Technical field of the invention
[0001] The present invention relates to a system and a method for avoiding misinterpretation
of images due to defective pixels present in matrix addressed electronic displays
during display time such as fixed format displays e.g. plasma displays, field emission
displays, liquid crystal displays, EL-displays, LED and OLED displays, especially
flat panel displays as used in projection or direct viewing concepts.
[0002] By a defective pixel is meant a pixel that always shows the same luminance (for instance,
but not limited to, always black or always full white) and/or colour behaviour independent
of the luminance stimulus applied to it, or shows a luminance or colour behaviour
that shows a severe distortion compared to non-defective pixels of the display. For
example a pixel that reacts to an applied drive signal, but that has a luminance behaviour
that is very different from the luminance behaviour of neighbouring pixels, for instance
significantly more dark or bright than surrounding pixels, can be considered a defect
pixel. Also pixels that are located on a wrong place can be considered as defective
pixels.
[0003] The present invention applies to emissive, transmissive, reflective and trans-reflective
display technologies fulfilling the feature that each pixel is individually addressable.
Background of the invention
[0004] At present, most matrix based display technologies are in its technological infancy
compared to long established electronic image forming technologies such as Cathode
Ray Tubes. As a result, many domains of image quality deficiency still exist and cause
problems for the acceptance of these technologies in certain applications.
[0005] Matrix based or matrix addressed displays are composed of individual image forming
elements, called pixels (Picture Elements), that can be driven (or addressed) individually
by proper driving electronics. The driving signals can switch a pixel to a first state,
the on-state (luminance emitted, transmitted or reflected), to a second state, the
off-state (no luminance emitted, transmitted or reflected), see for example EP-117335
which describes an LCD. For some displays, one intermediate state between the first
and the second state is used - see EP-462619 which also describes an LCD. For still
other displays, one or more intermediate states between the first and the second state
(modulation of the amount of luminance emitted, transmitted or reflected) are used
- see EP-117335.
[0006] Matrix addressed displays are typically composed of many millions of pixels and very
often pixels exist that are stuck in a certain state (on, off or anything in between).
Where sub-pixel elements are individually controllable then one of the sub-pixel elements
may become stuck in a certain state. For example, a pixel structure may comprise three
sub-pixel elements for red, green and blue colours. If one of these sub-pixels becomes
stuck, then the pixel structure has a permanent colour shift. Mostly such problems
are due to a malfunction in the driving electronics of the individual pixel (for instance
a defect transistor). Other possible causes are problems with various production processes
involved in the manufacturing of the displays, and/or by the physical construction
of these displays, each of them being different dependent on the type of technology
of the electronic display under consideration. It is also possible that a pixel or
sub-pixel element is not really stuck in a state, but shows a luminance or colour
behaviour that is significantly different from the pixels or sub-pixels in its neighbourhood.
For instance but not limited to: a defective pixel shows a luminance behaviour that
differs more than 20% (at one or more video-levels) from the pixels in its neighbourhood,
or a defective pixel shows a dynamic range (maximum luminance/minimum luminance) that
differs more than 15% from the dynamic range of pixels in its neighbourhood, or a
defective pixel shows a colour shift greater than a certain value comparing to an
average or desired value for the display. Of course other rules are possible to determine
if a pixel or sub-pixel is defective or not (any condition that has a potential danger
for image misinterpretation can be expressed in a rule to determine whether a pixel
is a defective pixel). The exact reason for a defective pixel is not important for
the present invention.
[0007] Defective pixels are very visible for a user of the display. This not only can be
very disturbing for the user, but it can also result in wrong interpretation of the
image being displayed. For applications where image fidelity is required to be high,
such as for example in medical applications, this situation is unacceptable.
[0008] US-5504504 describes a method to make some classes of defective pixels less visible
by changing the luminance of pixels in the neighbourhood of the defective pixel. This,
however, distorts the luminance of the whole image, and is not generally accepted
in the medical world when viewing medical images.
[0009] In prior art devices, some classes of defective pixels are located and burnt down
so as to always appear black and thus less visible to a user. This solution, however,
has the disadvantage that, e.g. in medical images, a black pixel in a region of interest
for a radiologist might indicate (or hide) for example a pathological defect such
as tumour cells. Therefore, there is a danger in such a situation that a radiologist
will not be able to discriminate between the defective pixel and a feature of therapeutic
relevance.
