Abstract:
A method and apparatus for processing video pictures for dynamic false contour effect compensation is provided wherein the video picture consists of pixels having at least one color component (RGB) the color component values being digitally coded with a digital code word, hereinafter called sub-field code word (SF-R, SF-G, F-B). To each bit of a sub-field code word (SF-R, SF-G, SF-B) a certain duration is assigned hereinafter called a sub-field, during which a color component of the pixel can be activated for light generation. The digital code words have n bits, characterized in that among a set p possible video levels for the at least one color component (RGB) a sub-set of m video levels with n&lt;m&lt;p is selected, which is used for light generation. The m values are selected according to the rule that the temporal center of gravity (CG 1,  CG 2,  CG 3 ) for the light generation of the corresponding sub-field code words grow continuously with the video level, apart from possible exceptions in the low video level range up to a first predefined limit and/or in the high video level range from a second predefined limit on.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a method and apparatus for processing video pictures, especially for dynamic false contour effect compensation. This method and apparatus is usable in display devices, such as matrix displays like plasma display panels (PDP), display devices with digital micro mirror arrays (DMD) and all kinds of displays based on the principle of duty cycle modulation (pulse width modulation) of light generation. 
   The plasma display technology now makes it possible to achieve flat colour panels of large size and with limited depth without any viewing angle constraints. The size of the displays may be much larger than the classical CRT picture tubes would have ever allowed. 
   A plasma display panel utilizes a matrix array of discharge cells, which could only be “on” or “off”. Also unlike a CRT or LCD in which grey levels are expressed by analogue control of the emission, a PDP controls the grey levels by modulating the number of light pulses per frame (sustain pulses). The eye will integrate this time-modulation over a period corresponding to the eye-time response. 
   Since the video amplitude determines the number of light pulses occurring at a given frequency, more amplitude means more light pulses and thus more “on time”. For this reason this kind of modulation is known as PWM, Pulse Width Modulation. To establish a concept for this PWM, each frame will be decomposed in sub-periods called “sub-fields”. For producing the small light pulses, an electrical discharge appears in a gas filled cell, called plasma cell and the produced UV radiation will excite a coloured phosphor, which emits light. In order to select which cell should be lighted, a first selected operation called “addressing” creates a charge in the cell to be lighted. Each plasma cell can be considered as a capacitor, which keeps the charge for a relative long time. Afterwards, a general operation called “sustaining” applied during the lighting period will accelerate the charges in the cell, produce further charges and excite some of the charges in the cell. Only in the cells addressed during the first selected operation, this excitation of charges takes place and UV radiation is generated when the excited charges go back to their neutral state. The UV radiation excites a phosphorous for light emission. The discharge of the cell is made in a very short period and some of the charges in the cell remain. With the next sustain pulse, the charge is utilized again for the generation of UV radiation and the next light pulse will be produced. During the whole sustain period of each specific sub-field, the cell will be lighted in small pulses. At the end an erase operation will remove all the charges to prepare a new cycle. 
   On one hand, the plasma display technology gives the possibility of nearly unlimited screen size, also of attractive thickness, but on the other hand, it generates new kinds of artefacts, which could damage the picture quality. Most of these artefacts are different from the known artefacts occurring on classical CRT colour picture tubes. It is mainly this different appearance of the artefacts that make them more visible to the viewer, since the viewer is used to see the well-known TV artefacts. 
   The invention mainly deals with a new specific artefact, which is called “dynamic false contour effect” since it corresponds to disturbances of grey levels and colours in form of an apparition of coloured edges in the picture when an observation point on the matrix screen moves. This kind of artefact is enhanced when the image has a smooth gradation, like when the skin of a person is being displayed (e.g. displaying of a face or of an arm, etc.). In addition, the same problem occurs on static images when observers are shaking their heads and that leads to the conclusion that such a failure depends on the human visual perception and happens on the retina of the eye. 
   In the prior art some approaches are already known to compensate for the false contour effect. As a false contour effect is directly related to the sub-field organization of the used plasma technology, one approach is to make an optimisation of the sub-field organization of the plasma display panels. The sub-field organization will be explained in greater detail below, but for the moment it should be noted that it is a kind of decomposition of the 8 bit grey level in 8 or more lighting sub-periods. An optimisation of such a picture encoding will have, indeed, a positive effect on the false contour effect. Nevertheless, such a solution can only slightly reduce the false contour effect amplitude but in any case the effect will still occur and will be perceivable. Furthermore, the sub-field organization is not a simple matter of design choice. The more sub-fields are allowed the less luminance the panel will be able to produce. So optimisation of the sub-field organization is only possible in a narrow range and will not eliminate this effect alone. 
   A second approach for the solution of above mentioned problem is known under the expression “pulse equalization technique”. This technique is a more complex one. It uses equalizing pulses, which are added or separated from the TV signal when disturbances of grey scale are foreseen. In addition, since the fact that the false contour effect is motion relevant, different pulses for each possible speed are necessary. That leads to the need of a big memory storing a large number of look-up tables (LUT) for each speed and there is a need of a motion estimator. Furthermore, since the false contour effect depends on the sub-field organization, the pulses have to be recalculated for each new sub-field organization. However, the big disadvantage of this technique results from the fact that the equalizing pulses add failures to the picture to compensate for a failure appearing on the eye retina. Additionally, when the motion increases in the picture, there is a need to add more pulses to the picture and that leads to conflicts with picture contents in case of very fast motion. 
   A further approach which is described in prior art documents, like EP-A-0 980 059, is based on a detection of the movements in the picture (displacement of the eye focus area) and the spreading of the right sub-field lighting periods over this displacement in order to ensure that the eye will only perceive the correct information through its movement. This solution requires a motion estimator, which delivers motion vector data for the pixels or pixel blocks. For each pixel the corresponding motion vector data is used to shift the entries in the sub-field code word in the direction of the motion vector. Thus the sub-field code words are corrected or recoded. The solution is good, and gives a good picture quality but, of course, has a need of an implementation of a motion estimator, which makes the high speed motion estimation. This motion estimator is relatively costly and not easy to implement. 
   Another approach for compensating the dynamic false contour effect is based on a new type of sub-field coding. Which is called “incremental sub-field coding”. The incremental sub-field coding method is disclosed for example in the European Patent Publication EP-A-0 952 569. In this type of sub-field coding method, there are only some basic sub-field code words used for the grey scale portrayal rendition. This means, that in the case of 8 bit video data there are not 256 different sub-field code words for the possible video levels, but instead only a few sub-field code words with specific characteristic for some distinct video levels and the remaining video levels are rendered by some optimised dithering or error diffusion technique. The speciality of the incremental code is that in each case there is never one sub-field inactivated between two consecutive activated sub-fields and never one sub-field activated between two consecutive inactivated sub-fields. With this characteristic the incremental code has the advantage, that false contour effect is no longer a problem due to the fact that sub-field code words for similar video levels cannot deviate at various bit positions. 
   The structure of such sub-field code words is very specific and varies from code word to code word in only one sub-field entry. This means that when there is a smooth transition of video levels like in a homogenous surface, as skin, then there will no longer occur the changes in the structure of sub-field code words, which can cause false contour effect. The number of available video levels is, however, substantively reduced so that a poor grey scale rendition results. To improve this grey scale rendition, a dithering technique is required, which brings back some of the lost video levels. It is hardly possible to bring back all the lost video levels with such a dithering technique or error diffusion technique in the case of this specific sub-field coding, where the number of grey levels is reduced to the number of sub-fields in the sub-field organization. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to disclose a method and an apparatus for processing video pictures, which achieve an efficient false contour effect compensation based on a new type of sub-field coding without the need of having a motion estimator but with an improved grey scale rendition, so that less dithering noise is produced. This object is achieved by the solution claimed in independent claims  1  and  7 . 
   According to the claimed solution in claim  1 , a new type of sub-field coding is used based on a sub-field organization with n sub-fields, in which among the set of p possible video levels for a colour component a sub-set of m video levels is selected with n&lt;m&lt;p wherein the m values are selected according to the rule that the temporal centre of gravity for the light generation of the corresponding sub-field code words grows continuously apart from exceptions in the low video level range up to a first predefined limit and/or in the high video level range from a second predefined limit on. While according to the incremental coding described above only very few video levels are admissible for sub-field coding, namely as much as sub-fields are available in the sub-field organization, the sub-field coding method according to the invention relies on more video levels and thus provides for a better grey scale rendition and less disturbing dithering noise. The admissible video levels according to the inventive solution cannot be arbitrarily taken from the full video level range but are selected with a specific rule, namely that the temporal centre of gravity of admissible sub-field code words grows smoothly when the video levels are ordered according to size. There are only some exceptions allowable in the low video level range and in the high video level range. 
   It is not possible to select video levels with growing gravity centre in the low level range because the number of possible video levels is low and so if only smoothly growing centre of gravity levels where selected, then there would not be enough video levels to have a good video quality in the black/dark video levels, since the human eye is very sensitive in the dark/black level range. But this is not so disturbing because the false contour effect is negligible in dark areas. 
   In the high video level range, there is a decrease of the centres of gravity anyhow, so here, also in the chosen video levels a decrease is accepted. Of course this can cause dynamic false contour effect, but this is not so important in this range since the human eye is not sensitive in the high video level range. This will be explained in more detail later on. 
   In between the low level and high video level range the admissible video levels and their corresponding sub-field code words follow a monotone rising curve and thus in this range the dynamic false contour effect does not occur. 
   In summary, with the sub-field coding method according to the invention a good compromise has been found in regards to dynamic false contour effect reduction and grey scale rendition. A very good picture quality is maintained. Advantageously additional embodiments of the inventive method are disclosed in the respective dependent claims. 
   It is very advantageous, if the sub-field coding process respects the rule that for all input video levels that are different from zero a sub-field code word is selected, in which never more than one consecutive sub-field is inactivated between two activated sub-fields. This rule drastically reduces the number of possible sub-field code words, so that for setting up the sub-set of m video levels the choice of video levels and corresponding sub-field code words is simplified. Taking only those sub-field code words and corresponding video levels respecting above mentioned rule has the additional advantage that the response fidelity of the plasma cells in case of a plasma display panel is subjectively increased. This is because the distance in time between two writing periods for a plasma cell is reduced, so that the probability of a correct pre-charging of the plasma cell during the writing period is increased. With sub-field coding methods not respecting this rule the problem can arise that some plasma cells show some sort of flickering because they are not correctly lighted in each video frame. 
   Advantageous features for an apparatus according to the invention are claimed in claims  5  to  7 . The sub-set of m video levels can be stored advantageously in a look-up table for the sub-field coding process. 
   As the plasma display panels have a linear response characteristic, it is advantageous to provide a Degamma unit, in which the input video levels are compensated for the gamma correction in the video source. 
   Also it is advantageous to provide a dithering unit, in which dithering values are added to the output values of the Degamma unit to increase the grey scale portrayal. As known from the dithering technique, in the dithering unit a truncation of the video level data is performed to the bit resolution, which is required for the number m of video levels in the selected sub-set. This video level data is input to the look-up table for the sub-field coding process. This look-up table can be designed to not include the sub-field code word but instead the full resolution video level word (preferably 8 bit). This allows implementing the dynamic false contour compensation method at the video level processing stage, i.e. before sub-field coding so that the method can be simply implemented very easily on any panel type. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are illustrated in the drawings and in more detail in the following description. 
     In the figures: 
       FIG. 1  shows the structure of a plasma display panel cell in the matrix technology; 
       FIG. 2  shows the conventional ADS addressing scheme during a frame period; 
       FIG. 3  shows the standard sub-field encoding principle for the ADS addressing scheme and priming; 
       FIG. 4  shows a video picture, in which the false contour effect is simulated; 
       FIG. 5  shows two different sub-field organization schemes; 
       FIG. 6  shows an illustration for explaining the false contour effect; 
       FIG. 7  illustrates the appearance of a dark edge when a display of two frames is made in the manner shown in  FIG. 6 ; 
       FIG. 8  illustrates that the temporal centre of gravity of the light generation is not growing monotonously with the video levels; 
       FIG. 9  shows the centre of gravity time positions for the sub-fields within a sub-field organization; 
       FIG. 10  illustrates the behaviour of the centre of gravity variation in a temporal gravity centre versus video level curve; 
       FIG. 11  shows a monotone rising curve with the selected points in the temporal centre of gravity versus video level coordinate system and the sub-set of selected points for sub-field encoding; 
       FIG. 12  illustrates all possible points in the temporal centre of gravity versus video level coordinate system for a sub-field organization with 11 sub-fields; 
       FIG. 13  illustrates a sub-set of points in the temporal centre of gravity versus video level coordinate system selected according to the minimum weight selection rule; 
       FIG. 14  shows the selection of points from the minimum weight sub-field code words for generating the monotonous rising curve; 
       FIG. 15  shows a first block diagram of the circuit implementation of the invention; and 
       FIG. 16  shows a more detailed block diagram for the implementation of the invention in the video processing stages before sub-field encoding. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The principle structure of a plasma cell in the so-called matrix plasma technology is shown in FIG.  1 . Reference number  10  denotes a face plate made of glass, with reference number  11  a transparent line electrode is denoted. The back plate of the panel is referenced with reference number  12 . There are  2  dielectric layers  13  for isolating face and back plate against each other. In the back plate column electrodes  14  are integrated being perpendicular to the line electrodes  11 . The inner part of the cells consists of a luminance substance  15  (phosphorous) and separator  16  for separating the different coloured phosphorous substances (green  15   a ) (blue  15   b ) (red  15   c ). The UV radiation caused by the discharge is denoted with reference number  17 . The light emitted from the green phosphorous  15   a  is indicated with an arrow having the reference number  18 . From this structure of a PDP cell it is clear that there are three plasma cells necessary, corresponding to the three colour components RGB to produce the colour of a picture element (pixel) of the displayed picture. 
   The grey level of each R, G, B component of a pixel is controlled in a PDP by modulating the number of light pulses per frame period. The eye will integrate this time modulation over a period corresponding to the human eye response. The most efficient addressing scheme should be to address n times if the number of video levels to be created is equal to n. In case of the commonly used 8 bit representation of the video levels, a plasma cell should be addressed 256 times according to this. But this is not technically possible, since each addressing operation requires a lot of time (around 2 μs per line&gt;960 μs for one addressing period&gt;245 ms for all 256 addressing operations), which is more than the 20 ms available time period for 50 Hz video frames. 
   From the literature a different addressing scheme is known, which is more practical. According to this addressing scheme a minimum of 8 sub-fields (in case of an 8 bit video level data word) are used in a sub-field organization for a frame period. With a combination of these 8 sub-fields it is possible to generate the 256 different video levels. This addressing scheme is illustrated in FIG.  2 . In this figure each video level for each colour component will be represented by a combination of 8 bits with the following weights:
         1/2/4/8/16/32/64/128       

