(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY(PCT)
`
`(19) World Intellectual Property Organization
`International Bureau
`
`(43) International Publication Date
`4 December 2003 (04.12.2003)
`
` (10) International Publication Number
`
`WO 03/100756 A2
`
`(51) International Patent Classification’:
`
`G09G 3/20
`
`Designated States (national): AE, AG, AL, AM, AT, AU,
`AZ, BA, BB, BG, BR, BY, BZ, CA, CH, CN, CO, CR, CU,
`
`(21) International Application Number:=PCT/IB03/01871 CZ, DE, DK, DM, DZ, EC, FR, BS, FI, GR, GD, GE, GH,
`GM,HR, HU,ID,IL, IN, IS, JP, KE, KG, KP, KR, KZ, LC,
`(22) International Filing Date:=29 April 2003 (29.04.2003)
`LK, LR, LS, LT, LU, LV, MA, MD, MG, MK, MN, Mw,
`MX, MZ, NI, NO, NZ, OM, PH, PL, PT, RO, RU, SC, SD,
`SE, SG, SK, SL, TJ, TM, TN, TR, TT, TZ, UA, UG, US,
`UZ, VC, VN, YU, ZA, ZM, ZW.
`
`(25) Filing Language:
`
`(26) Publication Language:
`
`English
`
`English
`
`(30) Priority Data:
`02077065.7
`
`27 May 2002 (27.05.2002)
`
`EP
`
`(71) Applicant (for all designated States except US): KONIN-
`KLIUKE PHILIPS ELECTRONICS N.V.
`[NL/NL];
`Groenewoudseweg 1, NI.-5621 BA Bindhoven (NT.).
`
`(72)
`(75)
`
`Inventors; and
`Inventors/Applicants (for US only): HEKSTRA, Ger-
`ben, J. [NL/NL]; c/o Prof. Holstlaan 6, NL-5656 AA
`Lindhoven (NL). KLOMPENHOUWER, Michiel, A.
`[NL/NL]; c/o Prof. Holstlaan 6, NL-5656 AA Eindhoven
`(NL).
`
`(74) Agent: RAAP, Adriaan, Y.; Internationaal Octrooibureau
`B.V., Prof. Holstlaan 6, NL-5656 AA Eindhoven (NL).
`
`(84) Designated States (regional): ARIPO patent (GH, GM,
`KE, LS, MW, MZ, SD, SL, SZ, TZ, UG, ZM, ZW),
`Kurasian patent (AM, AZ, BY, KG, KZ, MD, RU, ‘IJ, TM),
`European patent (AT, BE, BG, CH, CY, CZ, DE, DK, FE,
`ES, FI, FR, GB, GR, HU,IE, IT, LU, MC, NL, PT, RO,
`SE, SI, SK, TR), OAPI patent (BF, BJ, CF, CG, CI, CM,
`GA, GN, GQ, GW, ML, MR, NE, SN, TD, TG).
`
`Published:
`
`without international search report and to be republished
`upon receipt ofthat report
`
`For two-letter codes and other abbreviations, refer to the "Guid-
`ance Notes on Codes andAbbreviations” appearing at the begin-
`ning ofeach regular issue ofthe PCT Gazette.
`
`(54) Title: PLXEL FAULT MASKING
`
`
`
`WO03/100756A2
`
`(57) Abstract: A method for masking faulty sub-pixcls in a dis-
`play having a plurality of pixels formed of a numberof sub-
`pixels, wherein at least one pixel in said display is faulty and
`comprises at least one sub-pixel having a defect. The method
`comprises obtaining (S2) a set (15) of sub-pixel values (2, 3, 4)
`for generating desired perceptive characteristics for said pixel
`and determining (S3) a modified set (16) of sub-pixel values
`(2’, 3’, 4’) for generating modified perceptive characteristics for
`said pixcl. This modified sct of sub-pixel valucs is based on in-
`formation (14)regarding the sub-pixel defect so as to be imple-
`mentablein the display, and has values chosen to reduce an error
`perceived by a user. The modified values are then implemented
`(S4) in the display. The display is preferably of the kind where
`each pixel comprises a set of primary sub-pixels each emitting
`a primary colorandat least one additional, redundant sub-pixel
`for emitting an additional color, such as a RGBW display.