Summary of the invention
[0010] It is an object of the present invention to provide a method and device for avoiding
misinterpretation of images due to the "defective pixels" described above. It is not
a necessary intention to correct the defect pixels itself.
[0011] The above objectives is accomplished by a method and device according to the present
invention.
[0012] The present invention provides a method for avoiding misinterpretation of an image
displayed on a matrix display due to defective pixels in the matrix display. This
may be very important e.g. in the medical world. The method comprises obtaining information
on the presence of the defective pixels in the display, and modulating the operation
of the display so as to indicate, emphasise or warn for the presence of said defective
pixels on the actual display having defect pixels, or adapting the image content of
the defective pixels or of pixels in the neighbourhood of the defective pixels so
as to indicate, emphasise or warn for the presence of said defective pixels in a copy
of the displayed image. Such copy may be a hard copy or an electronic copy.
[0013] The information on the presence of defective pixels in the display may be obtained
from data previously stored in a memory device.
[0014] A method according to an the present invention comprises, while displaying the image
on the matrix display, supplying information on defective pixels to a user, based
on the stored data.
[0015] A method according to an embodiment of the present invention comprises visually marking
the at least one defective pixel on the display in order to indicate, emphasise or
warn its presence. This may e.g. be done highlighting the defective pixel by changing
the drive signal of neighbouring pixels.
[0016] A method according to an embodiment of the present invention may furthermore comprise
shifting, rotating or flipping the displayed image so that defective pixels are not
located in a region of interest. The method may also comprise shifting, rotating or
flipping the displayed image so that a defective pixel is located in a flat image
area, i.e. in an area of the image where there are not a lot of image transitions.
[0017] A method according to an embodiment of the present invention may furthermore comprise,
before modulating the operation of the display, selection of a desired indication
method, i.e. for example making all pixels around a defective pixel white (luminance
high), so as to obtain a kind of circle around the defective pixel, or making the
pixels around the defective pixel blink (driving it with e.g. subsequent drive levels
corresponding to a first, e.g. black, output level, and to a second, e.g. white, output
level).
[0018] The information on the presence of defective pixels is obtained by means of an image
capturing device such as a flatbed scanner, a camera, a CCD or a photodiode.
[0019] The present invention also provides a device for avoiding misinterpretation of an
image displayed on a matrix display due to defective pixels in the matrix display.
The device comprises an information retrieval device for obtaining information on
the presence of the defective pixels in the display, and a modulating device for modulating
the operation of the display so as to indicate, emphasise or warn for the presence
of said defective pixels on the actual display having defect pixels, or for adapting
the image content of the defective pixels or of pixels in the neighbourhood of the
defective pixels so as to indicate, emphasise or warn for the presence of said defective
pixels in a copy of the displayed image.
[0020] The information retrieval device may comprise a memory device where defective pixel
information data is stored. The device for avoiding misinterpretation may comprise
an information supply device for supplying information on defective pixels to a user,
based on the stored data, while displaying the image on the matrix display.
[0021] The information retrieval device may furthermore comprise marking means for visually
marking the defective pixels on the display. The marking means may comprise a driving
means for changing the drive signal of pixels neighbouring the defective pixel.
[0022] The information retrieval device may furthermore comprise a shifting device for shifting,
rotating or flipping the displayed image so that defective pixels are not located
in a region of interest. It may also comprise a shifting device for shifting, rotating
or flipping the displayed image so that a defective pixel is located in a flat image
area, i.e. in an area of the image where there are not a lot of image transitions..
[0023] The information retrieval device may furthermore comprise selection means for selecting
a desired modulation method.
[0024] The information retrieval device may furthermore comprise an image capturing device
for capturing images from individual display pixels, such as a scanner, a camera,
a CCD or a photodiode.
[0025] The present invention also provides a control unit for use with a device for avoiding
misinterpretation of an image displayed on a matrix display, due to defective pixels
in the matrix display. The control unit is adapted for controlling the obtaining of
information on the presence and characteristics of the defect pixels in the display,
and for controlling modulation of the operation of the display so as to indicate,
emphasise or warn for the presence of said defective pixels on the actual display
having defect pixels, or adaptation of the image content of the defective pixels or
of pixels in the neighbourhood of the defective pixels so as to indicate, emphasise
or warn for the presence of said defective pixels in a copy of the displayed image.