   To realize such a coding with the PDP technology, the frame period will be divided in 8 lighting periods called sub-fields, each one corresponding to a bit in a corresponding sub-field code word. The number of light pulses for the bit “ 2 ” is double as for the bit “ 1 ” and so forth. With these 8 sub-periods it is possible, through sub-field combination, to build the 256 grey levels. The standard principle to generate this grey level rendition is based on the ADS (Address Display Separated) principle, where all operations are performed at different times on the whole display panel. At the bottom of  FIG. 2  it is shown that in this addressing scheme each sub-field consists of three parts, namely an addressing period, a sustaining period and an erasing period. 
   In the ADS addressing scheme all the basic cycles follow one after the other. At first, all cells of the panel will be written (addressed) in one period, afterwards all cells will be lighted (sustained) and at the end all cells will be erased together. 
   The sub-field organization shown in  FIG. 2  is only a simple example and there are very different sub-field organizations known from the literature with e.g. more sub-fields and different sub-field weights. Often more sub-fields are used to reduce moving artefacts and “priming” could be used on more sub-fields to increase the response fidelity. Priming is a separate optional period, where the cells are charged and erased. This charge can lead to a small discharge, i.e. can create background light, which is in principle unwanted. After the priming period an erase period follows for immediately quenching the charge. This is required for the following sub-field periods, where the cells need to be addressed again. So priming is a period, which facilitates the following addressing period, i.e. it improves efficiency of the writing stage by regularly exciting all cells simultaneously. The addressing period length can be equal for all sub-fields, also the erasing period length. However, it is also possible that the addressing period length is different for a first group of sub-fields and a second group of sub-fields in a sub-field organization. In the addressing period, the cells are addressed line-wise from line  1  to line n of the display. In the erasing period all the cells will be discharged in parallel in one shot, which does not take as much time as for addressing. The example in  FIG. 3  shows the standard sub-field organisation with 8 sub-field inclusive the priming operation. At one point in time there is one of these operations active for the whole panel. 
     FIG. 4  shows the artefact due to the false contour effect. On the arm of the displayed woman two dark lines are shown, which for example are caused by this false contour effect. Also in the face of the woman such dark lines occur at the right side. 
   As mentioned above, a plasma display panel utilizes a matrix array of discharge cells, which can only be switched on or off. In a PDP modulating the number of light pulses per video frame controls the grey level of each colour component. The eye will integrate this time modulation over a period corresponding to the eye time response. Without motion, the eye of the observer will integrate over about a frame period these small light impulses and catch the impression of the correct grey level. 
   When an observation point (eye focus area) on the PDP screen moves, the eye will follow this movement. Consequently, it will no longer integrate the light from the same cell over a period (static integration) but it will integrate information coming from different cells located on the movement trajectory. Thus it will mix all the light pulses during this movement, which leads to a faulty signal information. This effect will now be explained in more detail below. In the field of plasma video encoding, the use of more than 8 sub-fields to represent the 256 original video levels is very common. This aims at reducing the weights of the MSBs, which are directly linked to the maximum level of false contour generated. The first example of such a sub-field organization based on 10 sub-fields is shown in the upper part of  FIG. 5. A  sub-field organization based on 12 sub-fields is shown in the lower part of FIG.  5 . Of course, the sub-field organizations shown in  FIG. 5  are only examples and the sub-field organization can be subject of modification for other embodiments. 
   The light emission pattern according to the sub-field organization introduces new categories of image quality degradation corresponding to disturbances of grey levels and colours. As already explained, these disturbances are defined as so called dynamic false contour effects, since the fact that they correspond to the appearance of coloured edges in the picture when an observation point on the PDP screen moves. The observer has the impression of a strong contour appearing on a homogeneous area like displayed skin. The degradation is enhanced when the image has a smooth gradation and also when the light emission period exceeds several ms. So, in dark scenes the effect is not so disturbing as in scenes with average grey level (e.g. luminance values from 32 to 223). In addition the same problem occurs in static images when observers shake their head, which leads to the conclusion that such a failure depends on the human visual perception. To better understand the basic mechanism of visual perception of moving images, a simple case will be considered. Let us assume a transition between the luminance levels  128  and  127  moving at a speed of 5 pixels per video frame and the eye is following this movement. 
     FIG. 6  shows a darker shaded area corresponding to the luminance level  128  and a lighter shaded area corresponding to the luminance level  127 . The sub-field organization shown in  FIG. 2  is used for building the luminance levels  128  and  127  as it is depicted in the right side of FIG.  6 . The three parallel lines in  FIG. 6  indicate the direction in which the eye follows the movement. The two outer lines show the area borders where a faulty signal will be perceived. Between them the eye will perceive a lack of luminance, which leads to the appearance of a dark edge in the corresponding area, which is illustrated in  FIG. 6  at the bottom. The effect that the lack of luminance will be perceived in the shown area is due to the fact that the eye will no longer integrate all lighting periods of one pixel when the point from which the eye receives light is in movement. Only part of the light pulses will probably be integrated when the point moves. Therefore, there is a lack of corresponding luminance and a dark edge will occur. 
   On the left side of  FIG. 7 , a curve is shown, which illustrates the behaviour of the eye cells during observing a moving picture depicted in FIG.  6 . The eye cells having a good distance from the horizontal transition will integrate enough light from the corresponding pixels. Only the eye cells, which are near the transition, will not be able to integrate a lot of light from the same pictures. In case of grey scale this effect corresponds to the apparition of artificial white or black edges. In the case of coloured pictures, since this effect will occur independently on the different colour components, it will lead to the apparition of coloured edges in homogeneous areas like skin. In a colour TV PDP, the same phenomenon will appear on the three components (RGB) but with different intensities depending on the colour level and its encoding in sub-fields. This will lead to coloured edges appearing on the picture and this is very annoying since they are unnatural. Furthermore, this effect will also occur in case of a sharp transition, e.g. a transition from white to black video level and combined with phosphor lag effect, this leads to a strong degradation of the sharpness of moving objects. 
   It is evident that from the explanation above that the false contour effect occurs when there is a transition from one level to another with a totally different sub-field code word. It is, therefore, an idea of the invention to make a specific selection of sub-field code words among the 2 n  possible sub-field arrangements, where n is the number of sub-fields in a sub-field organization, to verify that video levels with similar size will have sub-field code words with similar structure. The input video levels for the different colour components are usually given in an 8 bit binary code so that 256 different video levels are provided. The number p is the number of possible video levels, i.e. with 8 bit p=256. According to the invention, only a sub-set of these possible video levels will be used for sub-field coding, where m is the number of video levels in the selected sub-set. The relationship between m and p is m&lt;p. A problem is how to select the m grey levels for the sub-set and the corresponding sub-field code words among the 2 n  possible sub-field arrangements in order to avoid the occurrence of false contour effect. A compromise has to be found between selecting only those video levels and sub-field code words in order to avoid the false contour problematic on the one hand and on the other hand to keep a maximum of video levels in order to have the best video quality. Experiments have shown that an acceptable compromise between the number of video levels and a good false contour reduction is given if a minimum of selected video levels for the sub-set is equal to twice the number of sub-fields in the selected sub-field organization. 
   How to select the correct sub-field code words and corresponding video levels for the sub-set, is a more sophisticated problem but it can be solved relatively easily as will be shown hereinafter in the following explanations. 
   As described above a PDP emits light pulses in pulse width modulation form and the human eye integrates these light pulses during a frame period in order to perceive the correct brightness impression. In  FIG. 8  it is indicated how the temporal centre of gravity CG 1 , CG 2 , CG 3  of light emission varies when the video level is incremented one by one in case of a basic sub-field code like the well-known binary code. A vertical line indicates the temporal centre of gravity. A dark shaded sub-field means that during this sub-field the light generation is activated whereas a light shaded sub-field means that there is no light generation in this sub-field period. From  FIG. 8  it is evident that the temporal centre of gravity CG 1 , CG 2 , CG 3 , etc. is not growing smoothly (monotonously) with the video level. And it is this behaviour that makes this type of sub-field coding sensitive to false contour effect. The mathematical exact definition of the temporal centre of gravity of the light generation according to a sub-field code word is defined in the following formula: 
         CG   ⁡     (   code   )       =         ∑     i   =   1     n     ⁢       sfW   i     *       δ   i     ⁡     (   code   )       *     sfCG   i             ∑     i   =   1     n     ⁢       sfW   i     *       δ   i     ⁡     (   code   )                 
 