`
`

`

`WO 03/100756
`
`Pixel fault masking
`
`PCT/1IB03/01871
`
`The present invention relates to pixel fault masking in a display having a
`
`plurality of pixels formed of a number of sub-pixels. Aspects of the invention include a
`
`method, a control unit, and a display device.
`
`In conventional display systems, a number of sub-pixels, normally three for
`
`the red green and blue (RGB) primaries, make up a pixel. Mixing appropriate levels of each
`
`of the primaries makes up the desired color and intensity of a pixel. Recently, displays are
`
`emerging that make use of an additional, redundant sub-pixel in addition to the primary
`
`colors, such as a white sub-pixel (RGBW). The redundant sub-pixel can be used for
`
`enhancing the luminanceof the display, preferably without altering the chrominanceat all.
`
`10
`
`An example ofthis is described in WO 0137249, hereby incorporated by reference.
`
`When manufacturing displays such as liquid crystal displays, an important
`
`factor for determining the unit costis the yield, i.e., the number of defect displays produced
`
`for every functioning display. A display is defect if it contains faulty pixels,i.e., pixels that
`
`for some reason will not function appropriately, typically resulting from a defect sub-pixel.
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`15
`
`Normally, a certain numberoffaulty pixels can be accepted for a specific class
`
`of displays, and displays having a numberoffaulty pixels exceeding this numberare
`
`scrapped. However, even a single faulty sub-pixel can be a sourceofirritation, especially
`
`once it is spotted.
`
`To eliminate the occurrence offaulty pixels is very expensive,if at all
`
`20
`
`possible. Further, the difficulty of producing a perfect display is related to the number of
`
`pixels and the size of the display, and the problem with faulty pixels is therefore likely to
`
`increase as resolution and panel size increase.
`
`Therefore, it would be desirable to mask the effect of faulty pixels, hence
`
`reducing the risk of spotting them. This would also permit increasing the numberof accepted
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`25
`
`faulty pixels per display, and thereby reduce the number of scrapped displays. This increases
`
`the yield, and is beneficial in many aspects: more displays can be sold, less waste material is
`
`generated in the process, and the production cost per display is reduced.
`
`In camera systems, fault masking already exists, and has been implemented in
`commercially available chips. According to this technique, the surrounding of a defect sub-
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`

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`WO 03/100756
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`5
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`PCT/1IB03/01871
`
`pixel is used to compute its expected value, thus masking the fault. This techniqueis,
`
`however, not applicable to displays.
`
`Another approachis error diffusion, i.e., distributing the error in
`
`approximating a certain value over a set of neighbouring pixels. This is by itself not a
`
`suitable technique for fault masking, since the error to be distributed typically is too large,
`
`e.g., a sub-pixel stuck at level zero. In fact, the visibility of the fault appears increase due to
`
`the sharpening effect that occurs in the diffusion. Thus, so far, there is no available technique
`
`for masking of defect sub-pixels.
`
`10
`
`15
`
`An object of the present invention is to provide adequate masking of faulty
`
`pixels in a display.
`
`Another object is to provide a satisfying quality of the displayed image
`
`characteristics as perceived by a user.
`
`According to a first aspect of the present invention, these objects are achieved
`
`with a method according to the preamble of claim 1, further comprising obtaining, for each
`faulty pixel, information of said defect sub-pixel, obtaining a set of sub-pixel values for
`generating desired perceptivecharacteristics for said pixel, determining a modified set of
`
`sub-pixel values for generating modified perceptive characteristics for said pixel, said
`
`20
`
`modified set of sub-pixel values being based on said information so as to be implementable
`
`in the display, said modified set (16) of sub-pixel values being such as to reduce an error
`
`perceived by a userresulting from a difference between said desired perceptive
`
`characteristics and said modified perceptive characteristics, and implementing said modified
`
`set of sub-pixel values in the display.