[0026] The present invention also provides a computer program product for executing any
of the methods according to the present invention when executed on a computing device
associated with a system for avoiding misinterpretation of an image displayed on a
matrix display, due to defective pixels in the matrix display.
[0027] The present invention furthermore provides a machine readable data storage device
storing the computer program product according to the present invention.
[0028] The present invention also provides transmission of the computer program product
of the present invention over a local or wide area telecommunications network.
[0029] The present invention also provides an electronic data format that can be retrieved
from a hardcopy, softcopy or electronic medium and that allows:
- to identify a data set with respect to its applicability to the actual target display
having defect pixels, or in other words, the data set is for use only on that single
display device on which the defects are characterised by a method according to the
present invention,
- to indicate the position of the defect pixels on the actual display having defect
pixels, and
- to reconstruct or simulate the position of the defect pixels on copy of the image
rendered on a hardcopy or a softcopy on an electronic display device.
[0030] The present invention does not necessarily repair the defective pixels but rather
carries out corrective actions so as to avoid wrong image interpretation. This may
be done, according to an embodiment of the present invention, by warning the user
of the display that defective pixels are present in the display, by notifying the
location of defective pixels, for instance by generating a report or by marking them
visually on the display, on demand of the user or automatically. A quality system
according to the present invention is also able to take corrective actions to avoid
misinterpretation of the image, on the basis of the notified location of the defective
pixels. According to another embodiment, the misinterpretation may be avoided by shifting,
rotating or flipping the image so that no defective pixel is located in an area of
interest.
[0031] These and other characteristics, features and advantages of the present invention
will become apparent from the following detailed description, taken in conjunction
with the accompanying drawings, which illustrate, by way of example, the principles
of the invention. This description is given for the sake of example only, without
limiting the scope of the invention. The reference figures quoted below refer to the
attached drawings.
Brief description of the drawings
[0032]
Fig. 1 illustrates a grey scale matrix display having two defect pixels, all pixels
having an equal driving signal.
Fig. 2 illustrates a grey scale LCD based matrix display having two defect sub-pixels.
Fig. 3 illustrates a first embodiment of an image capturing device, the image capturing
device comprising a flatbed scanner.
Fig. 4 illustrates a second embodiment of an image capturing device, the image capturing
device comprising a CCD camera and a movement device.
Fig. 5 schematically illustrates an embodiment of an algorithm to identify matrix
display pixel locations.
Fig. 6 shows an example of a luminance response curve of a good and of a defective
pixel, the curves being constructed using sixteen characterisation points.
Fig. 7 illustrates defining classes of pixels to locate and characterise defect pixels.
[0033] In the different figures, the same reference figures refer to the same or analogous
elements.
Description of illustrative embodiments
[0034] The present invention will be described with respect to particular embodiments and
with reference to certain drawings but the invention is not limited thereto but only
by the claims. The drawings described are only schematic and are non-limiting. In
the drawings, the size of some of the elements may be exaggerated and not drawn on
scale for illustrative purposes. Where the term "comprising" is used in the present
description and claims, it does not exclude other elements or steps.
[0035] The invention will be described with reference to flat panel displays but is not
limited thereto. It is understood that a flat panel display does not have to be exactly
flat but includes shaped or bent panels. A flat panel display differs from displays
such as a cathode ray tube in that it comprises a matrix or array of "cells" or "pixels"
each producing or controlling light over a small area. Arrays of this kind are called
fixed format arrays. There is a relationship between the pixel of an image to be displayed
and a cell of the display. Usually this is a one-to-one relationship. Each cell may
be addressed and driven separately. It is not considered a limitation on the present
invention whether the flat panel displays are active or passive matrix devices. The
array of cells is usually in rows and columns but the present invention is not limited
thereto but may include any arrangement, e.g. polar or hexagonal. The invention will
mainly be described with respect to liquid crystal displays but the present invention
is more widely applicable to flat panel displays of different types, such as plasma
displays, filed emission displays, EL-displays, OLED displays, etc. In particular
the present invention relates not only to displays having an array of light emitting
elements but also displays having arrays of light emitting devices, whereby each device
is made up of a number of individual elements. The displays may be emissive, transmissive,
reflective, or trans-reflective displays.
[0036] Further the method of addressing and driving the pixel elements of an array is not
considered a limitation on the invention. Typically, each pixel element is addressed
by means of wiring but other methods are known and are useful with the invention,
e.g. plasma discharge addressing (as disclosed in US-6089739) or CRT addressing.
