[0001] The invention relates to a method and a device for encoding the luminance value of
a pixel of a picture into a subfield code word in a display device. It can be applied
to every display device using a PWM (Pulse Width Modulation) technology and subfields
for displaying video picture.
Background of the invention
[0002] The sub-field encoding part of a display using PWM technology is one of the most
important parts of the display device since the encoding is responsible of the gray-scale
portrayal (linearity and level of noise dithering) and of the motion rendition (level
of false contour).
[0003] The goal of the sub-field encoding is to fill up a sub-fields memory with subfields
data. The subfield data of a pixel is a code word wherein each bit is representative
of the state, "ON" or "OFF", of this pixel during a subfield. This sub-fields memory
will be read during the next frame, sub-field by sub-field, whereas it is written
pixel by pixel. This information is used directly to control the display device.
[0004] The subfield encoding step is generally done after a degamma function as shown in
Figure 1. The degamma function is first applied to the input luminance values. These
values are then coded by the sub-field encoding step into subfield code words. The
subfield encoding step is eventually preceded by a dithering step. The subfield code
words are then stored in a subfields memory.
[0005] In a standard approach, the encoding step is implemented by using a simple look-up
table. A subfield code word is associated to each luminance value.
[0006] Some problems can not be solved at all or in a simple way when using this standard
approach. This is the case of line load effect problem where the light emitted by
a current pixel for a given luminance value can vary according to the load of the
line of pixels to which the current pixel belongs. This problem can not be solved
completely by using the standard approach. It is the same for the linearity problem
when an average power level is controlled in the display device.
[0007] The line load effect is illustrated by figures 2 and 3. The figure 2 shows a test
picture to be displayed by a display device suffering from a problem of line load
effect. The first and the last lines are black on half of the pixels, and white on
the other half. The middle lines are white. The figure 3 shows the picture displayed
by the display device. The line load effect is visible on the middle lines. This effect
can be explained as follows: when a sub-field is used on a whole line its luminance
is decreased by 20% compared to its luminance on a line where it is not used. The
value of 20% is given as an example. The luminance value of the pixels of the middle
lines is thus 255·(1-(1-½) × 0.20) = 229,5 while the white pixels of the other lines
have a luminance of 255 · (1- (1-1) × 0.20) = 255.
Invention
[0008] It is an object of the present invention to disclose a subfield encoding method that
can solve in a simple way any luminance problem and notably the line load effect problem.
[0009] So the invention concerns a method for encoding the luminance value of a pixel of
a picture into a subfield code word in a display device, wherein each bit of the subfield
code word has a state "ON" or "OFF" and generates light emission during an own period
called subfield when its state is "ON", the light emitted for a pixel by its subfield
code word being representative of the luminance value of said pixel and the total
duration of the subfields associated to the bits of the subfield code word forming
the picture frame.
[0010] Each subfield has a weight proportional to its duration. According to the invention,
the bits of each subfield code word are computed recursively in the descending order
of the weights of the corresponding subfields from the bit associated to the subfield
having the most significant weight to the bit associated to the subfield having the
least significant weight.
More particularly, each bit of the subfield code word of a current pixel is computed
by :
- for the bit associated to the subfield having the most significant weight, allocating
a state "ON" to said bit if the luminance value of the current pixel is equal to or
greater than a threshold value associated to said bit, and
- for each following bit of the subfield code word, computing the remaining luminance
value to be encoded by said bit and the following bits of the subfield code word and
allocating a state "ON" to said bit if the remaining luminance value is equal to or
greater than a threshold value associated to said bit.
[0011] The threshold value associated to a bit is equal to the sum of the weights of the
subfields having a lower weight than the weight of the subfield corresponding to said
bit plus one.
[0012] In a specific embodiment, the remaining luminance value to be encoded by a bit for
a current pixel is computed as a function of the state of the pixels of the line of
the display device to which the current pixel belongs.