   In this formula sfW i  is a sub-field weight of the i th  sub-field, δ i  is equal to 1 if the i th  sub-field is “switched on” according to the sub-field code word and 0 otherwise. The temporal centre of gravity of the i th  sub-field is named sfCG i  in this formula.  FIG. 9  shows for each sub-field in a sub-field organization its corresponding temporal centre of gravity, again indicated by a vertical line. 
   In the next figure,  FIG. 10 , the temporal centres of gravity of all 256 video levels are shown in form of a curve for a sub-field organization with 11 sub-fields and sub-field weights as shown below:
     1 2 3 5 8 12 18 27 41 58 80   

   The temporal centre of gravity is calculated with the formula presented above. The curve in  FIG. 10  is by far not monotonous and includes a lot of jumps. It is the recognition of the invention that these jumps cause false contour effect. 
   In order to avoid this, it is therefore the idea of the invention to suppress these jumps by selecting only some video levels, for which the corresponding sub-field code words have temporal centres of gravity, which will grow smoothly. This can be done by drawing a monotone curve without jumps in the previous graphic and selecting in each case the nearest point. A lot of best fitting techniques are known for this purpose from the mathematics, e.g. Gaussian fit method, which relies on minimization of the square errors. Of course, this is only one embodiment of the invention. An example of a monotonous curve is shown in FIG.  11 . The selected video levels for the sub-set of video levels are indicated with small black squares. Next, a more sophisticated embodiment is described. 
   In the low video level range it is not always sufficient to respect the above mentioned rule to only select those video levels where the temporal centre of gravity is smoothly growing, because in this range the number of possible levels is low and so if only growing temporal centre of gravity levels were selected, there would not be enough video levels to provide a good video quality in dark pictures, since the human eye is very sensitive in the dark video picture range. On the other hand the false contour effect in the dark video picture range is negligible anyhow, so that it is acceptable that in this range the above-mentioned rule is violated. 
   In the high video level range, there is a decrease of the temporal centre of gravity, which is evident when looking at FIG.  10 . As soon as the sub-field with the highest sub-field weight is lighted, only some lower sub-fields can be lighted having a previous time position, which leads to a reduction of the overall temporal centre of gravity for the light emissions. Thus, also in this video level range the above given rule cannot be respected. In this area, the human eye is not very sensitive of distinguishing the different video levels and, therefore, it is not so important that the above-mentioned rule is respected. The occurring false contour effect is negligible in this video level range. This is in accordance with the Weber-Fechner law, which specifies that the eye is only sensitive to relative video amplitude changes. In the high video level range the relative video amplitude changes are low in comparison to the low or mid video level range. For these reasons, the above mentioned rule, that only those video levels and corresponding sub-field code words are selected for setting up the sub-set of video levels can be revised to the less strict rule that the monotony of the curve is only required in the video level range between a first and a second limit. With experiments it has been verified that for example 10% of the maximum video level is an appropriate level for the low video level range and 80% of the maximum video level is an appropriate level for the high video level range. 
   In the example shown in  FIG. 11  37 video levels (m=37) are selected for the sub-set among the 256 possible video levels. These 37 levels permit to keep a good video quality (grey scale portrayal). 
   Except for very simple sub-field organizations (up to 8 sub-fields) this selection can be directly made on the video level basis. For all other sub-field organizations with 9 and more sub-fields the choice is more difficult. This is illustrated in FIG.  12 . If there are p sub-fields in a sub-field organization, there are 2 p  different sub-field arrangements. 
   In  FIG. 12  all possible sub-field code words for a sub-field organization with 11 sub-fields are shown. In case of 11 sub-fields there are 2 11  sub-fields code words, which is equal to 2048 different sub-field arrangements. Of course, the curve can be simply fitted in this plurality of points as mentioned above, for example with the Gaussian fit algorithm and the nearest point can simply be taken. However, another embodiment will be described below which brings some advantages. 
   In this example the field of possible sub-field code words is reduced by only taking the minimum weight code words (mWC). These code words are all those code words, which have the smallest sub-fields activated for light emission for each video level, i.e. the one, which has the minimum binary value. This coding principle is better explained with an example. The following sub-field organization is considered also for this example:
     1 2 3 5 8 12 18 27 41 58 80   

   The numbers represent the sub-field weights. With this sub-field organization the video level  23  can be coded with the following codes: 
   
     
       
             
             
             
             
             
             
             
             
             
             
             
           
         
             
                 
             
           
           
             
               0 
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               1 
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               0 
               1 
               1 
               0 
               0 
               0 
               1 
               0 
               0 
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               0 
               1 
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               1 
               1 
               0 
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               1 
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                 1 
               
               
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                 1 
               
               
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                 1 
               
               
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                 0 
               
               
                 0 
               
             
             
                 
             
           
        
       
     
   
   From this set of sub-field code words the last one in bold letters is the minimum weight code word. This code has the most entries in the least significant bits. Please note that the LSB is on the left side in this table. 
   The centre of gravity positions for all possible 2 11 =2048 codes, are shown in FIG.  12 . From this set of code words the mWC words are indicated in white. From this graph, it is evident that mWC codes also have the minimum centres of gravity from all possible code words. Since the mWC codes make use of the smallest sub-fields in the sub-field organization, they introduce a minimum false contour effect. This is because the false contour effect is directly proportional to the sub-field weights. It is therefore, very advantageous in respect to dynamic false contour effect reduction that the sub-set of video levels is taken from the mWC codes. Of course, all the selected codes shall be on a monotonous rising curve as is explained above. The selection of the code words on the gravity centre curve can be made automatically. This can be done as illustrated in FIG.  13 .  FIG. 13  depicts all the mWC code words for the above given sub-field organization. It has also been used for FIG.  12  and FIG.  13 . In the gravity centre curve shown in  FIG. 13  the smallest structure that can be seen apart from the single points are the arches some of them have been marked in the figure with an ellipse. The idea is now to take only one point of each arch if possible. Of course, the created curve has to be monotonously. In fact from the code, it is possible to recognize the points which are on a specific arch. The sub-field code words of all points on an arch have identical entries in the MSBs (radical) but different entries in the LSBs. For example, the code words on the 3 rd  arch from the left have the following radical:
         X X X X X X X 1 0 1 0       

   The sub-field code words on the 4 th  arch from the left have the following radical:
         X X X X X X 1 0 1 1 0       

