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Gamma Curve Determination/Checking without Visual Equipment

IP.com Disclosure Number: IPCOM000108578D
Original Publication Date: 1992-Jun-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 5 page(s) / 219K

Publishing Venue

IBM

Related People

Todd, SJ: AUTHOR

Abstract

Disclosed is a method of visually matching known patterns to ascertain the gamma characteristic of display devices without using special equipment. It ensures an accurate intensity on any display device with at least six bits input to the digital/analog (D/A) converters.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 38% of the total text.

Gamma Curve Determination/Checking without Visual Equipment

       Disclosed is a method of visually matching known patterns
to ascertain the gamma characteristic of display devices without
using special equipment.  It ensures an accurate intensity on any
display device with at least six bits input to the digital/analog
(D/A) converters.

      Novel is the use of the eye to compare intensities generated by
a mixture of two known intensities and one pure unknown intermediate
intensity.  The resulting visual comparison verifies that a given
intensity has an expected value (gamma correction).  It also enables
by iteration an input look-up table value that gives the required
output intensity (to establish gamma correction).

      Three sets of values are involved: image ({im}), intermediate
({in}) and output ({out}).  The image values are those stored in the
image.  The intermediate values are the result of applying a lookup
table to the image values. The output values are the intensities
radiated by screen. Thus: in = lookup (im) and out = gam (in).  The
aim is to perform the following without the use of a lightmeter.
*    Determine the gamma mapping {gam};
*    Create a gamma correction lookup table {lookup} that is the
inverse of {gam}; out = gam(lookup(in)) = in
*    Verify whether a given lookup table {lookup} gives a reasonable
correction for the gamma of a given device.

      Assumed, except where otherwise stated, that all three sets of
values are in the range {0-1}.  The values actually used during
programming will typically have to be in the range {0-255}.  The
notation {1/2 = = 128} is used to show values in both ranges.

      All the tests described operate on the same principle. Where
two intermediate values {in1} and {in2}, produce, when used on large
areas of the display intensities {out1=gam(in1)} and {out2=gam(in2)},
respectively.  Then an area of screen display using a mix with
proportion {p1} of {in1} pixels, and {1-p1} of {in2} pixels will
produce an average intensity {out3}
       out3 = p1*out1 + (1-p1)*out2
Also when the mixed area is viewed from a distance next to a smooth
area of pixels with intermediate value {in3}, where {out3=gam(in3)},
there will be no noticeable boundary.

      Where the proportion {p} is a simple value such as {1/2}, a
mixed area of {in1} and {in2} values may be produced with a simple
pattern such as a chessboard called here a {fine} chessboard.  Where
less rational values of {p} are used, a suitable mixed area may be
generated using error diffusion.  This principle does not always
hold, but gives good results if used with care.

      The principle makes the assumption that a fine chessboard of
pixels with intensities {y} and {0} will have an average intensity of
{y/2}.  Where the fine chessboard consists of very small squares,
such as one pixel by one pixel, this is not true for most devices.
This may be seen by comparing fine chessboar...