[0013] For example, the remaining luminance value to be encoded by a current bit of the
subfield code word of a current pixel is computed by :
- calculating, for the subfield, called preceding subfield, computed just before the
subfield associated to said current bit, the number of pixels having the bit associated
to said preceding subfield in a "ON" state in the line of pixels to which the current
pixel belongs;
- estimating a subfield luminance value for said subfield on the basis of said number
of pixels, and
- subtracting said subfield luminance value from the luminance value to be encoded by
the bit associated to said preceding subfield and the following bits of the subfield
code word.
[0014] The invention concerns also to a device for encoding the luminance value of a pixel
of a picture into a subfield code word in a display device, wherein each bit of the
subfield code word has a state "ON" or "OFF" and generates light emission during an
own period called subfield when its state is "ON", the light emitted for a pixel by
its subfield code word being representative of the luminance value of said pixel and
the total duration of the subfields associated to the bits of the subfield code word
forming the picture frame. According the invention, this device computes the bits
of each subfield code word recursively in the descending order of the weights of the
corresponding subfields from the bit associated to the subfield having the most significant
weight to the bit associated to the subfield having the least significant weight.
[0015] For computing each bit of the subfield code word of a current pixel, the device comprises:
- a comparator circuit for allocating a state "ON" to the bit associated to the subfield
having the most significant weight if the luminance value of the current pixel is
equal to or greater than a threshold value associated to said bit, and for allocating
a state "ON" to said bit if the remaining luminance value to be encoded by said bit
and the following bits of the subfield code word is equal to or greater than a threshold
value associated to said bit, and
- a calculation circuit for calculating, for each bit of the subfield code word following
the bit associated to the most significant weight subfield, the remaining luminance
value to be encoded by said bit and the following bits of the subfield code word.
[0016] It further comprises a controller for outputting the threshold values associated
to the different bits of the subfield code word. Threshold value associated to a bit
is advantageously equal to the sum of the weights of the subfields having a lower
weight than the weight of the subfield corresponding to said bit plus one.
[0017] In a preferred embodiment, the calculation circuit calculates the remaining luminance
value to be encoded by a bit for a current pixel as a function of the state of the
pixels of the line of the display device to which the current pixel belongs. In that
case, for calculating the remaining luminance value to be encoded by a current bit
of the subfield code word of a current pixel, the calculation circuit comprises:
- means for calculating, for the subfield, called preceding subfield, computed just
before the subfield associated to said current bit, the number of pixels having the
bit associated to said preceding subfield in a "ON" state in the line of pixels to
which the current pixel belongs;
- means for estimating a subfield luminance value for said subfield on the basis of
said number of pixels, and
- means for subtracting said subfield luminance value from the luminance value to be
encoded by the bit associated to said preceding subfield and the following bits of
the subfield code word.
[0018] The invention relates also to a PWM display device comprising an encoding device
as mentioned hereinabove.
Brief description of the drawings
[0019] Exemplary embodiments of the invention are illustrated in the drawings and are explained
in more detail in the following description. In the drawings :
- Fig.1
- is a schematic diagram showing the steps to be applied to luminance values of pixels
to convert them into subfield code words,
- Fig.2
- is a test picture to be encoded with the inventive method;
- Fig.3
- illustrates the line load effect for the test picture of Fig.2,
- Fig.4
- is schematic diagram of a device implementing the inventive method,
- Fig.5
- is a more detailed schematic diagram of a device implementing the inventive method,
- Fig.6
- is a schematic diagram of a first part of the device of Fig.5 for generating the most
significant bits of the subfield code word, and
- Fig.7
- is a schematic diagram of a second part of the device of Fig.5 for generating the
other bits of the subfield code word.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0020] Although the invention is described in reference to a display device suffering from
a problem of line load effect when displaying some pictures, it can be used to compensate
for any luminance problem that can be estimated.