`
`25
`
`By taking the sub-pixel defect into consideration, the set of sub-pixel valuesis
`
`thus recalculated into a modified set, in order to minimize the error perceived by the user.
`
`Typical perceived characteristics include luminance (brightness) and chrominance (color).
`
`It is important to realize that this does not necessarily mean that the error in
`
`terms of absolute sub-pixel values is minimized. Minimizing the error in terms of absolute
`
`30
`
`sub-pixel values would minimize the chrominance crror, without taking luminance into
`
`consideration. In order to obtain a smaller perceived error, an adjustment might therefore be
`
`madeto better maintain desired luminance.
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`A requirement for effective fault masking is that the intended sub-pixel values
`
`can be adjusted both up and down to result in the actual sub-pixel values. In a case whereall
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`WO 03/100756
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`3
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`PCT/1IB03/01871
`
`sub-pixels are used in normal operation, some remaining capacity of these sub-pixels is
`
`preferably reserved, in order to enable optimal fault masking according to the invention.
`
`By this method, sub-pixel defects becomepractically invisible to the human
`
`visual system, and will hence no longerbe a sourceof irritation. By allowing more defects in
`
`a display, the yield can be improved drastically, with the advantages mentioned above.
`
`Considering that the numberoffaulty pixels is low compared to the total
`
`numberof pixels, the method will be low-cost, even in a case when the implemented method
`
`is computationally complex. If the fault masking is kept relatively simple, then the overhead,
`
`compared to normalpixel processing, is extremely low.
`
`10
`
`The information about faulty pixels can be obtained from a predefinedlist
`
`storing location and details of each faulty pixel. It may also be advantageous, as an
`
`alternative or in combination with thelist, to automatically detect sub-pixel defects. This
`
`eliminates the need for storing information about defects at the time of production, and also
`
`makes the fault masking adaptive to the occurrence of new faults. This in turn makesit
`
`15
`
`possible to enhancethe useful lifetime of displays for which defects appear overtime (¢.g.,
`PLED,but also LCD).
`
`The set of sub-pixel values can be obtained from a display memory, and the
`
`modified set of sub-pixel values can be returned to the memory. This offers an efficient way
`
`to interface with a conventional display driver.
`
`20
`
`The determination can include solving an approximation problem of
`
`constrained least square (CLS) type.
`
`The display is preferably of the kind where each pixel comprises a set of
`
`primary sub-pixels each emitting a primary color and at least one additional, redundant sub-
`
`pixel for emitting an additional color. The primary colors are chosen so as to enable
`
`25
`
`generation of any given color by combining them in adequate ratios. The most conventional
`
`combination of primaries is of course red, green and blue (RGB). Theadditional color can be
`
`chosen so as to include contributions from each of the primary colors. The example
`
`mentioned above was white (RGBW), but also other colors, such as cyan, magenta, or yellow
`
`can be useful. With more than three sub-pixels,it is also possible with an altogether different
`
`30
`
`set of colors, making division into primarics and non-primaries superfluous.
`
`The redundant sub-pixel can be shared by several pixels, for example by two
`
`pixels. This reduces the total numberofadditional sub-pixels, making the display less
`
`expensive.
`
`

`

`WO 03/100756
`
`A
`
`PCT/1IB03/01871
`
`The set of sub-pixel values and the modified set of sub-pixel values can each
`
`comprise values for sub-pixels adjacent to said defect sub-pixel. The sets are preferably
`
`related to the sub-pixels of a specific pixel, but may well be related to other neighborhoods of
`
`sub-pixels, if this is found advantageous.
`
`The original set of sub-pixel preferably comprises values for the primary color
`
`sub-pixels of a pixel. By only comprising these values, in a redundant sub-pixel type display,
`
`a certain “headroom”is guaranteed by the additional intensity that can be provided by
`
`activating the additional, redundant color sub-pixel. The modified set of sub-pixel values then
`
`also comprises values for any such redundant sub-pixel of the pixel.
