Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-05-07
2003-09-02
Chang, Kent (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S060000, C345S072000, C345S690000, C345S691000
Reexamination Certificate
active
06614414
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a display unit for displaying an image on a display device, wherein a plurality of periods, called sub-fields, are defined, each sub-field having a respective illumination level which is applied to the display device.
The invention further relates to an image display apparatus comprising such a display unit.
The invention further relates to method of displaying an image on a display device, wherein a plurality of periods, called sub-fields, are defined, each sub-field having a respective illumination level which is applied to the display device.
2. Description of the Related Art
U.S. Pat. No. 5,841,413 describes a plasma display panel driven in a plurality of sub-fields. A plasma display panel is made up of a number of cells that can be switched on and switched off. A cell corresponds with a pixel (picture element) of the image that is to be displayed on the panel. In the operation of the plasma display panel, three phases can be distinguished. The first phase is the erasure phase in which the memories of all cells of the panel are erased. The second phase is the addressing phase, in which the cells of the panel that are to be switched on are conditioned by setting appropriate voltages on their electrodes. The third phase is the sustain phase, in which sustain pulses are applied to the cells which cause the addressed cells to emit light for the duration of the sustain phase. The plasma display panel only emits light during this sustain phase. The three phases together are called a sub-field period or simply a sub-field. A single image, or frame, is displayed on the panel in a number of successive sub-field periods. A cell may be switched on for one or more of the sub-field periods. The light emitted by a cell in the sub-field periods in which it was switched on, is integrated in the eye of the viewer who perceives a corresponding intensity for that cell. In a particular sub-field period, the sustain phase is maintained for a particular time resulting in a particular illumination level of the activated cells. Typically, different sub-fields have a different duration of their sustain phase. A sub-field is given a coefficient of weight to express its contribution to the light emitted by the panel during the whole frame period. An example is a plasma display panel with 6 sub-fields having coefficients of weight of 1, 2, 4, 8, 16 and 32 respectively. By selecting the appropriate sub-fields in which a cell is switched on, 64 different intensity levels can be realized in displaying an image on this panel. The plasma display panel is then driven by using binary code words of 6 bits each, whereby a code word indicates the intensity level of a pixel in binary form.
In driving a plasma display panel, the frame period, i.e. the period between two successive images, is divided into a number of sub-field periods. During each of these sub-field periods, a cell may or may not be switched on and the integration over the sub-field periods results in a perceived intensity level of the pixel corresponding with this cell. Instead of displaying a pixel of an image as a single light flash that is proportional to its intensity level, on a plasma display panel, the pixel is displayed as a series of flashes shifted in time with respect to each other. This may cause artifacts if the eyes of the viewer move. Then it appears as if the light flashes do not originate from a single position and a blurring effect occurs. Furthermore, artifacts may occur in case the images show a moving object. The movement needs to be taken into account when displaying the object in a number of sub-fields. For each next sub-field, the object must be moved a little. Motion compensation techniques are used to calculate a corrected position for the sub-pixels in the sub-fields. In some circumstances, the motion compensation techniques are not fully reliable and may produce erroneous results, e.g. in an area of the image with little detail. The erroneous results lead to motion compensation where this should not be done. This gives so-called motion artifacts which are very visible.
An artifact is most noticeable if two neighboring pixels have a small difference in intensity level while for one of the pixels the sub-field with the largest coefficient of weight is on and for the other of the pixels this sub-field is off. In case of the example of the binary code above, the code word for one pixel has the most significant bit on and the code word for the other pixel has the most significant bit off. Any error in the calculated position of a sub-field, i.e., any motion artifact involving these pixels, will then give a relatively large artifact in the displayed image. The device described in U.S. Pat. No. 5,841,413 tries to mitigate these artifacts by restricting the code words that are used. This known device employs more sub-fields than necessary for realizing the required set of intensity values. The resulting set of code words for expressing the intensity value is redundant, i.e., for a given intensity value, more than one code word is available. From this redundant set, a subset is created whereby those code words are selected that give the least differences in the most significant bit for expressing a difference between the intensity values. This subset is created by searching the original set and determining what the effect on the artifacts may be for a difference between a given code word and each of the other code words.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a display unit as described in the preamble with an improved reduction of artifacts. This object is achieved, according to the invention, in that the display unit comprises:
an input for receiving respective input intensity values for sub-pixels of a particular pixel of the image,
a control unit for:
comparing at least one of the input intensity values with at least one predetermined value,
conditionally modifying, on the basis of said comparing, the at least one of the input intensity values to a desired value, and
modifying at least a further one of the input intensity values to compensate the effect on a property of the pixel caused by the modifying, if any, of the at least one of the input intensity values,
an output for sending respective output intensity values on the basis of the respective input intensity values potentially modified by the control unit, and
a coding unit for coding the output intensity levels into combinations of sub-fields for the respective sub-pixels.
The display unit of the invention makes it possible to control the intensity value of a sub-pixel, i.e., to modify it from its original intensity value to a desired value, while the effect that such modification would have on a given property of the pixel of which this sub-pixel forms a part, is compensated by a change of the intensity value for one of the other sub-pixels of that pixel. According to the invention, a flexibility is created to change the intensity value of one of the sub-pixels while this property does not change, by also changing the intensity value of one of the other colors. This property can be the luminance of a pixel, the color of a pixel, or some other characteristic of the pixel realized by the contribution of the sub-pixels of the pixel.
Controlling the intensity value that is sent to the display device for a certain sub-pixel, gives direct control whether a specific sub-field for that sub-pixel is switched on or not. This makes it possible to avoid the above problems where two nearby pixels have almost the same intensity value while one has a high weighted sub-field on while the other has not. The intensity value for one of the pixels is controlled in such a way that both have the high weighted sub-field on or off, whichever is most suitable in the situation at hand. The display unit of the invention has the advantage that it can be applied to a scheme of sub-field weights where the number of possible intensity level is maximal in view of the number of sub-fields,
De Haan Gerard
Klompenhouwer Michiel Adriaanszoon
Chang Kent
Goodman Edward W.
Koninklijke Philips Electronics , N.V.
Sheng Tom V.
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