   The sub-field code words on the 6 th  arch from the left have the following radicals:
         X X X X X X X X 1 0 1       

   Here X stands for the entry 0 or 1 and each X in the sub-field code words can be different from another X entry. 
   For achieving the best response fidelity for the plasma cells it is advantageous that the selected codes also respect the rule that in each sub-field code word there should never be more than one consecutive 0 entry between two 1 entries, which means that there is never more than one inactivated sub-field between two activated sub-fields for the plasma cell addressing. Such codes are also called refreshing codes because the plasma cells are activated in short succession, therefore, the charge in the cell cannot vanish during a relatively long inactivated period. This concept is already explained in another European Patent Application of the applicant, having the application number 00250066.8. For the disclosure of this refreshing concept it is, therefore, also expressively referred to this European Patent Application. The mWC code words already respect this rule so that every video level can be used which has a corresponding mWC code word. In case of a different sub-field organization, it may be necessary to further limit the mWC code words according to the “single inactivated sub-field rule” in order to get the same result. But this further limitation does not reduce the number of chosen levels a lot and, therefore, it does not cost a lot of flexibility. But on the other hand it brings the important advantage that the response fidelity of the plasma cells is subjectively increased. 
   For the further automatic selection of the video levels the following algorithm will be used: 
   The algorithm starts with a selection of the video level zero. Of course, the next video level is the video level  1  and the following video level is the level  2 . After this video level the next video level will be chosen which belongs to the next arch and has in addition the smallest centre of gravity superior to the centre of gravity of the previous selected video level. If all the centres of gravity of the next arch are inferior to the previous one, then the next video level will be chosen among the next arch, etc. 
   The next example better explains this selection process. For example by applying this method from video level  0  to video level  237 , which is also a selected GCC code (gravity centre code), having a centre of gravity equal to 6610 and a sub-field code word equal to 1 1 1 1 1 1 0 1 1 1 1, the next video level will be searched among the possible codes having the form:
     X X X 1 0 1 1 1 1 1. All possible codes with their centre of gravity are given below:   

   
     
       
             
             
             
             
             
             
           
         
             
                 
             
           
           
             
               level 
               238 
               Coded in 
               1 1 1 0 1 0 1 1 1 1 1 
               Center of gravity 
               6680 
             
             
               level 
               239 
               Coded in 
               0 1 0 1 1 0 1 1 1 1 1 
               Center of gravity 
               6677 
             
             
               level 
               240 
               Coded in 
               1 1 0 1 1 0 1 1 1 1 1 
               Center of gravity 
               6652 
             
             
               level 
               241 
               Coded in 
               1 0 1 1 1 0 1 1 1 1 1 
               Center of gravity 
               6636 
             
             
               
                 level 
               
               
                 242 
               
               
                 Coded in 
               
               
                 0 1 1 1 1 0 1 1 1 1 1 
               
               
                 Center of gravity 
               
               
                 6616 
               
             
             
               level 
               243 
               Coded in 
               1 1 1 1 1 0 1 1 1 1 1 
               Center of gravity 
               6591 
             
             
                 
             
           
        
       
     
   
   The lowest centre of gravity is from the video level  243 , however this video level cannot be chosen since it has a centre of gravity inferior to the centre of gravity of the previous video level  237 . Therefore, the next video level will be chosen to be the video level  242 . 
     FIG. 14  shows all selected GCC codes in form of a dark square among the mWC codes and the resulting monotonous curve. The curve does not monotonously grow only in the high video level range between  242  and  255 , which is the maximum video level that is selected. This level is also selected because it does not introduce very much false contour as explained above. From all  256  possible video levels only 37 video levels have been finally selected as the GCC codes. In the table below all mWC codes for all video levels from zero to  255  together with their centre of gravity values have been listed. The selected 37 GCC codes are highlighted with bold letters. 
   
     
       
             
           
             
             
             
             
             
           
         
             
                 
             
             
               All mWC codes with their Centers of Gravity: 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               
                 level 
               
               
                 0 
               
               
                 Coded in 0 0 0 0 0 0 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 0 
               
             
             
               
                 level 
               
               
                 1 
               
               
                 Coded in 1 0 0 0 0 0 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 575 
               
             
             
               
                 level 
               
               
                 2 
               
               
                 Coded in 0 1 0 0 0 0 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 1160 
               
             
             
               level 
               3 
               Coded in 1 1 0 0 0 0 0 0 0 0 0 
               Center of gravity 
               965 
             
             
               
                 level 
               
               
                 4 
               
               
                 Coded in 1 0 1 0 0 0 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 1460 
               
             
             
               
                 level 
               
               
                 5 
               
               
                 Coded in 0 1 1 0 0 0 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 1517 
               
             
             
               level 
               6 
               Coded in 1 1 1 0 0 0 0 0 0 0 0 
               Center of gravity 
               1360 
             
             
               level 
               7 
               Coded in 0 1 0 1 0 0 0 0 0 0 0 
               Center of gravity 
               2020 
             
             
               
                 level 
               
               
                 8 
               
               
                 Coded in 1 1 0 1 0 0 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 1840 
               
             
             
               
                 level 
               
               
                 9 
               
               
                 Coded in 1 0 1 1 0 0 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 1962 
               
             
             
               level 
               10 
               Coded in 0 1 1 1 0 0 0 0 0 0 0 
               Center of gravity 
               1941 
             
             
               level 
               11 
               Coded in 1 1 1 1 0 0 0 0 0 0 0 
               Center of gravity 
               1816 
             
             
               level 
               12 
               Coded in 1 0 1 0 1 0 0 0 0 0 0 
               Center of gravity 
               2486 
             
             
               level 
               13 
               Coded in 0 1 1 0 1 0 0 0 0 0 0 
               Center of gravity 
               2429 
             
             
               
                 level 
               
               
                 14 
               
               
                 Coded in 1 1 1 0 1 0 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 2297 
               
             
             
               level 
               15 
               Coded in 0 1 0 1 1 0 0 0 0 0 0 
               Center of gravity 
               2543 
             
             
               
                 level 
               
               
                 16 
               
               
                 Coded in 1 1 0 1 1 0 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 2420 
               
             
             
               
                 level 
               
               
                 17 
               
               
                 Coded in 1 0 1 1 1 0 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 2450 
               
             
             
               level 
               18 
               Coded in 0 1 1 1 1 0 0 0 0 0 0 
               Center of gravity 
               2411 
             
             
               level 
               19 
               Coded in 1 1 1 1 1 0 0 0 0 0 0 
               Center of gravity 
               2315 
             
             
               level 
               20 
               Coded in 1 1 0 1 0 1 0 0 0 0 0 
               Center of gravity 
               2938 
             
             
               level 
               21 
               Coded in 1 0 1 1 0 1 0 0 0 0 0 
               Center of gravity 
               2938 
             
             
               level 
               22 
               Coded in 0 1 1 1 0 1 0 0 0 0 0 
               Center of gravity 
               2884 
             
             
               
                 level 
               
               
                 23 
               
               
                 Coded in 1 1 1 1 0 1 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 2783 
               
             
             
               level 
               24 
               Coded in 1 0 1 0 1 1 0 0 0 0 0 
               Center of gravity 
               3078 
             
             
               level 
               25 
               Coded in 0 1 1 0 1 1 0 0 0 0 0 
               Center of gravity 
               3025 
             
             
               
                 level 
               
               
                 26 
               
               
                 Coded in 1 1 1 0 1 1 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 2930 
               
             
             
               level 
               27 
               Coded in 0 1 0 1 1 1 0 0 0 0 0 
               Center of gravity 
               3043 
             
             
               
                 level 
               
               
                 28 
               
               
                 Coded in 1 1 0 1 1 1 0 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 2955 
               
             
             
               level 
               29 
               Coded in 1 0 1 1 1 1 0 0 0 0 0 
               Center of gravity 
               2955 
             
             
               level 
               30 
               Coded in 0 1 1 1 1 1 0 0 0 0 0 
               Center of gravity 
               2915 
             
             
               level 
               31 
               Coded in 1 1 1 1 1 1 0 0 0 0 0 
               Center of gravity 
               2839 
             
             
               level 
               32 
               Coded in 1 1 1 0 1 0 1 0 0 0 0 
               Center of gravity 
               3474 
             
             
               level 
               33 
               Coded in 0 1 0 1 1 0 1 0 0 0 0 
               Center of gravity 
               3550 
             
             
               level 
               34 
               Coded in 1 1 0 1 1 0 1 0 0 0 0 
               Center of gravity 
               3462 
             
             
               level 
               35 
               Coded in 1 0 1 1 1 0 1 0 0 0 0 
               Center of gravity 
               3448 
             
             
               level 
               36 
               Coded in 0 1 1 1 1 0 1 0 0 0 0 
               Center of gravity 
               3400 
             
             
               
                 level 
               
               
                 37 
               
               
                 Coded in 1 1 1 1 1 0 1 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 3324 
               
             
             
               level 
               38 
               Coded in 1 1 0 1 0 1 1 0 0 0 0 
               Center of gravity 
               3625 
             
             
               level 
               39 
               Coded in 1 0 1 1 0 1 1 0 0 0 0 
               Center of gravity 
               3608 
             
             
               level 
               40 
               Coded in 0 1 1 1 0 1 1 0 0 0 0 
               Center of gravity 
               3561 
             
             
               
                 level 
               
               
                 41 
               
               
                 Coded in 1 1 1 1 0 1 1 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 3488 
               
             
             
               level 
               42 
               Coded in 1 0 1 0 1 1 1 0 0 0 0 
               Center of gravity 
               3640 
             
             
               level 
               43 
               Coded in 0 1 1 0 1 1 1 0 0 0 0 
               Center of gravity 
               3596 
             
             
               
                 level 
               
               
                 44 
               
               
                 Coded in 1 1 1 0 1 1 1 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 3527 
               