`
`10
`
`15
`
`Note that there is a trade-off between maximum luminance (no headroom
`
`reserved) and maximum fault masking performance (headroom available). This trade off can
`
`be very useful used in situation where produced displays are graded according to the number
`
`of faults and to their application (monitor, TV, video,still images, etc.) and market
`(professional or consumer). In expensive, essentially fault free displays, no headroom needs
`to be reserved, while in less expensive, faulty displays, headroom should be reserved in order
`to allow for the fault masking according to the present invention.
`Grading of displays according to the numberof defects/headroom in the
`
`described way can also work for non-redundant displays (e.g., conventional RGB).
`
`The method can further comprise compensating faulty pixels by error
`
`20
`
`diffusion. While inefficient for large errors such as sub-pixel stuck at zero, error diffusion
`
`may be advantageous for small errors remaining after fault masking according to the above
`
`method. This may be particularly advantageous in a case of limited headroom as described
`
`above.
`
`The method according to the invention is preferably implemented in a display
`
`25
`
`in which sub-pixels can be addressed accurately (matrix displays). Examples of such displays
`
`are active matrix LCD and PLEDs.
`
`According to a second aspect of the present invention, the above objects are
`
`achieved with a control unit for a display having a plurality of pixels formed of a number of
`
`sub-pixels, the control unit comprising means for obtaining, for each faulty pixel, information
`
`30
`
`of said defect sub-pixel, means for obtaining a set of sub-pixel values for generating desired
`
`perceptive characteristics for the faulty pixel, means for determining a modified set of sub-
`
`pixel values for generating actual perceptive characteristics for said faulty pixel, said
`
`modified set of sub-pixel values being based on information regarding said sub-pixel defect
`
`so as to be implementable in the display, said modified set of sub-pixel values being such as
`
`

`

`WO 03/100756
`
`5
`
`PCT/1IB03/01871
`
`to reduce an error perceived by a user resulting from a difference between said desired
`
`perceptive characteristics and said actual visual characteristics being such as to reduce an
`
`error perceived by a user, and means for implementing said modified set of sub-pixel values
`in the display.
`
`The control unit can further comprise a memoryfor storing information about
`
`sub-pixel defects. This provides the determining means with necessary information for
`
`determining the modified set of values.
`
`Alternatively, or in combination with the memory, the control unit comprises
`
`means for automatically detecting sub-pixel defects. With a higher yield mentioned above,it
`
`10
`
`becomes feasible to assemble the control unit on the panel before the (currently manual)
`
`panel test. Combined with active detection of defects in these drivers, a self-test can be
`
`performed, enabling more automation in testing, repair, and grading.
`
`Thecontrol unit can of course be implemented in a display device, and such a
`
`display is considered a third aspect of the present invention.
`
`15
`
`These and other aspects will be better understood by the following description
`of a currently preferred embodiment, with reference to the appended drawings.-
`
`Fig 1 illustrates alternative ways to generate the same perceptive
`
`20
`
`characteristics from a pixel having redundant sub-pixels.
`
`Fig 2 illustrates masking of a defect sub-pixel according to an embodiment of
`
`the invention.
`
`Fig 3 is a schematic block diagram of a control unit according to an
`
`embodiment of the invention communicating with a display driver.
`
`25
`
`30
`
`invention.
`
`invention.
`
`invention.
`
`Fig 4 is a flow chart of a method accordingto a first embodiment of the
`
`Fig 5 is a flow chart of a method according to a second embodimentofthe
`
`Fig 6a-6b illustrate remaining errors after masking.
`
`Fig 7 is a flow chart of a method according to a third embodiment of the
`
`Fig 8 illustrates several pixels sharing the same redundant sub-pixel.
`
`Fig 9a-9b illustrate several alternative pixel neighborhoods.
`
`

`

`WO 03/100756
`
`PCT/1IB03/01871
`
`The following description is related to a display having several pixels, each
`
`madeup of a numberofindividually addressable sub-pixels. Examples of such displays are
`
`active matrix liquid crystal displays and PLED displays.