[0021] The general idea of the recursive coding is to encode one sub-field after the other
in order to be able to compensate for problems occurring on one sub-field with the
other sub-fields. More particularly, the bits of a subfield code word are computed
recursively one after the other such that the defects (e.g. line load and/or linearity)
in the light emission or luminance generated by a bit of the subfield code word can
be compensated by the following bits of the subfield code word. In a particular embodiment,
the bits of each subfield code word will be computed recursively in the descending
order of the weights of the corresponding subfields from the most significant bit
(MSB) associated to the subfield having the most significant weight to the least significant
bit (LSB) associated to the subfield having the least significant weight.
[0022] According to the invention, a state "1" (= "ON") is allocated to the MSB of a current
pixel if the luminance value of this pixel is equal to or greater than a threshold
associated to this bit (or to the corresponding subfield). For each following bit,
a remaining luminance value to be encoded by said bit and the following bits is computed
and a state "1" is allocated to said bit if the remaining luminance value is equal
to or greater than a threshold value associated to said bit.
[0023] An example will be described with the following sub-field weights and the corresponding
switching values :
Subfield SFi |
SF1 |
SF2 |
SF3 |
SF4 |
SF5 |
SF6 |
SF7 |
SF8 |
SF9 |
SF10 |
Weight Wi |
1 |
2 |
4 |
7 |
12 |
19 |
29 |
42 |
59 |
80 |
Switching value SVi |
1 |
2 |
4 |
8 |
15 |
27 |
46 |
75 |
117 |
176 |
[0024] The switching value SV
i of a current subfield SF
i (or of a bit of a subfield code word) is for example the sum of the weights W
j of the subfields preceding said current subfields plus one :

[0025] The remaining luminance value to be encoded by a current bit of a subfield code word
of a current pixel can be computed by :
- estimating the luminance generated by the bit preceding the current bit and
- subtracting said luminance value from the luminance value to be encoded by the bit
preceding the current bit and the bits following said preceding bit.
[0026] As each bit of a subfield code word refers to a particular subfield among the plurality
of subfields of the video frame, we will use indifferently in the following specification
the expressions "luminance of a bit" or "luminance of a subfield". The luminance of
a bit for a current pixel is calculated on the basis of the number of pixels having
the preceding bit in the state "1" in the line of pixels to which the current pixel
belongs.
[0027] As explained previously, the luminance of a subfield can vary depending on the line
load of the line of pixels to be displayed (number of pixels in a "ON" state in the
considered line of pixels). So it is evaluated as soon as all required information
is known. It can be evaluated at the end of the loading of the picture but in order
to limit the time delay, it usually will be evaluated at the latest after each line.
For a perfect display where the luminance of a sub-field on a pixel is only a function
of the pixel itself and the luminance of the pixel can be evaluated directly, the
luminance of the sub-field is roughly the same for all pixels of the picture. For
a display where the luminance on a line is dependent on the load distribution on this
line (e.g. line load effect), the luminance of a sub-field can only be evaluated when
the sub-field has been encoded for the whole line.
[0028] According to the invention, the problem of line load effect is processed in a new
way. Generally, the line-load effect is seen as a luminance loss on a line. Nevertheless
it is equivalent to say that when a sub-field is used on a whole line its luminance
is decreased by 20% in comparison to its luminance on a line where it is not used
and to say that when the sub-field is not used on a line its luminance is increased
by 25% compared to its luminance when it is used on the whole line. The reference
luminance is different, but the effect is the same. Thus, in the figures 2 and 3,
the luminance value of the pixels of the first and last lines is 255·(1+(1-½)×0.25)
= 287 while the white pixels of the middle lines have a luminance of 255·(1+(1-1)×0.25)
=255.
[0029] It has to be noted that since the luminance of each sub-field is evaluated, the linearity
problem due to the APL control can also be compensated by the inventive method.
[0030] This inventive method will be better explained by means of an example using the sub-field
weights and the switching values given in the previous table.