[0037] A matrix addressed display 2 comprises individual pixels 4. These pixels 4 can take
all kinds of shapes, e.g. they can take the forms of characters. The examples of matrix
displays 2 given in Fig. 1 and Fig. 2 have rectangular or square pixels 4 arranged
in rows and columns. Fig. 1 illustrates the image of a display 2 with two defective
pixels (one pixel 3a is stuck in a dark state and another pixel 3b is stuck in a bright
state). Every pixel 4 is driven with the same signal. Except for the defective pixels
3a, 3b, all pixels have the same luminance output level. The spatial distribution
of the defect pixels 3a, 3b is arbitrary, as can be seen.
[0038] In order to be able to avoid misinterpretation of the image due to defective pixels,
in first instance the location of the defective pixels has to be known, and thus detected.
[0039] The present invention provides a vision measurement system, a set-up for automated,
electronic vision of the individual pixels of the matrix addressed display, i.e. for
measuring the colour or luminance emitted or reflected (depending on the type of display)
by individual pixels 4, using a vision measurement set-up. The vision measurement
system comprises an image capturing device 6, 12 and possibly a movement device 5
for moving the image capturing device 6, 12 and the display 2 with respect to each
other. Two embodiments are given as an example, although other electronic vision implementations
may be possible reaching the same result: an electronic image of the pixels 4.
[0040] According to a first embodiment, as represented in Fig. 3, the matrix addressed display
2 is placed with its light emitting side against an image capturing device, for example
is placed face down on a flat bed scanner 6. The flat bed scanner 6 may be a suitably
modified document or film scanner. The spatial resolution (pixels/inch) of the scanner
6 is so as to allow for adequate vision of the individual pixels 4 of the display
2 under test, i.e. the spatial resolution is high enough to allow for precisely locating
defective pixels. This spatial resolution is dependent on the algorithm used to extract
the location of defective pixels. The sensor 8 and image processing hardware of the
flat bed scanner 6 also have enough luminance sensitivity and resolution in order
to give a precise quantisation of the luminance emitted by the pixels 4. For an emissive
display 2, the light source 10 or lamp of the scanner 6 is switched off: the luminance
measured is emitted by the display 2 itself. For a reflective type of display 2, the
light source 10 or lamp of the scanner 6 is switched on: the light emitted by the
display 2 is light from the scanner's light source 10, modulated by the reflective
properties of the display 2, and reflected, and is subsequently measured by the sensor
8 of the scanner 6.
[0041] The output file of the image capturing device, in the embodiment described scanner
6, is an electronic image file giving a detailed picture of the pixels 4 of the complete
electronic display 2.
[0042] According to a second embodiment of the vision measurement system, as illustrated
in Fig. 4, an image capturing device, such as e.g. a high resolution CCD camera 12,
which may be a colour camera or a monochrome camera, is used to take a picture of
the pixels 4 of the display 2. The resolution of the CCD camera 12 is so as to allow
to adequately define the individual pixels 4 of the display 2 to be characterised.
In current state of the art of CCD cameras, it is not possible to image large matrix
displays 2 at once. As an example, high resolution electronic displays 2 with an image
diagonal of more than 20" require that the CCD camera 12 and the display 2 are moved
with respect to each other, e.g. the CCD camera 12 is scanned (in X-Y position) over
the image surface of the display 2, or vice versa: the display 2 is scanned over the
sensor area of the CCD camera 12, in order to take several pictures of different parts
7 of the display area 2. The pictures obtained in this way are thereafter preferably
stitched to obtain one image of the complete active image surface of the display 2.
A colour CCD-camera can be used to extract colour information on the pixels, but also
a monochrome CCD-camera with added colour filters (for instance on a filter wheel)
can be used.
[0043] Again, the resulting electronic image file, i.e. the output file of the image capturing
device, which is in the embodiment described a CCD camera 12, gives a detailed picture
of the pixels 4 of the display 2 that needs to be characterised. An example of an
image 13 of the pixels 4 of a matrix display 2 is visualised in Fig. 5a.
[0044] Once an image 13 of the pixels 4 of the display 2 has been obtained, a process is
run to find the exact location of defective pixels 3a, 3b and to extract pixel characterisation
data from the electronic image 13 obtained from the image capturing device 6, 12 to
characterise these defective pixels 3a, 3b.