`
`Further, a preferred embodimentrelates to a display in which the sub-pixels of
`
`a pixel are redundant, i.e. can emit at least one additional color apart from the required
`
`primary colors. As mentioned above, an RGBWpixelstructure is an example of such a set of
`
`redundant sub-pixels, having a white sub-pixel in addition to the primary red, green and blue
`
`sub-pixels.
`
`10
`
`15
`
`With redundant sub-pixels there are multiple ways to drive the individual sub-
`
`pixels to achieve the same chrominance and luminance. An example ofthis is shown
`
`graphically in fig 1, where the same color and intensity is achieved on both sides in this
`
`figure. On the left hand side is indicated a set 1 of sub pixel values red 2, green 3, blue 4 and
`
`white 5. The white sub-pixel 5 is set to zero. On the right handside isillustrated a set 6 of
`different values red 2’, green 3’, blue 4’ and white 5°, In this case, the white level 5’ is taken
`as the minimum of the RGBlevels 2, 3, 4, being the green level 3. This level is then
`
`subtracted from all RGBlevels 2, 3, 4, as shown onthe right, with the result that the green
`
`sub-pixel level 3” is set to zero.
`
`With this approach, both sets 1, 6 of pixel values result in the same color and
`
`20
`
`intensity. Note that, in this example, if the green sub-pixel would have been defect (stuck-at-
`
`off), it could have been compensated without introducing any error.
`
`The principles of the invention are illustrated with reference to fig 2, where
`
`identical objects have been given the same referencesasin fig 1. In this case, the pixelis
`
`defect, and moreprecisely the sub-pixel for the blue primary is stuck-at-off. Therefore, the
`
`25
`
`desired set of sub-pixel values 2, 3, 4, indicated on the left hand side of fig 2, can not be
`
`implemented by the display panel. According to the present invention, the intensity values for
`
`the remaining sub-pixels (in this case red, green and white) are modified to compensate for
`
`the absent blue contribution, so that the perceived error is minimized, or at least reduced.
`
`As an example, such error minimization can be include that the overall
`
`30
`
`luminance of the error is close to zero, while the chrominance ofthe error is as close as
`
`possible to white. There is a preference to approximate the luminancebetter than the
`
`chrominance, since the human visual system (HVS) is known to be more sensitive to
`
`luminance differences, and to have a lowerresolution for chrominance.
`
`

`

`WO 03/100756
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`7
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`PCT/1IB03/01871
`
`Returning to fig 2, the modified sub-pixel values 2’, 3’, 4’, 5° are shown on the
`
`right hand side, together with the error 7, 8, 9. As can be seen, the white sub-pixel 5’ has
`
`been activated, and manages to compensate for the majority of the lacking blue contribution.
`
`At the sametime, the white sub-pixel 5’ contributes in the red and green areas, and these sub-
`
`pixel values have to be reduced. As the desired blue value 3 exceeds the desired green value
`
`2, there will be an error in the green color, or in the blue color, or in both. In the illustrated
`
`case, an error is introducedin the green color 8, and a small error 9 also remains in the blue
`
`color.
`
`If the absolute error in sub-pixel values were to be minimized, the red color
`
`10
`
`could be modified so as to avoid error in the red. However, due to the fact that it is the
`
`perceived characteristics, resulting from the sub-pixel values, that are minimized, an error 8
`
`is introduced also in the red colorin order to minimize the luminanceerror.
`
`The general problem can be described in the following, mathematical, way:
`
`Let #1 be a vectorofthe desired pixel value, defined in an n-dimensional
`
`15
`
`linear space, such as the CIE1931 XYZ color space or the Lu ‘y* luminance/chrominance
`
`space. Let p be the vector of the values (normalized, and display gamma independent) for the
`
`k sub-pixels, and let Af be an nxk matrix to transform a point in the k-dimensional sub-
`pixel space to the n-dimensional perceptive space. Thej" column in M isthe location ofthe
`7” sub-pixel in the perceptive space.
