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Length Value Estimator for Vector Pattern Generation

IP.com Disclosure Number: IPCOM000035381D
Original Publication Date: 1989-Jul-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 5 page(s) / 86K

Publishing Venue

IBM

Related People

Gonzalez-Lopez, J: AUTHOR [+2]

Abstract

Fig. 1 depicts a block diagram of a typical image display system. Graphics orders are downloaded from a host system and stored in System Memory. A display Processor interprets those orders and writes the appropriate pixels of the Frame Buffer. The content of the Frame Buffer is visualized in the monitor. The System Controller supervises the operation of the different components.

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Length Value Estimator for Vector Pattern Generation

Fig. 1 depicts a block diagram of a typical image display system. Graphics orders are downloaded from a host system and stored in System Memory. A display Processor interprets those orders and writes the appropriate pixels of the Frame Buffer. The content of the Frame Buffer is visualized in the monitor. The System Controller supervises the operation of the different components.

A simplified implementation of the Display Processor, which is suitable for the sake of the explanation, is given in Fig. 2. A microprocessor with local storage for program and data has access to the System Memory through an appropriate interface so that the graphics orders become accessible to the microprocessor. The display processor contains a vector generator (VG) that implements a suitable vector generation algorithm, such as the well known Bresenham's algorithm. The output of VG provides the X-Y coordinates of the Frame Buffer locations that belong to the vector being generated.

The Length Value Generator (LVG) is used when the line is to be written with some associated pattern (i.e., dots, dashes, etc.). For this purpose the LVG is fed with appropriate information by the VG to allow the estimation of the current length of the vector. The current

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length is defined as the geometrical distance from the initial point of the vector to the current position. A look-up table (LUT) is then used to determine if the current pixel should be written into the frame buffer or not according to the defined pattern stored in the table. The LUT provides the signal WE (Write Enable) to the frame buffer for this purpose. The content of the LUT can be fixed or programmable. Description of the Problem

The current length at point P of the vector MN (see Fig. 3) is estimated as: L = Au + (B-A)v where A=1 and B=3/2. The expression means that for a line in the first octant (i.e., the angle alpha is in the interval 0-45 degrees), the current length estimate is obtained by adding A each time a horizontal move occurs, or by adding B if a diagonal move takes place, to the previous estimate. The initial value for the estimate is zero.

In the general case, B is added if the vector generator indicates a diagonal move. Otherwise A is added.

The exact current length has the value:

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Lex = SQRT(U**2 + v**2).

The problem results from the fact that the accuracy of the estimate is a function of the vector orientation (angle ALPHA). If a set of dotted lines are drawn with different orientations from a common origin so that they virtually cover the whole plane (i.e., 360 degrees around the center), corresponding dots of the

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different lines should lie on concentric circles around the origin. Any error in the estimation of the true current length would result in distortion of the circles. E = Lex SQRT(1 + h**2) L A + (B-A)h with h = v = tan(ALPHA) u

(Image Omitted)

as the ratio of the true current...