification or deletion of graphic features can result in the screen buffering cells having all their bits reset and thus becoming effectively unused. Methods for recycling such cells for reuse by the drawing mechanism are described with reference to the figures. A pel string selected to represent a requested vector on a screen is derived by means of a Bresenham pel generator. Thus, as illustrated in Fig.1 for a vector draw operation from coordinates (X1 Y1) to (X2 Y2), the refresh buffer entry 1 for the screen area containing the vector start pel is allocated a PS cell (1) and the pel string from the vector start (X1 Y1) to the first cell boundary ins...