`The approximation problem is expressed in matrix form as:
`
`20
`
`m=M-pré,
`
`where @ is the error in approximation, defined in the perceptive space. The equation is
`
`written out in full as:
`
`M,, My,
`mM,
`My | M, My
`
`“* M,,
`M,,
`
`P|
`&
`| Pe 4 &,
`
`m
`
`n Mi Mn2
`
`wae
`
`Mnk
`
`.
`Px
`
`&
`n
`
`25
`
`Anysolution to the approximation problem mustsatisfy the constraints:
`
`0<p, <1,
`
`ieG
`
`DP, = Si
`
`ieF,
`
`where G and F are the sets of indices of the functioning (G) and faulty (F) sub-pixels within a
`
`given pixel respectively. Each of the faulty primaries can be stuck at a given,fixed level /,.
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`

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`WO 03/100756
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`3
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`PCT/1IB03/01871
`
`Our objective is to minimize the approximation error ¢ , for which we propose to minimize
`
`the L2-norm of ¢ , which can be expressed:
`min >) (¢,)’.
`
`The approximation error can be weighed, so to minimize > (w,é,)’ . This enables
`
`prioritizing perceptive measures, such as luminance over chrominance. The weighingis
`
`achieved byleft-multiplying all terms in the equation with the weighting matrix W, given by:
`
`Ww,
`
`We-=
`
`Wy
`
`Ww
`
`The weighted problem is then given by:
`
`W-m=W-M-pt+W-é
`
`10
`
`The weights w,of the approximation error can be made adaptive to the image
`
`content around the defect. For example, the surroundingsof the faulty pixel can be analyzed
`
`to detect smooth or textured luminance, smooth or textured chrominance, or edges. Based on
`
`this, the weights can be adapted to minimize the perceived error, given the surroundings.
`
`The entire problem as stated aboveis a constrained least squares (CLS)
`
`15
`
`problem, which can readily be solved by known techniques, using for example Optimization
`
`Toolbox for use with Matlab, distributed by MathWorks. The complexity of solving the
`
`problem is relatively low, since the dimensionsof the matrix M are quite small (typically k=4
`
`and n=2). Moreover, since the matrix J/is known, and the sameforall pixels, dedicated and
`
`faster solvers can be developed.
`
`20
`
`Typically, there are tens of defects in a display having millions of sub-pixels.
`
`Asthe above problem only needsto be solved for the defect pixels, there is relatively much
`
`time available to solve the approximation problem. This makesit feasible to use general
`
`purpose, low power and low-complexity hardware to solve the approximation problem.
`
`The proposed scheme has been simulated and has been found to work
`
`25
`
`exceptionally well. These tests were performed for a numberofstill images, with an
`
`emulated RGBW display with 500 defect sub-pixels.
`
`A schematic illustration of a control unit 12 implementing a fault masking
`
`process according to the invention implemented together with a display system 13 is shown
`
`in the flow chart in fig 3. The control unit 12 comprises a memory 11 storingalist of
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`

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`WO 03/100756
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`9
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`PCT/1IB03/01871
`
`information about faulty pixels. It is here assumed that any defects of the display in question
`
`are specified, both regarding position and type. Typically this could be achieved byletting
`
`the list 11 include the location ofthe faulty pixels, the faulty sub-pixels within that pixel, and
`
`the details of each faulty sub-pixel. The details of the sub-pixel defect can consist of an
`
`intensity level at which the sub-pixel is stuck. Typically the level is zero, i.e., the sub-pixel
`
`does not emit any light (is black). The list of faults can preferably be generated beforehand,
`for example during production ofthe display. However, it would be advantageousif the
`display automatically could detect which sub-pixels are defect and what the characteristic of
`the defect is. This would ensure an updated and correctlist 11 at all times. For this purpose,
`the control unit can be provided with a module 19 for automatically detecting defects in the
`
`10
`
`sub-pixels of the display. Such a module 19 can be connected to the memory 11, and can be
`
`arranged to update thelist if needed.