[0045] In a first step, the actual location of the matrix display pixels 4 is identified
and related to the pixels of the electronic image 13, for example of the CCD or scanner
image.
[0046] In matrix displays 2, individual pixels 4 can be separated by a black matrix raster
14 that does not emit light. Therefore, in the image 13, a black raster 15 can be
distinguished. This characteristic can be used in the algorithms to clearly separate
and distinguish the matrix display pixels 4. The luminance distribution on an imaginary
line in a first direction, e.g. vertical line 16 in a Y-direction, and across an imaginary
line in a second direction, e.g. horizontal line 18 in an X-direction, through a pixel
4 can be extracted using imaging software, as illustrated in Fig. 5a to Fig. 5c. Methods
of extracting features from images are well known, e.g. as described in "Intelligent
Vision Systems for Industry", B. G. Batchelor and P. F. Whelan, Springer-Verlag, 1997,
"Traitement de l'Image sur Micro-ordinateur", Toumazet, Sybex Press, 1987; "Computer
vision", Reinhard Klette and Karsten Schlüns, Springer Singapore, 1998; "Image processing:
analysis and machine vision", Milan Sonka, Vaclaw Hlavac and Roger Boyle, 1998.
[0047] Supposing that the image generated by the matrix display 2 when the image was acquired
by the image capturing device 6, 12 was set on all pixels 4 having a first value,
e.g. all white pixels 4 or all pixels 4 fully on. Then the luminance distribution
across vertical line 16 and horizontal line 18, in the image 13 acquired by the image
capturing device 6, 12, shows peaks 19 and valleys 21, that correspond with the actual
location of the matrix display pixels 4, as shown in Fig. 5b and Fig. 5c respectively.
As noted before, the spatial resolution of the image capturing device, e.g. the scanner
6 or the CCD camera 12, needs to be high enough, i.e. enough to allow precisely locating
the matrix display pixels. Therefor, the resolution of the image capturing device
may be higher than the resolution of the matrix display 2 (over-sampling). A lower
degree of over-sampling is also possible. By using for instance optical filters, the
required oversampling degree can further be decreased. Because of the over-sampling,
it will be possible to express the horizontal and vertical distance of the matrix
display pixels 4 precisely in units of pixels of the image capturing device 6, 12
(not necessarily integer numbers).
[0048] A threshold luminance level 20 is constructed that is located at a suitable value
between the maximum luminance level measured at the peaks 19 and minimum luminance
level measured at the valleys 21 across the vertical lines 16 and the horizontal lines
18, e.g. approximately in the middle. All pixels of the image capturing device 6,
12 with luminance below the threshold level 20 indicate the location of the black
raster 15 in the image, and thus of a corresponding black matrix raster 14 in the
display 2. These locations are called in the present description "black matrix locations"
22. The most robust algorithm will consider a pixel location of the image capturing
device 6, 12 which is located in the middle between two black matrix locations 22
as the centre of a matrix display pixel 4. Such locations are called "matrix pixel
centre locations" 24. Depending on the amount of over-sampling, an amount of image
capturing device pixels located around the matrix pixel centre locations 24 across
vertical line 16 and horizontal line 18, can be expected to represent the luminance
of one matrix display pixel 4. In Fig. 5a, these image capturing device pixels, e.g.
CCD pixels, are located in the hatched area 26 and are indicated with numbers 1 to
7 in Fig. 5b. These CCD pixels are called "matrix pixel locators" 28 in the following.
The matrix pixel locators 28 are defined for one luminance level of the acquired image
13. To make the influence of noise minimal, the luminance level is preferably maximised
(white flat field when acquiring the image).
[0049] Other algorithms to determine the exact location of the matrix display pixels 4 are
included within the scope of the present invention. By means of example a second embodiment,
which describes an alternative using markers, is discussed below.
[0050] A limited number of marker pixels (i.e. matrix display pixels 4 with a driving signal
which is different from the driving signal of the other matrix pixels 4 of which an
electronic image is being taken), for instance four, is used to allow precise localisation
of the matrix display pixels 4. For example, four matrix display pixels 4 ordered
in a rectangular shape can be driven with a higher driving level than the other matrix
display pixels 4. When taking an electronic image 13 of this display area, it is easy
to determine precisely the location of those four marker pixels 4 in the electronic
image 13. This can be done for instance by finding the four areas in the electronic
image 13 that have the highest local luminance value. The centre of each marker pixel
can then be defined as the centre of the local area with higher luminance. Once those
four marker pixels have been determined, interpolation can be used to determine the
location of the other matrix display pixels present in the electronic image. This
can be done easily since the location of the other matrix display pixels is known
relative to the marker pixels a priori (defined by the matrix display pixel structure).
