Computer graphics processing and selective visual display system – Display peripheral interface input device – Light pen for fluid matrix display panel
Reexamination Certificate
1998-11-13
2001-04-10
Luu, Matthew (Department: 2672)
Computer graphics processing and selective visual display system
Display peripheral interface input device
Light pen for fluid matrix display panel
C345S156000
Reexamination Certificate
active
06215468
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject invention relates to the display of color video signals, and more particularly, to displaying color video signals on a linear display.
All video and data graphic sources assume that the information will be displayed on a CRT-like device, i.e., a display device with a non-linear luminance output/electrical input function. The assumed function is the so-called gamma function, where V
OUT
=V
IN
&ggr;
, where &ggr;=2.2-2.8. However, various flat displays, including plasma display panels (PDP's), have a linear response, i.e., &ggr;=1.0. If this discrepancy is not corrected, the displayed picture has a pale appearance with little color saturation.
2. Description of the Related Art
Typically, digital video signal processing is carried out using 8 bits. However, some PDP's utilize a 10-bit digital input. A gamma look-up table can be used to interface between the 8-bit processing and the 10-bit PDP. Hence, the look-up table uses a list of 256 10-bit values for each color, each of these values being corrected with respect to gamma. The most common way of realizing the look-up table is by using a (P)ROM or a RAM. Since the typical width of RAM and ROM is 8 bits, it is necessary to use 2 8-bit ROMs or RAMs in parallel to store the look-up table.
SUMMARY OF THE INVENTION
It is an object of the invention to store the look-up table in a more economical fashion. This object is achieved in a circuit for converting an 8-bit input video signal into a 10-bit gamma corrected output video signal, said circuit comprising input means for receiving the 8-bit input video signal; means for gamma correcting and for converting the 8-bit input video signal into a 10-bit gamma corrected video signal; and output means for supplying the 10-bit gamma corrected video signal.
In a particular embodiment of the invention, the gamma correcting and converting means comprises a look-up table containing 8-bit gamma table values; and means for converting the 8-bit gamma table values into the 10-bit gamma corrected video signal.
One approach in generating the 8-bit gamma table values is to use a direct conversion where:
V
OUT
=V
IN
&ggr;
(1)
in which the 8-bit gamma table values are actually 8-bit gamma corrected video signal values, and then linearly converting the 8-bit gamma corrected video signal values into 10-bit gamma corrected video signal values.
However, Applicant notes that human vision has a non-linear perceptual response to luminance. The perceptual response to luminance is called Lightness. In an approximation, this response is logarithmic, i.e., the human eye is sensitive to ratios of luminance rather than absolute values. Hence, the human eye can distinguish luminance levels down to 1%. Relating this to the 10-bit PDP, the human eye can clearly distinguish the differences between, e.g., 0 and 1 and 2; just barely distinguish the difference between 100 and 101, but not distinguish the difference between 1015 and 1023. Hence, linear coding of the PDP input is not optimal, but rather, for low luminance levels, 10 bits, or preferably 12 bits are needed, while for high luminance levels, 8 bits are more than sufficient.
Using the Lightness theory, Applicant has found that 10 bits are not needed for the full range of the PDP; just for the lower part of the range. On the other hand, since &ggr;=2.2-2.8, it is known that &ggr;>1. Hence, for the lower input values, the most significant bits (MSB's) of the output are always 0. Based on these facts, instead of 8-bit gamma corrected signal values, 8-bit gamma table values V
CLUT
may be generated by the following formulae:
V
10
(
i
)=4*
i
&ggr;
for
i=
0 to 255; (2)
V
CLUT
(
i
)=
V
10
(
i
), for
i=
0 to 63; (3)
V
CLUT
(
i
)=
V
10
(
i
)/2, for
i=
64 to 127; and (4)
V
CLUT
(
i
)=
V
10
(
i
)/4, for
i=
128 to 255. (5)
Correspondingly, the converting means constructs the 10-bit gamma corrected output values V
OUT
(i) by shifting the look-up table output depending on the MSB's of the input signal as follows:
V
IN
(7, 6)
V
OUT
(9-0)
00
0, 0, V
CLUT
(7-0)
01
0, V
CLUT
(7-0), 0
10
V
CLUT
(7-0), V
CLUT
(7), V
CLUT
(6)
11
V
CLUT
(7-0), V
CLUT
(7), V
CLUT
(6)
The basic assumption is that &ggr;>1.0, therefore V
OUT
≦V
IN
. If V
IN
(
7
,
6
)=00, then V
OUT
(
9
,
8
)=00 and these bits do not need to be coded. Similarly, if V
IN
(
7
,
6
)=01, then V
OUT
(
9
,
8
)≦01, so V
OUT
(
9
)=0 and this bit does not need to be coded. Hence, for V
IN
≦63, the full 10-bit range of the PDP is used; for 64≦V
IN
≦127, 9 bits are used; and for V
IN
≧128, 8 bits are used. The extension by V
CLUT
(
7
), V
CLUT
(
6
) rather than 0, 0, in the latter case is theoretical so that a continuous range up to the maximum PDP input of 1023 is achieved.
Since the circuit is to be used in a color system, the look-up table contains three such tables, one for each color signal.
In addition to correcting for gamma, it is desirable that the stored values in the tables take into consideration white point correction. To this end, the above formula (1) is modified for each color:
V
10
(
i
)=4*GAIN*
i
65
for
i=
0 to 255, (6)
where GAIN is a factor equal to or less than 1. Hence, for each color, the GAIN factor is adjusted to achieve a desired white point.
REFERENCES:
patent: 5764216 (1998-06-01), Tanaka et al.
patent: 5917959 (1999-06-01), Kagawa et al.
patent: 07182507 (1995-07-01), None
patent: 9209065 (1992-05-01), None
patent: 9214336 (1992-08-01), None
Good-Johnson Motilewa
Goodman Edward W.
Luu Matthew
Philips Electronics North America Corporation
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