`
`Further, an input/output module 17 is arranged to communicate with the
`
`display system 13. The display system in fig 3 is only represented by a display memory 13,
`
`15
`
`while other components are left out for the sake of clarity. In contact with the memory 11 and
`
`the I/O-module is a module 18 for solving the approximation problem described above.
`
`Such a control unit 12 for performing the steps in the flow charts of figs 4, 5
`and 7 can be implemented by any combination of software and/or hardware components, and
`
`be incorporated in the circuitry of a conventional display driver.
`
`20
`
`A flow chart of the process performed by the control unit 12 in fig 3 is
`
`illustrated in fig 4.
`
`In step S1, program control obtains, from the list 11 of defect pixels, the
`
`location and details 14 of a defect, i.e., the faulty sub-pixel(s) and the stuck-at level(s). Then,
`
`in step S2, a set of desired sub-pixel values 15 is obtained from display memory 13, e.g.,
`from a frame memory, pixel stream or likewise. In step $3, the set of desired sub-pixel values
`
`25
`
`15 and the sub-pixel defect 14 are used as inputs to an optimization, which delivers an
`approximation in the form of a modified set of sub-pixel values 16. As described above,this
`
`modified set may include additional sub-pixel values, e.g., for a white sub-pixel. In step S4
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`the modified set of values 16 is then returned to the display memory 13, or communicated
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`directly to the display driver (not shown). The above steps S1-S4 are repeated forall pixel
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`defects in the list 11 and for each picture frame, by a program loop effected in step S5.
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`The fault masking can be run out of synch with the regular pixel processing, or
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`be part of the same processing flow.
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`An alternative to the flow chart in fig 4 is given in fig 5. In this case, after the —
`desired sub pixel values have been obtained in step S2, the surroundingsofthe defect pixel
`are analyzed in step S8. This can be accomplished by obtaining the pixel values for adjacent
`pixels from the display memory 13. Then, in step S9, weights are computed, and then used as
`input to the optimization in step $3. Such weights can be used to favor selected perceptive
`characteristics. The weights can be adaptive, in order to enable adjustment to changing image
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`characteristics.
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`Fig 6a-b showa typical distribution of errors in both the image with defects
`(fig 6a), and the image with fault masking (fig 6b). Clearly the large errors are eliminated,
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`and only errors with smaller values remain, which makes the approximationerror eligible for
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`error diffusion.
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`The schemefor this is known, and consists of adapting the intensity of pixels
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`adjacent to a faulty pixel, thereby compensating the error. All known methods perform some
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`form of a 1-D scanning over the image, resulting in a directed error diffusion (to the bottom-
`right). If error diffusion is implemented after fault masking according to the described
`method, the error can be distributed equally in all possible directions.
`
`Therefore, a novel ring diffusion scheme is proposed. Any residual erroris
`first distributed over the immediate surroundingin all dimensions(a first ring ofpixels).
`Preference can be given to correct overall luminanceerrors, possibly at the cost of
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`introducing additional chrominanceerrors.If thereis still a luminanceerrorafterthis, pixels
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`forming the next “ring” can be used to correct this error, and so on within reasonable limits.
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`By giving preferenceto first correcting the luminance, and then the chrominanceerror,
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`minimalvisibility of the defect is expected.
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`A flow chart of the method includingthe error diffusion is illustratedin fig 7,
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`with error diffusion performed in step $12, after the modified values have been calculated in
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`step S3.
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`Notethatit is not necessary that each pixel has its own individual redundant
`
`sub-pixel. To limit the redundancy, a redundant sub-pixel 21 can be shared over a group of
`surrounding pixels, as illustrated in fig 8 for the case of one white sub-pixel shared by two
`pixels 22 and 23. The shared redundant sub-pixel 21 is then used by the control unit 12 to
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`mask a defect in any one ofthese pixels 22, 23.
