Browse Prior Art Database

Algorithm for Eliminating Ghosting on LCD Displays

IP.com Disclosure Number: IPCOM000109712D
Original Publication Date: 1992-Sep-01
Included in the Prior Art Database: 2005-Mar-24
Document File: 2 page(s) / 110K

Publishing Venue

IBM

Related People

Balasubramanian, PS: AUTHOR [+2]

Abstract

Ghosting, or the change in average background level in the columns that contain objects that are in extreme contrast with the background, are a very common and annoying phenomenon in many current LCD displays. This disclosure teaches algorithms, usable in either software or implementable in hardware, to reduce and minimize the effect on the display.

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This is the abbreviated version, containing approximately 51% of the total text.

Algorithm for Eliminating Ghosting on LCD Displays

       Ghosting, or the change in average background level in
the columns that contain objects that are in extreme contrast with
the background, are a very common and annoying phenomenon in many
current LCD displays.  This disclosure teaches algorithms, usable in
either software or implementable in hardware, to reduce and minimize
the effect on the display.

      The phenomenon of ghosting, or a streaked change in background
level in the normally vertical columns of an LCD display containing
an object that is in contrast to the nominal background level of the
surrounding pels, is well known and very common on the LCD displays
currently used in notebook, laptop, and other portable or flat panel
displays employing several classes of LCD technology.  The  essential
cause, simplified, is the change in average background brightness
level for a specified column when it contains a large block of pels
that are significantly different in average brightness level to the
rest of the background pels.  This will make the pels in that column
average either slightly brighter or slightly dimmer, depending upon
the color (level) of the large block of contrasting pels.  The column
drivers are unable to compensate for the difference to maintain the
same background level, therefore creating a ghost, or area, usually
vertical to the object, of a slightly different background level than
columns that do not contain a strongly contrasting block of data.

      The problem of ghosting can be solved in several ways.  The
conventional solution would be to improve the drivers for the column
so that the phenomenon does not  exist.  This is difficult to achieve
with present technology and does not solve the problem for existing
displays.

      A solution that can be implemented in software, for existing
systems or in the controller hardware for new equipment, is to adjust
the gray-scale background bias level on either the affected columns
or the background to compensate for the non-linearity of the levels
produced by the column drivers.  As an example, if we assume a
standard 32- level VGA LCD panel with conventional gray-scale
emulating controller, any pel can have an apparent value of from 0 to
31.  If we assume that 0 is total black and 31 is total white, the
gray scale can be divided into 32 increments of brightness.  The
column driver for any one of, for the example of VGA  standard
displays, 640 vertical columns will be trying to maintain analog 1 or
0 values for each or the 480-pel high (240 in the case of a split
display) column depending upon the data shifted into that column.
The actual level maintained for a 0 or a 1 will depend to some de...