[0051] An advantage of this algorithm compared to the one of the previous embodiment is
that a lower degree of over-sampling is necessary since it is not necessary anymore
to be able to isolate the black matrix in the electronic image. Therefore, lower resolution
image capturing devices 6, 12 can be used. The algorithm can also be used for matrix
displays where no black matrix structure is present or for matrix displays that also
have black matrix between sub-pixels or parts of sub-pixels, such as a colour pixel
for example.
[0052] After having determined the location of each individual matrix pixel 4, its luminance
is calculated.
[0053] The luminance of the matrix pixel locators 28 across the X-direction and Y-direction
that describe one pixel location, are averaged to one luminance value using a suitable
calculation method, e.g. the standard formula for calculation of a mean. As a result,
every pixel 4 of the matrix display 2 that is to be characterised is assigned a pixel
value (a representative or averaged luminance value). Other more complex formulae
are included within the scope of the present invention: e.g. harmonic mean can be
used, or a number of pixel values from the image 13 can be rejected from the mean
formula as "outliers or noisy image capturing device pixels".
[0054] It will be well understood by people skilled in the art that the luminance of the
individual pixels 4 can be calculated in any of the described ways or any other way
for various test images or luminances, i.e. for a plurality of test images in which
the pixels are driven by different driving levels. Supposing that, in order to obtain
a test image, all pixels are driven with the same information, i.e. with the same
drive signal or the same driving level, then the displayed image represents a flat
field with luminance of the pixels ranging from black to white depending on the drive
signal. For each percentage of drive between 0% (zero drive, black field) and 100%
(full drive or white field) a complete image 13 of the matrix display 2 under test
can be acquired, and the luminance of each individual pixel 4 can be calculated from
the acquired image 13 with any of the described algorithms or any other suitable algorithm.
If all response points (video level vs. luminance level) of a given pixel i are then
grouped, then the luminance response function of that given pixel i is obtained. The
response function may be represented by a number of suitable means for storage and
retrieval, e.g. in the form of an analytical function, in the form of a look-up table
or in the form of a curve. An example of such a luminance response curve 30 is illustrated
in Fig. 6. An example of a luminance response curve 31 of a defective pixel is also
illustrated in Fig. 6. The luminance response curve of a defective pixel has substantially
the same luminance level for any drive level.
[0055] The luminance response function can be constructed with as many points as desired
or required for evaluating the rules that define defective pixels. The curves 30,
31 in the example of Fig. 6 are constructed using sixteen characterisation points
32, which result from the display and acquisition of images, and the calculation of
luminance levels for a given pixel 4.
[0056] It is to be remarked that a luminance response function is thus available for every
individual pixel 4 of the matrix display 2 to be characterised. The luminance response
functions of individual pixels 4 may all be different or the response functions may
be reduced to a smaller number of typical or representative functions, and each pixel
may be assigned to one of these typical functions.
[0057] For modern colour liquid crystal displays (LCDs) with a resolution up to three million
pixels, each pixel e.g. composed of a number of colour sub-pixels such as red, green
and blue sub-pixels, this means 9 million functions are obtained, each defined by
a set of e.g. sixteen values (luminance in function of drive level). If a sub-pixel
becomes stuck in one state the result is a colour shift for that pixel. This can be
identified by carrying out the characterisation of each pixel on a single colour basis.
That is all the red sub-pixels are switched on, viewed and analysed, followed by all
the blue and all the green.
[0058] The luminance response of the individual (sub)pixels 4 completely describes that
pixel's luminance behaviour as a function of the applied drive signal. By examining
this luminance behaviour, defective pixels can be localised, bearing in mind the rules
that define the defective pixels. The luminance response of a defect pixel may be
for instance a substantially constant value, independent of the level of the drive
signal, as shown by curve 31 in Fig. 6. Alternatively, the luminance response of a
defect pixel may for example have values that are substantially higher or lower than
the values of the luminance responses of non-defective pixels in the neighbourhood
of the defective pixel.
[0059] In a second step, the behaviour of the defect (sub)pixels 3a, 3b is characterised.