             
             
               
                 level 
               
               
                 45 
               
               
                 Coded in 0 1 0 1 1 1 1 0 0 0 0 
               
               
                 Center of gravity 
               
               
                 3582 
               
             
             
               level 
               46 
               Coded in 1 1 0 1 1 1 1 0 0 0 0 
               Center of gravity 
               3516 
             
             
               level 
               47 
               Coded in 1 0 1 1 1 1 1 0 0 0 0 
               Center of gravity 
               3504 
             
             
               level 
               48 
               Coded in 0 1 1 1 1 1 1 0 0 0 0 
               Center of gravity 
               3468 
             
             
               level 
               49 
               Coded in 1 1 1 1 1 1 1 0 0 0 0 
               Center of gravity 
               3409 
             
             
               level 
               50 
               Coded in 1 1 1 1 0 1 0 1 0 0 0 
               Center of gravity 
               4080 
             
             
               level 
               51 
               Coded in 1 0 1 0 1 1 0 1 0 0 0 
               Center of gravity 
               4193 
             
             
               level 
               52 
               Coded in 0 1 1 0 1 1 0 1 0 0 0 
               Center of gravity 
               4146 
             
             
               level 
               53 
               Coded in 1 1 1 0 1 1 0 1 0 0 0 
               Center of gravity 
               4079 
             
             
               level 
               54 
               Coded in 0 1 0 1 1 1 0 1 0 0 0 
               Center of gravity 
               4114 
             
             
               level 
               55 
               Coded in 1 1 0 1 1 1 0 1 0 0 0 
               Center of gravity 
               4050 
             
             
               level 
               56 
               Coded in 1 0 1 1 1 1 0 1 0 0 0 
               Center of gravity 
               4030 
             
             
               level 
               57 
               Coded in 0 1 1 1 1 1 0 1 0 0 0 
               Center of gravity 
               3990 
             
             
               
                 level 
               
               
                 58 
               
               
                 Coded in 1 1 1 1 1 1 0 1 0 0 0 
               
               
                 Center of gravity 
               
               
                 3931 
               
             
             
               level 
               59 
               Coded in 1 1 1 0 1 0 1 1 0 0 0 
               Center of gravity 
               4257 
             
             
               level 
               60 
               Coded in 0 1 0 1 1 0 1 1 0 0 0 
               Center of gravity 
               4286 
             
             
               level 
               61 
               Coded in 1 1 0 1 1 0 1 1 0 0 0 
               Center of gravity 
               4225 
             
             
               level 
               62 
               Coded in 1 0 1 1 1 0 1 1 0 0 0 
               Center of gravity 
               4204 
             
             
               level 
               63 
               Coded in 0 1 1 1 1 0 1 1 0 0 0 
               Center of gravity 
               4165 
             
             
               
                 level 
               
               
                 64 
               
               
                 Coded in 1 1 1 1 1 0 1 1 0 0 0 
               
               
                 Center of gravity 
               
               
                 4109 
               
             
             
               level 
               65 
               Coded in 1 1 0 1 0 1 1 1 0 0 0 
               Center of gravity 
               4273 
             
             
               level 
               66 
               Coded in 1 0 1 1 0 1 1 1 0 0 0 
               Center of gravity 
               4253 
             
             
               level 
               67 
               Coded in 0 1 1 1 0 1 1 1 0 0 0 
               Center of gravity 
               4215 
             
             
               
                 level 
               
               
                 68 
               
               
                 Coded in 1 1 1 1 0 1 1 1 0 0 0 
               
               
                 Center of gravity 
               
               
                 4162 
               
             
             
               level 
               69 
               Coded in 1 0 1 0 1 1 1 1 0 0 0 
               Center of gravity 
               4244 
             
             
               
                 level 
               
               
                 70 
               
               
                 Coded in 0 1 1 0 1 1 1 1 0 0 0 
               
               
                 center of gravity 
               
               
                 4209 
               
             
             
               level 
               71 
               Coded in 1 1 1 0 1 1 1 1 0 0 0 
               Center of gravity 
               4157 
             
             
               level 
               72 
               Coded in 0 1 0 1 1 1 1 1 0 0 0 
               Center of gravity 
               4183 
             
             
               level 
               73 
               Coded in 1 1 0 1 1 1 1 1 0 0 0 
               Center of gravity 
               4133 
             
             
               level 
               74 
               Coded in 1 0 1 1 1 1 1 1 0 0 0 
               Center of gravity 
               4117 
             
             
               level 
               75 
               Coded in 0 1 1 1 1 1 1 1 0 0 0 
               Center of gravity 
               4086 
             
             
               level 
               76 
               Coded in 1 1 1 1 1 1 1 1 0 0 0 
               Center of gravity 
               4040 
             
             
               level 
               77 
               Coded in 0 1 1 1 1 0 1 0 1 0 0 
               Center of gravity 
               4835 
             
             
               level 
               78 
               Coded in 1 1 1 1 1 0 1 0 1 0 0 
               Center of gravity 
               4780 
             
             
               level 
               79 
               Coded in 1 1 0 1 0 1 1 0 1 0 0 
               Center of gravity 
               4907 
             
             
               level 
               80 
               Coded in 1 0 1 1 0 1 1 0 1 0 0 
               Center of gravity 
               4882 
             
             
               level 
               81 
               Coded in 0 1 1 1 0 1 1 0 1 0 0 
               Center of gravity 
               4844 
             
             
               level 
               82 
               Coded in 1 1 1 1 0 1 1 0 1 0 0 
               Center of gravity 
               4791 
             
             
               level 
               83 
               Coded in 1 0 1 0 1 1 1 0 1 0 0 
               Center of gravity 
               4852 
             
             
               level 
               84 
               Coded in 0 1 1 0 1 1 1 0 1 0 0 
               Center of gravity 
               4815 
             
             
               level 
               85 
               Coded in 1 1 1 0 1 1 1 0 1 0 0 
               Center of gravity 
               4766 
             
             
               level 
               86 
               Coded in 0 1 0 1 1 1 1 0 1 0 0 
               Center of gravity 
               4780 
             
             
               level 
               87 
               Coded in 1 1 0 1 1 1 1 0 1 0 0 
               Center of gravity 
               4731 
             
             
               level 
               88 
               Coded in 1 0 1 1 1 1 1 0 1 0 0 
               Center of gravity 
               4711 
             
             
               level 
               89 
               Coded in 0 1 1 1 1 1 1 0 1 0 0 
               Center of gravity 
               4678 
             
             
               
                 level 
               
               
                 90 
               
               
                 Coded in 1 1 1 1 1 1 1 0 1 0 0 
               
               
                 Center of gravity 
               
               
                 4632 
               
             
             
               level 
               91 
               Coded in 1 1 1 1 0 1 0 1 1 0 0 
               Center of gravity 
               4988 
             
             
               level 
               92 
               Coded in 1 0 1 0 1 1 0 1 1 0 0 
               Center of gravity 
               5040 
             
             
               level 
               93 
               Coded in 0 1 1 0 1 1 0 1 1 0 0 
               Center of gravity 
               5005 
             
             
               level 
               94 
               Coded in 1 1 1 0 1 1 0 1 1 0 0 
               Center of gravity 
               4958 
             
             
               level 
               95 
               Coded in 0 1 0 1 1 1 0 1 1 0 0 
               Center of gravity 
               4969 
             
             
               level 
               96 
               Coded in 1 1 0 1 1 1 0 1 1 0 0 
               Center of gravity 
               4923 
             
             
               level 
               97 
               Coded in 1 0 1 1 1 1 0 1 1 0 0 
               Center of gravity 
               4903 
             
             
               level 
               98 
               Coded in 0 1 1 1 1 1 0 1 1 0 0 
               Center of gravity 
               4870 
             
             
               
                 level 
               
               
                 99 
               
               
                 Coded in 1 1 1 1 1 1 0 1 1 
               
               
                 Center of gravity 
               
               
                 4827 
               
             
             
               level 
               100 
               Coded in 1 1 1 0 1 0 1 1 1 0 0 
               Center of gravity 
               5010 
             
             
               level 
               101 
               Coded in 0 1 0 1 1 0 1 1 1 0 0 
               Center of gravity 
               5020 
             
             
               level 
               102 
               Coded in 1 1 0 1 1 0 1 1 1 0 0 
               Center of gravity 
               4976 
             
             
               level 
               103 
               Coded in 1 0 1 1 1 0 1 1 1 0 0 
               Center of gravity 
               4957 
             
             
               level 
               104 
               Coded in 0 1 1 1 1 0 1 1 1 0 0 
               Center of gravity 
               4926 
             
             
               
                 level 
               
               
                 105 
               
               
                 Coded in 1 1 1 1 1 0 1 1 1 0 0 
               
               
                 Center of gravity 
               
               
                 4884 
               
             
             
               level 
               106 
               Coded in 1 1 0 1 0 1 1 1 1 0 0 
               Center of gravity 
               4978 
             
             
               level 
               107 
               Coded in 1 0 1 1 0 1 1 1 1 0 0 
               Center of gravity 
               4959 
             
             
               level 
               108 
               Coded in 0 1 1 1 0 1 1 1 1 0 0 
               Center of gravity 
               4929 
             
             
               
                 level 
               
               
                 109 
               
               
                 Coded in 1 1 1 1 0 1 1 1 1 0 0 
               
               
                 Center of gravity 
               
               
                 4889 
               
             
             
               level 
               110 
               Coded in 1 0 1 0 1 1 1 1 1 0 0 
               Center of gravity 
               4934 
             
             
               
                 level 
               
               
                 111 
               
               
                 Coded in 0 1 1 0 1 1 1 1 1 0 0 
               
               
                 Center of gravity 
               
               
                 4905 
               
             
             
               level 
               112 
               Coded in 1 1 1 0 1 1 1 1 1 0 0 
               Center of gravity 
               4867 
             
