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Browse Prior Art Database

Light Pen Emulation on a Raster Display Device

IP.com Disclosure Number: IPCOM000043086D
Original Publication Date: 1984-Jul-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Gay, AC: AUTHOR

Abstract

The problem of emulating a directed-beam display light pen on a raster display is well known. Briefly, the problem is as follows: The light pen of a directed beam display detects light from the vector pointed at, at the time the vector is displayed. Therefore, the vector 'detected' is simply that being drawn when a light pen detect interrupt occurs. On a raster display, pels (picture elements) of a vector are displayed when the corresponding bit in one or more bit plane(s) is set. It is not possible to correlate an illuminated pel pointed at by a pen or, more usually, a tablet with a given vector as the pel may have been drawn at any time previously and may have been set by more than one vector.

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Light Pen Emulation on a Raster Display Device

The problem of emulating a directed-beam display light pen on a raster display is well known. Briefly, the problem is as follows: The light pen of a directed beam display detects light from the vector pointed at, at the time the vector is displayed. Therefore, the vector 'detected' is simply that being drawn when a light pen detect interrupt occurs. On a raster display, pels (picture elements) of a vector are displayed when the corresponding bit in one or more bit plane(s) is set. It is not possible to correlate an illuminated pel pointed at by a pen or, more usually, a tablet with a given vector as the pel may have been drawn at any time previously and may have been set by more than one vector. Thus to determine which vector is selected, the display list must be inspected until a vector is found which would cause a pel to be set on at the position pointed to. The problem is that this correlation process takes time, and immediate feedback is required so that the operator knows which vector was selected. The correlation process may be speeded up considerably by clipping vectors to a small trap window before checking to see if they generate a pel at the selected position. This eliminates many vectors, but still takes time. A fast selection/feedback procedure which requires no correlation for operator feedback can be devised by observing IBM 3250 directed-beam display light-pen operation. If the 3250 operator points the light pen at a part of the screen in which nothing is displayed, then clearly nothing happens. When the pen is positioned on a displayed vector, a 'blip' appears on the vector at the pen position, i.e., where light was detected. It is this blip which is used by the operator as pointing feedback . Moving the pen across the screen causes the blip to appear only on the vector pointed at, at any instant in time. The actual selection of the vector is performed by depressing the tip-switch while pointing to the vector. Thus, it is only nec...