[0031] The recursive encoding method is done line by line and sub-field by sub-field. The
goal is to determine for each pixel value an appropriate subfield code word. At the
beginning of the method, the subfield code word of all the pixel values can be represented
like this: X X X X X X X X X X.
First line : the luminance value of the white pixels (value to encode: 255) is encoded
in X X X X X X X X X X ;
First line : the luminance value of the black pixels (value to encode: 0) is encoded
in X X X X X X X X X X ;
Middle line : the luminance value of the pixels (value to encode: 255) is encoded
in X X X X X X X X X X ;
Last line : the luminance value of the white pixels (value to encode: 255) is encoded
in X X X X X X X X X X ;
Last line : the luminance value of the black pixels (value to encode: 0) is encoded
in X X X X X X X X X X;
[0032] According to the invention, the bit of the subfield code word corresponding to the
most significant subfield (10
th subfield) is determined for all the pixels of the picture. On the first line and
during the first recursive step (encoding of the bit related to the 10
th sub-field) the white pixels, with an original luminance value of 255, will use the
sub-field 10 since 255≥176 (switching value for the 10
th subfield). Therefore, their subfield code word will be of the type of X X X X X X
X X X 1. The black pixels will not use this sub-field since 0<176, so their subfield
code word will be of the type of X X X X X X X X X 0.
[0033] Once this subfield code bit has been determined for this line, its load can be evaluated
and its effective luminance can be estimated. In the example of figure 2, the load
of this line for this sub-field is equal to ½ since one half of the pixels are white
and use this sub-field, and the other half is black and so do not use this sub-field.
The sub-field will be brighter than expected and its luminance on this line will be
equal to: 80-(1+(1-½)×0.25)=90. For the pixels where this sub-field is used, the luminance
of this sub-field is subtracted to the luminance value to encode. So at the end of
the first recursive step for the first line, we have:
First line: the luminance value of the white pixels is encoded in X X X X X X X X
X 1 ; the remaining value to encode is 165;
First line, the luminance value of the black pixels is encoded in X X X X X X X X
X 0 ; the remaining value to encode is 0;
[0034] For the first line, the second recursive step corresponding to the encoding of the
9
th sub-field (the most significant remaining sub-field) is carried out. The white pixels
have now a luminance value of 165, so they will use the 9
th sub-field since 165≥117. The black pixels will not use the 9
th sub-field since 0<117. The load of this line for this sub-field is equal to ½. The
effective luminance of the 9
th sub-field for this first line is equal to: 59·(1+(1-½)×0.25)=66. So at the end of
the second recursive step on the first line we have:
First line : the luminance value of the white pixels is encoded in X X X X X X X X
1 1 ; the remaining value to encode is 99 ;
First line : the luminance value of the black pixels is encoded in X X X X X X X X
0 0 ; the remaining value to encode is 0 ;
[0035] For the first line, the third recursive step corresponding to the encoding of the
8
th sub-field is then carried out. The white pixels will use the 8
th sub-field since 99≥75. The effective luminance of this sub-field for the first line
is equal to 42. (1 + (1-½) × 0.25) = 47. So at the end of the third recursive step
on the first line we have:
First line : the luminance value of the white pixels is encoded in X X X X X X X 1
1 1 ; the remaining value to encode is 52 ;
First line : the luminance value of the black pixels is encoded in: X X X X X X X
0 0 0 ; the remaining value to encode is 0 ;
[0036] For the first line, the fourth recursive step corresponding to the encoding of the
7