             
               level 
               113 
               Coded in 0 1 0 1 1 1 1 1 1 0 0 
               Center of gravity 
               4876 
             
             
               level 
               114 
               Coded in 1 1 0 1 1 1 1 1 1 0 0 
               Center of gravity 
               4839 
             
             
               level 
               115 
               Coded in 1 0 1 1 1 1 1 1 1 0 0 
               Center of gravity 
               4822 
             
             
               level 
               116 
               Coded in 0 1 1 1 1 1 1 1 1 0 0 
               Center of gravity 
               4796 
             
             
               level 
               117 
               Coded in 1 1 1 1 1 1 1 1 1 0 0 
               Center of gravity 
               4760 
             
             
               level 
               118 
               Coded in 0 1 0 1 1 0 1 1 0 1 0 
               Center of gravity 
               5698 
             
             
               level 
               119 
               Coded in 1 1 0 1 1 0 1 1 0 1 0 
               Center of gravity 
               5655 
             
             
               level 
               120 
               Coded in 1 0 1 1 1 0 1 1 0 1 0 
               Center of gravity 
               5633 
             
             
               level 
               121 
               Coded in 0 1 1 1 1 0 1 1 0 1 0 
               Center of gravity 
               5600 
             
             
               level 
               122 
               Coded in 1 1 1 1 1 0 1 1 0 1 0 
               Center of gravity 
               5559 
             
             
               level 
               123 
               Coded in 1 1 0 1 0 1 1 1 0 1 0 
               Center of gravity 
               5634 
             
             
               level 
               124 
               Coded in 1 0 1 1 0 1 1 1 0 1 0 
               Center of gravity 
               5612 
             
             
               level 
               125 
               Coded in 0 1 1 1 0 1 1 1 0 1 0 
               Center of gravity 
               5581 
             
             
               level 
               126 
               Coded in 1 1 1 1 0 1 1 1 0 1 0 
               Center of gravity 
               5542 
             
             
               level 
               127 
               Coded in 1 0 1 0 1 1 1 1 0 1 0 
               Center of gravity 
               5576 
             
             
               level 
               128 
               Coded in 0 1 1 0 1 1 1 1 0 1 0 
               Center of gravity 
               5546 
             
             
               level 
               129 
               Coded in 1 1 1 0 1 1 1 1 0 1 0 
               Center of gravity 
               5507 
             
             
               level 
               130 
               Coded in 0 1 0 1 1 1 1 1 0 1 0 
               Center of gravity 
               5511 
             
             
               level 
               131 
               Coded in 1 1 0 1 1 1 1 1 0 1 0 
               Center of gravity 
               5473 
             
             
               level 
               132 
               Coded in 1 0 1 1 1 1 1 1 0 1 0 
               Center of gravity 
               5454 
             
             
               level 
               133 
               Coded in 0 1 1 1 1 1 1 1 0 1 0 
               Center of gravity 
               5426 
             
             
               
                 level 
               
               
                 134 
               
               
                 Coded in 1 1 1 1 1 1 1 1 0 1 0 
               
               
                 Center of gravity 
               
               
                 5390 
               
             
             
               level 
               135 
               Coded in 0 1 1 1 1 0 1 0 1 1 0 
               Center of gravity 
               5834 
             
             
               level 
               136 
               Coded in 1 1 1 1 1 0 1 0 1 1 0 
               Center of gravity 
               5795 
             
             
               level 
               137 
               Coded in 1 1 0 1 0 1 1 0 1 1 0 
               Center of gravity 
               5860 
             
             
               level 
               138 
               Coded in 1 0 1 1 0 1 1 0 1 1 0 
               Center of gravity 
               5839 
             
             
               level 
               139 
               Coded in 0 1 1 1 0 1 1 0 1 1 0 
               Center of gravity 
               5810 
             
             
               level 
               140 
               Coded in 1 1 1 1 0 1 1 0 1 1 0 
               Center of gravity 
               5773 
             
             
               level 
               141 
               Coded in 1 0 1 0 1 1 1 0 1 1 0 
               Center of gravity 
               5801 
             
             
               level 
               142 
               Coded in 0 1 1 0 1 1 1 0 1 1 0 
               Center of gravity 
               5773 
             
             
               level 
               143 
               Coded in 1 1 1 0 1 1 1 0 1 1 0 
               Center of gravity 
               5737 
             
             
               level 
               144 
               Coded in 0 1 0 1 1 1 1 0 1 1 0 
               Center of gravity 
               5738 
             
             
               level 
               145 
               Coded in 1 1 0 1 1 1 1 0 1 1 0 
               Center of gravity 
               5703 
             
             
               level 
               146 
               Coded in 1 0 1 1 1 1 1 0 1 1 0 
               Center of gravity 
               5684 
             
             
               level 
               147 
               Coded in 0 1 1 1 1 1 1 0 1 1 0 
               Center of gravity 
               5657 
             
             
               
                 level 
               
               
                 148 
               
               
                 Coded in 1 1 1 1 1 1 1 0 1 1 0 
               
               
                 Center of gravity 
               
               
                 5623 
               
             
             
               level 
               149 
               Coded in 1 1 1 1 0 1 0 1 1 1 0 
               Center of gravity 
               5833 
             
             
               level 
               150 
               Coded in 1 0 1 0 1 1 0 1 1 1 0 
               Center of gravity 
               5860 
             
             
               level 
               151 
               Coded in 0 1 1 0 1 1 0 1 1 1 0 
               Center of gravity 
               5833 
             
             
               level 
               152 
               Coded in 1 1 1 0 1 1 0 1 1 1 0 
               Center of gravity 
               5798 
             
             
               level 
               153 
               Coded in 0 1 0 1 1 1 0 1 1 1 0 
               Center of gravity 
               5799 
             
             
               level 
               154 
               Coded in 1 1 0 1 1 1 0 1 1 1 0 
               Center of gravity 
               5765 
             
             
               level 
               155 
               Coded in 1 0 1 1 1 1 0 1 1 1 0 
               Center of gravity 
               5747 
             
             
               level 
               156 
               Coded in 0 1 1 1 1 1 0 1 1 1 0 
               Center of gravity 
               5721 
             
             
               
                 level 
               
               
                 157 
               
               
                 Coded in 1 1 1 1 1 1 0 1 1 1 0 
               
               
                 Center of gravity 
               
               
                 5689 
               
             
             
               level 
               158 
               Coded in 1 1 1 0 1 0 1 1 1 1 0 
               Center of gravity 
               5799 
             
             
               level 
               159 
               Coded in 0 1 0 1 1 0 1 1 1 1 0 
               Center of gravity 
               5800 
             
             
               level 
               160 
               Coded in 1 1 0 1 1 0 1 1 1 1 0 
               Center of gravity 
               5768 
             
             
               level 
               161 
               Coded in 1 0 1 1 1 0 1 1 1 1 0 
               Center of gravity 
               5750 
             
             
               level 
               162 
               Coded in 0 1 1 1 1 0 1 1 1 1 0 
               Center of gravity 
               5725 
             
             
               
                 level 
               
               
                 163 
               
               
                 Coded in 1 1 1 1 1 0 1 1 1 1 0 
               
               
                 Center of gravity 
               
               
                 5694 
               
             
             
               level 
               164 
               Coded in 1 1 0 1 0 1 1 1 1 1 0 
               Center of gravity 
               5749 
             
             
               level 
               165 
               Coded in 1 0 1 1 0 1 1 1 1 1 0 
               Center of gravity 
               5732 
             
             
               
                 level 
               
               
                 166 
               
               
                 Coded in 0 1 1 1 0 1 1 1 1 1 0 
               
               
                 Center of gravity 
               
               
                 5708 
               
             
             
               level 
               167 
               Coded in 1 1 1 1 0 1 1 1 1 1 0 
               Center of gravity 
               5677 
             
             
               level 
               168 
               Coded in 1 0 1 0 1 1 1 1 1 1 0 
               Center of gravity 
               5702 
             
             
               level 
               169 
               Coded in 0 1 1 0 1 1 1 1 1 1 0 
               Center of gravity 
               5679 
             
             
               level 
               170 
               Coded in 1 1 1 0 1 1 1 1 1 1 0 
               Center of gravity 
               5649 
             
             
               level 
               171 
               Coded in 0 1 0 1 1 1 1 1 1 1 0 
               Center of gravity 
               5651 
             
             
               level 
               172 
               Coded in 1 1 0 1 1 1 1 1 1 1 0 
               Center of gravity 
               5621 
             
             
               level 
               173 
               Coded in 1 0 1 1 1 1 1 1 1 1 0 
               Center of gravity 
               5606 
             
             
               level 
               174 
               Coded in 0 1 1 1 1 1 1 1 1 1 0 
               Center of gravity 
               5584 
             
             
               level 
               175 
               Coded in 1 1 1 1 1 1 1 1 1 1 0 
               Center of gravity 
               5555 
             
             
               level 
               176 
               Coded in 1 1 0 1 1 1 0 1 1 0 1 
               Center of gravity 
               6512 
             
             
               level 
               177 
               Coded in 1 0 1 1 1 1 0 1 1 0 1 
               Center of gravity 
               6492 
             
             
               level 
               178 
               Coded in 0 1 1 1 1 1 0 1 1 0 1 
               Center of gravity 
               6465 
             
             
               level 
               179 
               Coded in 1 1 1 1 1 1 0 1 1 0 1 
               Center of gravity 
               6433 
             
             
               level 
               180 
               Coded in 1 1 1 0 1 0 1 1 1 0 1 
               Center of gravity 
               6525 
             
             
               level 
               181 
               Coded in 0 1 0 1 1 0 1 1 1 0 1 
               Center of gravity 
               6522 
             
             
               level 
               182 
               Coded in 1 1 0 1 1 0 1 1 1 0 1 
               Center of gravity 
               6490 
             
             
               level 
               183 
               Coded in 1 0 1 1 1 0 1 1 1 0 1 
               Center of gravity 
               6470 
             
