Browse Prior Art Database

Automatic BitBlt Direction Calculation

IP.com Disclosure Number: IPCOM000102857D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 1 page(s) / 60K

Publishing Venue

IBM

Related People

Johnson, L: AUTHOR [+4]

Abstract

Disclosed is a device which calculates in hardware the direction of a BitBlt to ensure that no pixels are ever lost during overlapping BitBlts. It makes use of hardware which is commonly used in a rasterization processor to calculate Bresenham line draw parameters and so is not costly to implement in terms of logical gates used. Normally the overlap calculation is performed by a Front End Processor and the result is written to the rasterization processor before the start of the BitBlt. That method is slower and uses more system resources. It requires extra front- end processor cycles to set up a BitBlt and calls for an extra instruction to be passed along the command bus connecting the front-end processor and rasterization processor.

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This is the abbreviated version, containing approximately 87% of the total text.

Automatic BitBlt Direction Calculation

      Disclosed is a device which calculates in hardware the
direction of a BitBlt to ensure that no pixels are ever lost during
overlapping BitBlts.  It makes use of hardware which is commonly used
in a rasterization processor to calculate Bresenham line draw
parameters and so is not costly to implement in terms of logical
gates used.  Normally the overlap calculation is performed by a Front
End Processor and the result is written to the rasterization
processor before the start of the BitBlt. That method is slower and
uses more system resources.  It requires extra front- end processor
cycles to set up a BitBlt and calls for an extra instruction to be
passed along the command bus connecting the front-end processor and
rasterization processor.

      As shown in the figure, the BitBlt parameter calculation uses
as input, X1,Y1 and X2,Y2 where X and Y are screen coordinates.
X1,Y1 is the address of source pixel 1 and X2,Y2 is the address of
destination pixel 1. X1 is subtracted from X2 and Y1 is subtracted
from Y2.  The signs of the results of the two subtractions are used
to calculate the BitBlt direction.

      In a rasterization processor, the registers used to hold X1,Y1
and X2,Y2 can also be used to hold the end points of a line when
the operation being performed by the processor is line draw rather
than BitBlt.  The first step for the calculation of Bresenham
Parameters for the Bresenham line draw algorithm requir...