th sub-field is then carried out. The white pixels will use the 7
th sub-field since 52≥46. The effective luminance of this sub-field for the first line
is equal to 29-(1+(1-½)×0.25)=33. So at the end of the fourth recursive step on the
first line we have:
First line : the luminance value of the white pixels is encoded in X X X X X X 1 1
1 1 ; the remaining value to encode is 19 ;
First line : the luminance value of the black pixels is encoded in X X X X X X 0 0
0 0 ; the remaining value to encode is 0 ;
[0037] For the first line, the fifth recursive step corresponding to the encoding of the
6
th sub-field is then carried out. The white pixels will not use the 6
th sub-field since 19<27. So, the load of this line for this sub-field will be equal
to 0. So the effective luminance of this sub-field for the first line is equal to
19.(1+(1-0)×0.25)=24. So at the end of the fifth recursive step on the first line
we have:
First line : the luminance value of the white pixels is encoded in X X X X X 0 1 1
1 1 ; the remaining value to encode is 19 ;
First line : the luminance value of the black pixels is encoded in X X X X X 0 0 0
0 0 ; the remaining value to encode is 0 ;
[0038] For the first line, the sixth recursive step corresponding to the encoding of the
5
th sub-field is then carried out. The white pixels will use the 5
th sub-field since 19≥15. The effective luminance of this sub-field for the first line
is equal to 12·(1+(1-½) × 0.25) =14. So at the end of the sixth recursive step on
the first line we have:
First line : the luminance value of the white pixels is encoded in: X X X X 1 0 1
1 1 1 ; the remaining value to encode is 5 ;
First line : the luminance value of the black pixels is encoded in X X X X 0 0 0 0
0 0 ; the remaining value to encode is 0 ;
[0039] The first line will not use the 4
th sub-field since its highest remaining value (5 which is the remaining value for the
white pixels) is smaller than its switching value (8). So at the end of the seventh
recursive step, we have
First line : the luminance value of the white pixels is encoded in X X X 0 1 0 1 1
1 1 ; the remaining value to encode is 5;
First line : the luminance value of the black pixels is encoded in X X X 0 0 0 0 0
0 0 ; the remaining value to encode is 0 ;
[0040] For the first line, the eighth recursive step corresponding to the encoding of the
3
th sub-field is then carried out. The white pixels will use the 3
rd sub-field since 5≥4. The luminance of this sub-field for the first line is equal
to 4·(1+(1-½)×0.25) = 4. So at the end of the eighth recursive step, we have
First line : the luminance value of the white pixels is encoded in: X X 1 0 1 0 1
1 1 1 ; the remaining value to encode is 1 ;
First line, the luminance value of the black pixels is encoded in X X 0 0 0 0 0 0
0 0 ; the remaining value to encode is 0 ;
[0041] For the first line, the ninth recursive step corresponding to the encoding of the
2
nd sub-field is then carried out. The white pixels will not use the 2
nd sub-field. The luminance of this sub-field for the first line is equal to 2 · (1
+ (1- 0) × 0.25) = 2. So at the end of the ninth recursive step, we have
First line, the luminance value of the white pixels is encoded in X 0 1 0 1 0 1 1
1 1 ; the remaining value to encode is 1;
First line, the luminance value of the black pixels is encoded in X 0 0 0 0 0 0 0
0 0 ; the remaining value to encode is 0 ;
[0042] Finally, the tenth recursive step corresponding to the encoding of the 1
st sub-field is then carried out. The white pixels will use the 1
st sub-field since 1≥1. So at the end of the tenth recursive step, we have
First line, the luminance value of the white pixels is encoded in 1 0 1 0 1 0 1 1
1 1 ;
First line, the luminance value of the black pixels is encoded in 0 0 0 0 0 0 0 000.
[0043] The luminance value of the white pixels and black pixels of the other lines identical
to the first line are encoded as defined for the first line.