             
               level 
               184 
               Coded in 0 1 1 1 1 0 1 1 1 0 1 
               Center of gravity 
               6445 
             
             
               level 
               185 
               Coded in 1 1 1 1 1 0 1 1 1 0 1 
               Center of gravity 
               6413 
             
             
               level 
               186 
               Coded in 1 1 0 1 0 1 1 1 1 0 1 
               Center of gravity 
               6458 
             
             
               level 
               187 
               Coded in 1 0 1 1 0 1 1 1 1 0 1 
               Center of gravity 
               6439 
             
             
               level 
               188 
               Coded in 0 1 1 1 0 1 1 1 1 0 1 
               Center of gravity 
               6414 
             
             
               level 
               189 
               Coded in 1 1 1 1 0 1 1 1 1 0 1 
               Center of gravity 
               6383 
             
             
               level 
               190 
               Coded in 1 0 1 0 1 1 1 1 1 0 1 
               Center of gravity 
               6402 
             
             
               level 
               191 
               Coded in 0 1 1 0 1 1 1 1 1 0 1 
               Center of gravity 
               6377 
             
             
               level 
               192 
               Coded in 1 1 1 0 1 1 1 1 1 0 1 
               Center of gravity 
               6347 
             
             
               level 
               193 
               Coded in 0 1 0 1 1 1 1 1 1 0 1 
               Center of gravity 
               6345 
             
             
               level 
               194 
               Coded in 1 1 0 1 1 1 1 1 1 0 1 
               Center of gravity 
               6315 
             
             
               level 
               195 
               Coded in 1 0 1 1 1 1 1 1 1 0 1 
               Center of gravity 
               6298 
             
             
               level 
               196 
               Coded in 0 1 1 1 1 1 1 1 1 0 1 
               Center of gravity 
               6275 
             
             
               
                 level 
               
               
                 197 
               
               
                 Coded in 1 1 1 1 1 1 1 1 1 0 1 
               
               
                 Center of gravity 
               
               
                 6246 
               
             
             
               level 
               198 
               Coded in 0 1 0 1 1 0 1 1 0 1 1 
               Center of gravity 
               6798 
             
             
               level 
               199 
               Coded in 1 1 0 1 1 0 1 1 0 1 1 
               Center of gravity 
               6766 
             
             
               level 
               200 
               Coded in 1 0 1 1 1 0 1 1 0 1 1 
               Center of gravity 
               6747 
             
             
               level 
               201 
               Coded in 0 1 1 1 1 0 1 1 0 1 1 
               Center of gravity 
               6722 
             
             
               level 
               202 
               Coded in 1 1 1 1 1 0 1 1 0 1 1 
               Center of gravity 
               6692 
             
             
               level 
               203 
               Coded in 1 1 0 1 0 1 1 1 0 1 1 
               Center of gravity 
               6732 
             
             
               level 
               204 
               Coded in 1 0 1 1 0 1 1 1 0 1 1 
               Center of gravity 
               6713 
             
             
               level 
               205 
               Coded in 0 1 1 1 0 1 1 1 0 1 1 
               Center of gravity 
               6689 
             
             
               level 
               206 
               Coded in 1 1 1 1 0 1 1 1 0 1 1 
               Center of gravity 
               6659 
             
             
               level 
               207 
               Coded in 1 0 1 0 1 1 1 1 0 1 1 
               Center of gravity 
               6675 
             
             
               level 
               208 
               Coded in 0 1 1 0 1 1 1 1 0 1 1 
               Center of gravity 
               6651 
             
             
               level 
               209 
               Coded in 1 1 1 0 1 1 1 1 0 1 1 
               Center of gravity 
               6622 
             
             
               level 
               210 
               Coded in 0 1 0 1 1 1 1 1 0 1 1 
               Center of gravity 
               6619 
             
             
               level 
               21 1 
               Coded in 1 1 0 1 1 1 1 1 0 1 1 
               Center of gravity 
               6590 
             
             
               level 
               212 
               Coded in 1 0 1 1 1 1 1 1 0 1 1 
               Center of gravity 
               6573 
             
             
               level 
               213 
               Coded in 0 1 1 1 1 1 1 1 0 1 1 
               Center of gravity 
               6550 
             
             
               
                 level 
               
               
                 214 
               
               
                 Coded in 1 1 1 1 1 1 1 1 0 1 1 
               
               
                 Center of gravity 
               
               
                 6522 
               
             
             
               level 
               215 
               Coded in 0 1 1 1 1 0 1 0 1 1 1 
               Center of gravity 
               6796 
             
             
               level 
               216 
               Coded in 1 1 1 1 1 0 1 0 1 1 1 
               Center of gravity 
               6767 
             
             
               level 
               217 
               Coded in 1 1 0 1 0 1 1 0 1 1 1 
               Center of gravity 
               6804 
             
             
               level 
               218 
               Coded in 1 0 1 1 0 1 1 0 1 1 1 
               Center of gravity 
               6786 
             
             
               level 
               219 
               Coded in 0 1 1 1 0 1 1 0 1 1 1 
               Center of gravity 
               6763 
             
             
               level 
               220 
               Coded in 1 1 1 1 0 1 1 0 1 1 1 
               Center of gravity 
               6735 
             
             
               level 
               221 
               Coded in 1 0 1 0 1 1 1 0 1 1 1 
               Center of gravity 
               6749 
             
             
               level 
               222 
               Coded in 0 1 1 0 1 1 1 0 1 1 1 
               Center of gravity 
               6727 
             
             
               level 
               223 
               Coded in 1 1 1 0 1 1 1 0 1 1 1 
               Center of gravity 
               6699 
             
             
               level 
               224 
               Coded in 0 1 0 1 1 1 1 0 1 1 1 
               Center of gravity 
               6696 
             
             
               level 
               225 
               Coded in 1 1 0 1 1 1 1 0 1 1 1 
               Center of gravity 
               6669 
             
             
               level 
               226 
               Coded in 1 0 1 1 1 1 1 0 1 1 1 
               Center of gravity 
               6652 
             
             
               level 
               227 
               Coded in 0 1 1 1 1 1 1 0 1 1 1 
               Center of gravity 
               6631 
             
             
               
                 level 
               
               
                 228 
               
               
                 Coded in 1 1 1 1 1 1 1 0 1 1 1 
               
               
                 Center of gravity 
               
               
                 6604 
               
             
             
               level 
               229 
               Coded in 1 1 1 1 0 1 0 1 1 1 1 
               Center of gravity 
               6737 
             
             
               level 
               230 
               Coded in 1 0 1 0 1 1 0 1 1 1 1 
               Center of gravity 
               6750 
             
             
               level 
               231 
               Coded in 0 1 1 0 1 1 0 1 1 1 1 
               Center of gravity 
               6728 
             
             
               level 
               232 
               Coded in 1 1 1 0 1 1 0 1 1 1 1 
               Center of gravity 
               6702 
             
             
               level 
               233 
               Coded in 0 1 0 1 1 1 0 1 1 1 1 
               Center of gravity 
               6699 
             
             
               level 
               234 
               Coded in 1 1 0 1 1 1 0 1 1 1 1 
               Center of gravity 
               6673 
             
             
               level 
               235 
               Coded in 1 0 1 1 1 1 0 1 1 1 1 
               Center of gravity 
               6657 
             
             
               level 
               236 
               Coded in 0 1 1 1 1 1 0 1 1 1 1 
               Center of gravity 
               6636 
             
             
               
                 level 
               
               
                 237 
               
               
                 Coded in 1 1 1 1 1 1 0 1 1 1 1 
               
               
                 Center of gravity 
               
               
                 6610 
               
             
             
               level 
               238 
               Coded in 1 1 1 1 1 1 0 1 1 1 1 
               Center of gravity 
               6680 
             
             
               level 
               239 
               Coded in 0 1 0 1 1 0 1 1 1 1 1 
               Center of gravity 
               6677 
             
             
               level 
               240 
               Coded in 1 1 0 1 1 0 1 1 1 1 1 
               Center of gravity 
               6652 
             
             
               level 
               241 
               Coded in 1 0 1 1 1 0 1 1 1 1 1 
               Center of gravity 
               6636 
             
             
               
                 level 
               
               
                 242 
               
               
                 Coded in 0 1 1 1 1 0 1 1 1 1 1 
               
               
                 Center of gravity 
               
               
                 6616 
               
             
             
               level 
               243 
               Coded in 1 1 1 1 1 0 1 1 1 1 1 
               Center of gravity 
               6591 
             
             
               
                 level 
               
               
                 244 
               
               
                 Coded in 1 1 0 1 0 1 1 1 1 1 1 
               
               
                 Center of gravity 
               
               
                 6625 
               
             
             
               level 
               245 
               Coded in 1 0 1 1 0 1 1 1 1 1 1 
               Center of gravity 
               6610 
             
             
               level 
               246 
               Coded in 0 1 1 1 0 1 1 1 1 1 1 
               Center of gravity 
               6590 
             
             
               level 
               247 
               Coded in 1 1 1 1 0 1 1 1 1 1 1 
               Center of gravity 
               6566 
             
             
               level 
               248 
               Coded in 1 0 1 0 1 1 1 1 1 1 1 
               Center of gravity 
               6579 
             
             
               level 
               249 
               Coded in 0 1 1 0 1 1 1 1 1 1 1 
               Center of gravity 
               6559 
             
             
               level 
               250 
               Coded in 1 1 1 0 1 1 1 1 1 1 1 
               Center of gravity 
               6535 
             
             
               level 
               251 
               Coded in 0 1 0 1 1 1 1 1 1 1 1 
               Center of gravity 
               6533 
             
             
               level 
               252 
               Coded in 1 1 0 1 1 1 1 1 1 1 1 
               Center of gravity 
               6510 
             
             
               level 
               253 
               Coded in 1 0 1 1 1 1 1 1 1 1 1 
               Center of gravity 
               6496 
             
             
               level 
               254 
               Coded in 0 1 1 1 1 1 1 1 1 1 1 
               Center of gravity 
               6477 
             
             
               
                 level 
               
               
                 255 
               
               
                 Coded in 1 1 1 1 1 1 1 1 1 1 1 
               
               
                 Center of gravity 
               
               
                 6454 
               
             
             
                 
             
           
        
       
     
   
   The sub-field code words for the GCC coding are also listed in the next table. 
   