[0060] A defective pixel 3a, 3b can be characterised by following attributes:
* a location: a row and column number and possibly a subpixel index that indicates
which subpixel(s) is (are) defect. The subpixel index is only applicable if the pixel
is composed out of a number (typically 3) of subpixels (for instance R, G, and B for
colour or A, B and C for monochrome), as represented in Fig. 2.
* an identification for the rule that defined this pixel as being defective
* one or more digital driving levels (DDL) that describe the corresponding luminance/colour
of the defect (sub)pixels. The nummber of DDL's needed can be dependent on the rule
that was used.
For example: a pixel with location (320,334) has two defect subpixels (A and C) that
have a luminance behaviour corresponding to DDL (16 and 111).
[0061] Other algorithms are also possible. One such other algorithm is described below as
an example.
[0062] The same setup (CCD-camera or scanner) as described before can be used. Instead of
driving all pixels of the display with the same driving signal and using (severe)
over-sampling to locate the pixels of the matrix display, one can drive the pixels
in groups requiring less resolution. For instance four classes of pixels can be defined:
class A that groups all pixels in the even columns and even rows of the display, class
B that groups all pixels in even columns and odd rows of the display, class C that
groups all pixels in the odd columns and even rows of the display and class D that
groups all pixels in the odd rows and odd columns of the display. This is illustrated
in Fig. 7.
[0063] All pixel classes can then be driven separately, resulting in less resolution requirements.
The classes that are "not driven" can be set to a fixed or chosen DDL. For instance
if class A is driven with DDL=255, all other classes can be driven with DDL=0 to be
able to easily locate the pixels of class A from the resulting CCD-camera or scanner
image. The size of the classes can be chosen at will (from 1 pixel/class to all display
pixels in a single class). Suppose one has classes that hold only a single pixel,
then a CCD-camera with very low resolution will be sufficient.
[0064] All information on defect pixels of the display may be stored in a database. This
database will be called in the present description "display characteristics data".
[0065] Information on defective pixels can be entered in the database in any of two ways:
- the manufacturer of the display (or a service center) extracts information on the
defective pixels and updates the "display characteristics data", or
- the user of the display updates the "display characteristics data" based on his visual
perception.
[0066] In the first case, the manufacturer can test the display with a reliable vision system
that extracts information on defective pixels, for example a vision system as described
above. This information is stored in the "display characteristics data" and may be
supplied to the customer as a kind of quality certificate.
[0067] In the second case, the user of the display can update the "display characteristics
data" for example by means of a graphical user interface. The graphical user interface
can show a number of test-images on the display that allow the user to easily locate
defective pixels. If the user detects new defective pixels then he can enter the location
of these defective pixels (for instance by means of a pointing device such as e.g.
a mouse, a cursor or an optical pointing device. Once the defective pixels have been
located, a new series of test-images will allow the user to identify the characteristics
of the defect. These characteristics then are saved into the database for each defective
pixel of the display.
[0068] According to a first embodiment of the present invention, based on the "display characteristics
data" information can be supplied to the user of the display on a data storage device
such as a diskette or a CD-ROM for example. This can be done in several ways, for
example:
- The user can explicitly ask to display the defective pixels on the screen, or to give
information about the defective pixels, for example by pressing a button. In this
case, software algorithms can mark the defective pixels visually on the screen or
can display a report in any desired format that gives the desired information. This
can be done during actual display of an image or when no image is displayed.
- A number of triggers can cause this information to be displayed automatically. These
triggers can be very simple, for example the launch of a certain program (for instance
a radiologic image viewer). Also more complex triggers are possible: one exampie is
that a defective pixel is visually marked on the screen, e.g. by adapting the image
content of pixels in the neighbourhood of a defective pixel, such as for example by
making its neighbouring pixels blink, when the defective pixel is in the current area
of interest of a radiologic viewer that is active at that moment, which area of interest
may be indicated by means of a pointing device such as a mouse or by a touch screen
for example. Also this automatic display of defective pixel information may be done,
but does not need to be done, during actual displaying of an image.
[0069] Of course, a graphical user interface can be provided which will make it possible
to easily manage these triggers and trigger actions. Another possibility is using
command-line options or configuration files.
[0070] According to a second embodiment of the present invention, defective pixels of the
matrix display are indicated on an electronic copy of the displayed image.