[0044] For the middle lines, the first recursive step corresponding to the encoding of the
10
th sub-field is carried out as follows. The pixels (all of them are white) use the 10
th sub-field since 255≥176. The load of this line for this sub-field is equal to 1 since
all pixels of this line use this sub-field. So the luminance of this sub-field for
the middle lines is equal to the expected value 80 · (1 + (1-1) × 0.25) = 80. So at
the end of the first recursive step, we have
Middle line : the luminance value of the pixels is encoded in X X X X X X X X X 1
; the remaining value to encode is 175 ;
[0045] At the end of the second recursive step (encoding of the 9
th subfield) of the middle line, since 175≥117, we have :
Middle line : the luminance value of the pixels is encoded in X X X X X X X X 1 1
; the remaining value to encode is 116;
[0046] At the end of the third recursive step (encoding of the 8
th subfield) of the middle line, since 116≥75, we have :
Middle line : the luminance value of the pixels is encoded in X X X X X X X 1 1 1
; the remaining value to encode is 74;
[0047] At the end of the fourth recursive step (encoding of the 7
th subfield) of the middle line, since 74≥46, we have:
Middle line : the luminance value of the pixels is encoded in X X X X X X 1 1 1 1
; the remaining value to encode is 45;
[0048] At the end of the fifth recursive step (encoding of the 6
th subfield) of the middle line, since 45≥27, we have:
Middle line : the luminance value of the pixels is encoded in X X X X X 1 1 1 1 1
; the remaining value to encode is 26;
[0049] At the end of the sixth recursive step (encoding of the 5
th subfield) of the middle line, since 26≥15, we have:
Middle line : the luminance value of the pixels is encoded in X X X X 1 1 1 1 1 1
; the remaining value to encode is 14;
[0050] At the end of the seventh recursive step (encoding of the 4
th subfield) of the middle line, since 14≥8, we have:
Middle line : the luminance value of the pixels is encoded in X X X 1 1 1 1 1 1 1,
the remaining value to encode is 7;
[0051] At the end of the eighth recursive step (encoding of the 3
rd subfield) of the middle line, since 7≥4, we have:
Middle line: the luminance value of the pixels is encoded in X X 1 1 1 1 1 1 1 1 ;
the remaining value to encode is 3;
[0052] At the end of the ninth recursive step (encoding of the 2
nd subfield) of the middle line, since 3≥2, we have:
Middle line: the luminance value of the pixels is encoded in X 1 1 1 1 1 1 1 1 1 ;
the remaining value to encode is 1;
[0053] At the end of the tenth recursive step (encoding of the 1
st subfield) of the middle line, since 1≥1, we have :
Middle line: the luminance value of the pixels is encoded in 1 1 1 1 1 1 1 1 1
[0054] So finally we get for the whole picture:
First line : the luminance value of the white pixels is encoded in 1 0 1 0 1 0 1 1
1 1
First line : the luminance value of the black pixels is encoded in 0 0 0 0 0 0 0 000
Middle line : the luminance value of the pixels is encoded in 1 1 1 1 1 1 1 1 1 1
Last line: the luminance value of the white pixels is encoded in 1 0 1 0 1 0 1 1 1
1
Last line : the luminance value of the black pixels is encoded in 0 0 0 0 0 0 0 000
[0055] It is possible to check that the load effect is not visible with this encoding. On
the first and last lines, the white pixels have a sub-field weight sum of 227.
[0056] But the used sub-fields have a load of 50% on these lines, so the luminance is equivalent
to 227 · (1+ (1-½) × 0.25) = 255. And on the middle lines, the white pixels have a
sub-field weight sum of 255. But since the used sub-fields have a load of 100% on
these lines, the luminance is equal to the expected luminance (255). So no line load
will be visible.
[0057] A device for implementing the inventive method is shown at figure 4. This device
comprises a recursive encoding circuit 10 and a controller 11 for controlling said
circuit. The recursive encoding circuit 10 receives video coming from a degamma circuit
12 and outputs subfield code words SF[15:0] in a subfields memory 13. In the example
of figure 4, the circuit 10 applies also a dithering function to the data coming from
a degamma circuit 12.
