International Business Machines Corporation

Jack E Bresenham

Adrian C Gay

Nicholas David Butler

Ronald John Bowater

Donald George Grice

Shing-Chou Pi

Norman R Sheen

John P Richards

In a line generator and a method for determining the individual pixels to be plotted for a line to be drawn in a display system, coded representations of a plurality of lines are stored in a line definition table. The coded representation of each individual line is a string of data items representing the transitions between adjacent pixels to be plotted for drawing the individual line. Preferably, only coded representations of lines up to a predetermined size (i.e. the length of the line in the case of a straight line) are stored in the line definition table. Strings of data items for representing the pixels to be plotted for longer lines to be drawn are still calculated as in the prior art. In this case, control logic determines whether there are coded representations of a line to be drawn in the line definition table, or not, and passes control to appropriate processing logic for determining the pixels to be plotted. In a preferred embodiment, the string of data items forming the coded representation of a line to be drawn is a string of binary digits and the value of each bit in the string represents a transition in one of two directions. This provides a very compact representation of the line.

Line generation in a display system

In a method of storing characters in a display system having a display device with an orthogonal matrix of addressable points, each character is represented as a succession of strokes each constrained to lie in one of the eight fundamental directions of the matrix and, except for the first stroke, each starting at the end of the previous stroke. Each such stroke is stored in a binary coded form which includes a first binary number (direction code) defining the angular direction of the stroke, a second binary number (length code) defining a number of matrix steps from one addressable point to the next along the stroke in that direction, and a third binary number (move/draw code) defining the visibility of the stroke. In order to facilitate character rotation by any multiple of 45.degree. the direction code defining each fundamental direction corresponds to the addition modulo 2.sup.n of a binary constant to the direction code which defines the fundamental direction of 45.degree. thereto in a given direction of rotation, where n is the number of bits in the direction code.

Method of storing characters in a display system

The invention concerns a graphics display system of the kind having a refresh raster display device, a line clipping mechanism for clipping line primitives for viewing in a selected rectangular viewport having horizontal and vertical edges parallel to the X and Y axes respectively of the display area of the display device, and a vector-to-raster converter for converting the clipped lines to patterns of discrete pels in a refresh buffer, the vector-to-raster converter including an incremental line drawing mechanism responsive to line information supplied by the clipping mechanism to draw the line starting at one endpoint and proceeding towards the other. In such a system, for at least some lines of a kind which have a first endpoint in the viewport and the second endpoint outside the viewport, the clipping mechanism is arranged to avoid calculating the intersection point of the line with the viewport edge intersected by the line, and the vector-to-raster converter is arranged to start the line drawing mechanism at the first endpoint and halt the line drawing mechanism when the currently generated coordinate along the display axis normal th the said viewport edge equals the coordinate of the viewport edge along the same axis.

Graphics display system and method having improved clipping technique

An improved method and apparatus are disclosed for generating circular arcs of any arc length greater than zero up to and including a full circle of 360 degrees. The method and apparatus can locate nonsymmetrical closest points for noninteger radius and arc center values. The method and apparatus is capable of generating the incremental move commands for drawing an arc either in the clockwise or counter clockwise direction which is a distinct advantage when used to drive a pen type electromechanical plotter. By dividing the generation of a 360 degree arc into eight octants, only two of the original eight directions need to be considered as candidate directions toward the next integer display matrix value to be displayed. The method and apparatus employ a novel initialization which provides a simple stopping test for any circular arc of arbitrary length and direction. Only simple addition and sign testing is used to display the circular arc.

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Chapel Hill, NC, United States of America

Jack E Bresenham