[0071] According to another embodiment of the present invention, for liability reasons,
defects of the matrix display may always be recorded with any copy of an actually
displayed image going to be stored. In that case, at any moment in time, anyone is
always able to see what has actually been seen on the display. This may be of importance
for example in the medical world, where a doctor may make a diagnosis based on the
displayed images.
[0072] According to a further embodiment of the present invention, the information about
defective pixels can also be used to take a number of corrective actions to avoid
image misinterpretation. By corrective actions is meant performing certain actions
that changes the current state of the display image to a state were there is no (less)
risk of image misinterpretation.
[0073] Some examples will make this more clear:
- It is supposed that a defective pixel is in the current area of interest of a medical
image viewer. Then the corrective action could be to automatically shift the image
to the left, right, up or down until the defective pixel is no longer in the region
of interest.
- Defective pixels are especially risky when they occur in an image area were much image
transition occurs (no flat image area). Then the corrective action could be shifting
the image in such a way that the defect pixel(s) will be in a flat image area.
[0074] Instead of immediately performing corrective actions, a user can also select the
desired action from a list that can be displayed.
[0075] Of course any combination of corrective actions and displaying of image information
is also possible and can be configured with the graphical user interface or by command-line
options or configuration files.
[0076] It is to be understood that although preferred embodiments, specific constructions
and configurations have been discussed herein for devices according to the present
invention, various changes or modifications in form and detail may be made without
departing from the scope and spirit of this invention.
1. Method for avoiding misinterpretation of an image displayed on a matrix display due
to defective pixels in the matrix display, the method comprising:
obtaining information on the presence of the defective pixels in the display, and
modulating the operation of the display so as to indicate, emphasise or warn for the
presence of said defective pixels on the actual display having defect pixels, or adapting
the image content of the defective pixels or of pixels in the neighbourhood of the
defective pixels so as to indicate, emphasise or warn for the presence of said defective
pixels in a copy of the displayed image.
2. Method according to claim 1, wherein, the copy is a hard copy or an electronic copy.
3. Method according to claim 1 or 2, wherein the information is obtained from data previously
stored in a memory device.
4. Method according to claim 3, comprising, while displaying the image on the matrix
display, supplying information on defective pixels to a user, based on the stored
data.
5. Method according to any of the previous claims, wherein indicating, emphasising or
warning for the presence of at least one defective pixel comprises visually marking
the at least one defective pixel on the display.
6. Method according to any of the previous claims, furthermore comprising shifting the
displayed image so that defective pixels are not located in a region of interest.
7. Method according to any of the previous claims, furthermore comprising shifting the
displayed image so that a defective pixel is located in a flat image area.
8. Method according to any of the previous claims, wherein the information on the presence
of defective pixels is obtained by means of an image capturing device.
9. A device for avoiding misinterpretation of an image displayed on a matrix display
due to defective pixels in the matrix display, the device comprising:
an information retrieval device for obtaining information on the presence of the defective
pixels in the display, and
a modulating device for modulating the operation of the display so as to indicate,
emphasise or warn for the presence of said defective pixels on the actual display
having defect pixels, or for adapting the image content of the defective pixels or
of pixels in the neighbourhood of the defective pixels so as to indicate, emphasise
or warn for the presence of said defective pixels in a copy of the displayed image.
10. A device according to claim 9, wherein the information retrieval device comprises
a memory device where defective pixel information data is stored.
11. A device according to claim 10, comprising an information supply device for supplying
information on defective pixels to a user, based on the stored data, while displaying
the image on the matrix display.
12. A device according to any of claims 9 to 11, furthermore comprising marking means
for visually marking the defective pixels on the display.
13. A device according to any of claims 9 to 12, furthermore comprising a shifting device
for shifting the displayed image so that defective pixels are not located in a region
of interest.
14. A device according to any of claims 9 to 13, furthermore comprising a shifting device
for shifting the displayed image so that a defective pixel is located in a flat image
area.
15. A control unit for use with a device for avoiding misinterpretation of an image displayed
on a matrix display, due to defective pixels in the matrix display, the control unit
being adapted for controlling the obtaining of information on the presence and characteristics
of the defect pixels in the display, and for controlling modulation of the operation
of the display so as to indicate, emphasise or warn for the presence of said defective
pixels on the actual display having defect pixels, or adaptation of the image content
of the defective pixels or of pixels in the neighbourhood of the defective pixels
so as to indicate, emphasise or warn for the presence of said defective pixels in
a copy of the displayed image.