[0058] A schematic diagram of the recursive encoding circuit 10 is given at figure 5. It
comprises n encoding blocks, one for each subfield (n being the number of subfield).
In the following description, each subfield is denoted SF
i, i being the number of the subfield. SF
n designates the subfield with the highest weight (also denoted most significant subfield)
and SF
1 designates the subfield with the lowest weight (also denoted least significant subfield).
Each encoding block receives from the controller 11 the switching value SV
i associated to the subfield SF
i that it encodes and a luminance value coming from the preceding encoding block or
the degamma circuit and outputs a subfield code bit B
i corresponding to the subfield SF
i and a remaining luminance value RV
i to be encoded by the following encoding blocks. The subfield code bit B
i is stored in the subfields memory 13.
[0059] More particularly, the encoding block associated to the subfield SF
n receives luminance value coming from the degamma circuit 12 and the switching value
of the subfield SF
n from the controller 11 and outputs a subfield code bit B
n and the remaining luminance value RV
n to be encoded by the following encoding blocks. The encoding block associated to
the subfield SF
i, i∈[2...n-1] receives the remaining luminance value RV
i and the associated switching value SV
i from the controller 11 and outputs the subfield code bit B
i and the remaining luminance value RV
i to be encoded by the following encoding blocks. The last encoding block associated
to the subfield SF
1 receives the remaining luminance value RV
2 and the switching value SV
1 and outputs the subfield code bit B
1.
[0060] A possible schematic diagram of the encoding block associated to the subfield SF
i, i∈[2...n], is shown at figure 6. It comprises :
- a line memory 20i for storing the luminance values for a line of pixels coming from the degamma circuit
12 for the subfield SFn and the remaining values RVi for the subfields SFi with i∈[2...n-1];
- a dithering block 21i for applying a dithering function to said luminance values or remaining values RVi,
- a comparison circuit 22i for comparing the dithered luminance values to the switching value SVi and outputting a bit Bi to "1" when said dithered luminance values are equal to or greater than SVi, the bit Bi being the subfield code bit that is stored in the subfields memory 13,
- a load evaluation circuit 23i for computing the load Loadi of the considered line for the subfield SFi,
- a luminance estimation circuit 24i for estimating the effective luminance Li of the subfield SFi for the considered line of pixels on the basis on the load values Loadi,
- a one bit line memory 25i for delaying the bit Bi of one line period, said delayed bit be denoted B'i,
- a circuit 26i for multiplying the bit B'i and the effective luminance Li;
- a circuit 27i for subtracting the output value of the multiplying circuit 26i from the luminance value stored in the line memory 20i, the result value being the remaining value to be encoded by the following encoding
blocks.
[0061] A possible schematic diagram of the encoding block associated to the subfield SF
1 is shown at figure 7. It comprises :
- a dithering block 211 for applying a dithering function to the remaining value RV2, and
- a comparison circuit 221 for comparing the dithered luminance values to the switching value SV1 and outputting a bit B1 to "1" when said dithered luminance values are equal to or greater than SV1, the bit B1 being the subfield code bit that is stored in the subfields memory 13.
[0062] All these circuit diagrams are only given as examples of implementation. The different
line memories of the device can be combined in one single memory. It can also comprise
no dithering circuit.
1. Method for encoding the luminance value of a pixel of a picture into a subfield code
word in a display device, wherein each bit of the subfield code word has a state "ON"
or "OFF" and generates light emission during an own period called subfield when its
state is "ON", the light emitted for a pixel by its subfield code word being representative
of the luminance value of said pixel and the total duration of the subfields associated
to the bits of the subfield code word forming the picture frame, each sub-field having
a weight proportional to its duration, wherein the bits of each subfield code word
are computed recursively one after the other from the bit associated to the subfield
having the most significant weight to the bit associated to the subfield having the
least significant weight,
characterized in that each bit of the subfield code word of a current pixel is computed by :
- for the bit associated to the subfield having the most significant weight, allocating
a state "ON" to said bit if the luminance value of the current pixel is equal to or
greater than a threshold value associated to said bit, and
- for each following bit of the subfield code word, computing a remaining luminance
value to be encoded by said bit and the following bits of the subfield code word and
allocating a state "ON" to said bit if said remaining luminance value is equal to
or greater than a threshold value associated to said bit.