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`Further, the optimization need not be restricted to the sub-pixels within the
`tight boundariesof a single pixel. Any set of close neighboring sub-pixels could suffice, as
`illustrated in fig 9a-b. In fig 9a, instead of modifying the sub-pixel values for the pixel 25
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`comprising the defect sub-pixel 26, a group of sub-pixels 27 is defined comprising one sub-
`pixel from each of four neighboring pixels 25, 28, 29, 30. In fig 9b, the selected group of sub-
`pixels 31 comprises nine sub-pixels, including two white 32, 33. It can even be preferred to
`test several different neighborhoods (groups of sub-pixels) in order to determine which one
`provides the best masking. For example, as mentioned above, a sub-pixel stuck at zero can be
`completely corrected if the defect sub-pixel has the lowest value in the group (see fig 1). It
`can therefore be useful to investigate whether a group of sub-pixels can be defined wherein
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`the defect sub-pixel has the lowest value.
`Theoretically, the invention is applicable also to displays with non-redundant
`sub-pixels (standard RGB). Trial experiments have shown improvement, albeit not as much
`as for redundant sub-pixels. The performance could be improved by including more
`surrounding sub-pixels in the optimization, as mentioned above.
`In parts of the above description, only one faulty sub-pixel has been assumed.
`In order to achieve satisfying fault masking, it can then be preferred to have multiple
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`redundant sub-pixels.
`A numberof additional variations to the described embodiments are possible
`
`within the scope of the appended claims. For example, other computational schemes than the
`proposed CLS optimization are possible, as long as they try to minimize the perceived error
`in luminance and chrominance. The optimization problem can also be extended to include the
`distance to surrounding sub-pixels. This could be used to favor sub-pixels which are spatially
`close to the defect, and so to minimize any perceived spatial errors. Such an extension could
`be implemented by addinga single vector ofthe distances d, as an extra row in the matrix M.
`
`Also, in the above description, the distance between pixel defects has been
`assumedlarge enough that only independentdefects have to be considered. However,thisis
`not a restriction of the invention, which could be adapted to handle dependent defects.
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`CLAIMS:
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`PCT/1IB03/01871
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`Method for masking faulty sub-pixels in a display having a plurality ofpixels
`1.
`formed of a numberof sub-pixels, wherein at least one pixel in said display is faulty and
`comprises at least one sub-pixel having a defect, said method being characterized by
`obtaining, for each faulty pixel, information of said defect sub-pixel,
`obtaining a set of sub-pixel values for generating desired perceptive
`
`characteristics for said pixel,
`
`determining a modified set of sub-pixel values for generating modified
`perceptive characteristics for said pixel, said modified set of sub-pixel values being based on
`
`said information so as to be implementablein the display, said modified set of sub-pixel
`values being chosen to reduce an error perceived by a user resulting from a difference
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`10
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`between said desired perceptive characteristics and said modified perceptive characteristics,
`
`and
`
`implementing said modified set of sub-pixel values in the display.
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`2.
`
`Method according to claim 1, wherein said information is obtained from a
`
`predefinedlist storing location and details of each faulty pixel.
`
`3.
`
`Methodaccording to claim 1 or 2, further comprising automatically detecting
`
`sub-pixel defects.
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`4,
`
`Method according to claim 1 — 3, wherein said set of sub-pixel valuesis
`
`obtained from a display memory, and said modified set of sub-pixel values is returned to said
`
`memory.
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`5.
`Method according to any one of the preceding claims, wherein said
`determination includes solving an approximation problem of constrained least square type.
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`6.
`
`Methodaccording to any one of the preceding claims, wherein each pixel
`
`comprises a set of primary sub-pixels each emitting a primary color and at least one
`
`additional sub-pixel each emitting an additional color.
`
`7.
`
`Method according to claim 6, wherein said additional sub-pixel is shared by
`
`several pixels.
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`8.
`
`Method according to any one of the preceding claims, wherein said set of sub-
`
`pixel values and said modified set of sub-pixel values each comprise values for sub-pixels
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`adjacent to said defect sub-pixel.
`
`9.
`
`Method acc