2. Method according to claim 1, characterized in that the threshold value (SVi) associated to a bit is equal to the sum of the weights
of the subfields having a lower weight than the weight of the subfield corresponding
to said bit plus one.
3. Method according to claim 1 or 2, characterized in that the remaining luminance value to be encoded by a bit for a current pixel is computed
as a function of the state of the pixels of the line of the display device to which
the current pixel belongs.
4. Method according to claim 3,
characterized in that the remaining luminance value to be encoded by a current bit of the subfield code
word of a current pixel is computed by :
- calculating, for the subfield, called preceding subfield, computed just before the
subfield associated to said current bit, the number of pixels having the bit associated
to said preceding subfield in a "ON" state in the line of pixels to which the current
pixel belongs;
- estimating a subfield luminance value for said subfield on the basis of said number
of pixels, and
- subtracting said subfield luminance value from the luminance value to be encoded
by the bit associated to said preceding subfield and the following bits of the subfield
code word.
5. Device for encoding the luminance value of a pixel of a picture into a subfield code
word in a display device, wherein each bit of the subfield code word has a state "ON"
or "OFF" and generates light emission during an own period called subfield when its
state is "ON", the light emitted for a pixel by its subfield code word being representative
of the luminance value of said pixel and the total duration of the subfields associated
to the bits of the subfield code word forming the picture frame, each sub-field having
a weight proportional to its duration, wherein the bits of each subfield code word
are computed recursively one after the other from the bit associated to the subfield
having the most significant weight to the bit associated to the subfield having the
least significant weight,
characterized in that, for computing each bit of the subfield code word of a current pixel, it comprises:
- a comparator circuit (22i) for allocating a state "ON" to the bit associated to the subfield having the most
significant weight if the luminance value of the current pixel is equal to or greater
than a threshold value associated to said bit, and for allocating a state "ON" to
said bit if the remaining luminance value to be encoded by said bit and the following
bits of the subfield code word is equal to or greater than a threshold value associated
to said bit, and
- a calculation circuit (23i,24i,25i,26i,27i) for calculating, for each bit of the subfield code word following the bit associated
to the most significant weight subfield, the remaining luminance value to be encoded
by said bit and the following bits of the subfield code word.
6. Device according to claim 5, characterized in that it further comprises a controller (11) for outputting the threshold values associated
to the different bits of the subfield code word.
7. Device according to claim 5 or 6, wherein the threshold value (SVi) associated to
a bit is equal to the sum of the weights of the subfields having a lower weight than
the weight of the subfield corresponding to said bit plus one.
8. Device according to any one of claims 5 to 7, wherein said calculation circuit (23i,24i,25i,26i,27i) calculates the remaining luminance value to be encoded by a bit for a current pixel
as a function of the state of the pixels of the line of the display device to which
the current pixel belongs.
9. Device according to claim 8, wherein the calculation circuit, for calculating the
remaining luminance value to be encoded by a current bit of the subfield code word
of a current pixel, comprises :
- means (23i) for calculating, for the subfield, called preceding subfield, computed just before
the subfield associated to said current bit, the number of pixels having the bit associated
to said preceding subfield in a "ON" state in the line of pixels to which the current
pixel belongs;
- means (24i) for estimating a subfield luminance value for said subfield on the basis of said
number of pixels, and
- means (27i) for subtracting said subfield luminance value from the luminance value to be encoded
by the bit associated to said preceding subfield and the following bits of the subfield
code word.
10. Display device wherein it comprises an encoding device as claimed in any one of claims
5 to 9.