     
       
             
           
             
             
             
             
             
           
         
             
                 
             
             
               GCC codes with their Centers of Gravity: 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               level 
               0 
               Coded in 0 0 0 0 0 0 0 0 0 0 0 
               Center of gravity 
               0 
             
             
               level 
               1 
               Coded in 1 0 0 0 0 0 0 0 0 0 0 
               Center of gravity 
               575 
             
             
               level 
               2 
               Coded in 0 1 0 0 0 0 0 0 0 0 0 
               Center of gravity 
               1160 
             
             
               level 
               4 
               Coded in 1 0 1 0 0 0 0 0 0 0 0 
               Center of gravity 
               1460 
             
             
               level 
               5 
               Coded in 0 1 1 0 0 0 0 0 0 0 0 
               Center of gravity 
               1517 
             
             
               level 
               8 
               Coded in 1 1 0 1 0 0 0 0 0 0 0 
               Center of gravity 
               1840 
             
             
               level 
               9 
               Coded in 1 0 1 1 0 0 0 0 0 0 0 
               Center of gravity 
               1962 
             
             
               level 
               14 
               Coded in 1 1 1 0 1 0 0 0 0 0 0 
               Center of gravity 
               2297 
             
             
               level 
               16 
               Coded in 1 1 0 1 1 0 0 0 0 0 0 
               Center of gravity 
               2420 
             
             
               level 
               17 
               Coded in 1 0 1 1 1 0 0 0 0 0 0 
               Center of gravity 
               2450 
             
             
               level 
               23 
               Coded in 1 1 1 1 0 1 0 0 0 0 0 
               Center of gravity 
               2783 
             
             
               level 
               26 
               Coded in 1 1 1 0 1 1 0 0 0 0 0 
               Center of gravity 
               2930 
             
             
               level 
               28 
               Coded in 1 1 0 1 1 1 0 0 0 0 0 
               Center of gravity 
               2955 
             
             
               level 
               37 
               Coded in 1 1 1 1 1 0 1 0 0 0 0 
               Center of gravity 
               3324 
             
             
               level 
               41 
               Coded in 1 1 1 1 0 1 1 0 0 0 0 
               Center of gravity 
               3488 
             
             
               level 
               44 
               Coded in 1 1 1 0 1 1 1 0 0 0 0 
               Center of gravity 
               3527 
             
             
               level 
               45 
               Coded in 0 1 0 1 1 1 1 0 0 0 0 
               Center of gravity 
               3582 
             
             
               level 
               58 
               Coded in 1 1 1 1 1 1 0 1 0 0 0 
               Center of gravity 
               3931 
             
             
               level 
               64 
               Coded in 1 1 1 1 1 0 1 1 0 0 0 
               Center of gravity 
               4109 
             
             
               level 
               68 
               Coded in 1 1 1 1 0 1 1 1 0 0 0 
               Center of gravity 
               4162 
             
             
               level 
               70 
               Coded in 0 1 1 0 1 1 1 1 0 0 0 
               Center of gravity 
               4209 
             
             
               level 
               90 
               Coded in 1 1 1 1 1 1 1 0 1 0 0 
               Center of gravity 
               4632 
             
             
               level 
               99 
               Coded in 1 1 1 1 1 1 0 1 1 0 0 
               Center of gravity 
               4827 
             
             
               level 
               105 
               Coded in 1 1 1 1 1 0 1 1 1 0 0 
               Center of gravity 
               4884 
             
             
               level 
               109 
               Coded in 1 1 1 1 0 1 1 1 1 0 0 
               Center of gravity 
               4889 
             
             
               level 
               111 
               Coded in 0 1 1 0 1 1 1 1 1 0 0 
               Center of gravity 
               4905 
             
             
               level 
               134 
               Coded in 1 1 1 1 1 1 1 1 0 1 0 
               Center of gravity 
               5390 
             
             
               level 
               148 
               Coded in 1 1 1 1 1 1 1 0 1 1 0 
               Center of gravity 
               5623 
             
             
               level 
               157 
               Coded in 1 1 1 1 1 1 0 1 1 1 0 
               Center of gravity 
               5689 
             
             
               level 
               163 
               Coded in 1 1 1 1 1 0 1 1 1 1 0 
               Center of gravity 
               5694 
             
             
               level 
               166 
               Coded in 0 1 1 1 0 1 1 1 1 1 0 
               Center of gravity 
               5708 
             
             
               level 
               197 
               Coded in 1 1 1 1 1 1 1 1 1 0 1 
               Center of gravity 
               6246 
             
             
               level 
               214 
               Coded in 1 1 1 1 1 1 1 1 0 1 1 
               Center of gravity 
               6522 
             
             
               level 
               228 
               Coded in 1 1 1 1 1 1 1 0 1 1 1 
               Center of gravity 
               6604 
             
             
               level 
               237 
               Coded in 1 1 1 1 1 1 0 1 1 1 1 
               Center of gravity 
               6610 
             
             
               level 
               242 
               Coded in 0 1 1 1 1 0 1 1 1 1 1 
               Center of gravity 
               6616 
             
             
               level 
               244 
               Coded in 1 1 0 1 0 1 1 1 1 1 1 
               Center of gravity 
               6625 
             
             
               level 
               255 
               Coded in 1 1 1 1 1 1 1 1 1 1 1 
               Center of gravity 
               6454 
             
             
                 
             
           
        
       
     
   
   A further reduction of this sub-set of m video levels can be advantageous in order to optimise the linearity of the response characteristic. E.g. the two video levels  44  and  45  are very close together but their code words differ in three bit positions. This can result in a different perception of the video levels on the human eye, more different than necessary from the bare video level values. Therefore, it is reasonable to further decimate the m video levels and take either video level  44  or  45  for the sub-field coding. Once the video levels (V i , 0&lt;= i&lt;m) of the sub-set of video levels have been chosen, the image picture has to be encoded with these levels. A circuit implementation of this process is shown in FIG.  15 . In the first block, the input video data coded on 8 bit standard binary code needs to be applied to a Degamma function. This is because the PDP has a linear response characteristic whereas the CRT displays rather have a quadratic response characteristic to the beam intensity. This is well-known in the art and for that reason at the video source, for example in the studio or in the camera itself, the video signal is Gamma corrected so that the picture seen by the human eye via a CRT display will get the correct brightness impression. These pre-corrected pictures are broadcast and in the TV receivers the pictures are automatically displayed with the correct linear response because of the gamma function-like response characteristic of the picture tubes. The human eye will observe the correct colour impressions. The Degamma function will be applied to the input video data in block  100 . In block  100  also a resealing task is performed. This means that the Degamma data due to calculation accuracy being 16 bit data words is rescaled to the range between 0 and m, where m is the number of levels used during GCC coding. However, each video level V i  of the set of m levels needs to be represented with 3 bits of precision. In case that m is equal to 37 as in the example above, 6 bits are required to differentiate between these levels. However, as every level shall be represented with 3 bits of precision, in total 9 bits, are output from the Degamma and resealing look-up table in block  100 . In decimal values the output values will have the form X.0; X.125; X.25; . . . X.875; X+1.0. In the next block  200  three dithering bits are added to the input values. 
   Dithering is a well-known technique for increasing the grey level resolution. With dithering, some artificial levels are added in between the existing video levels. This improves the grey scale portrayal but on the other hand adds high frequency low amplitude dithering noise, which is perceptible to the human viewer only at a small viewing distance. A full description of the dithering technique, which also is adapted to the PDP technology is known from the further European Patent Application of the applicant 00250099.9. For the disclosure of the dithering technique it is, therefore, expressively referred also to this patent application. The resulting 9 bit data words are truncated in block  200  to the final bit resolution for the 37 video levels. The final bit resolution is 6 bits and, therefore, 3 bits are truncated after addition of 3 dithering bits. 
   The final 6 bit video data is input to an optional video coding look-up table in block  300 . This look-up table is used to assign to each of the 37 video levels the corresponding correct 8 bit video level. This is done in order to leave the sub-field coding unit relatively unchanged. With this structure it is possible to implement the GCC coding according to the invention completely on the video level-processing block. Of course, in the sub-field coding unit which follows block  300 , there needs to be a corresponding sub-field coding look-up table, which assigns to each of the output video levels the correct GCC code word for addressing the plasma display panel. In an alternative embodiment block  300  can be omitted and the 6 bit output video data in block  200  can be directly input into the sub-field encoding unit if the sub-field encoding unit shall be designed in new form. This is not necessary in case of the first above-mentioned embodiment. 
   In  FIG. 16  a circuit implementation of the invention is illustrated. Input R, G, B video data is forwarded to Degamma unit  100  and a dither evaluation unit  500 . The Degamma unit  100  performs the 16 bit Degamma function and resealing and delivers 9 bit video data R, G, B at the output. The dither evaluation unit  500  computes the dithering numbers DR for red, DG for green and DB for the blue colour component. To do that it requires the Sync-signals HV to determine which pixel is currently processed and which line and frame number is valid. A full description of how the dithering numbers are calculated and what dithering pattern is used is contained in above-mentioned EP application of the applicant. In block  200  the resulting dithering numbers and the Degamma output values are added and the 3 least significant bits of the result are truncated so that the final output values are R, G and B are achieved. These values are forwarded to a sub-field coding unit  400 , which performs sub-field coding under control of control unit  900 . The sub-field code words are read out of the look-up table  410  in sub-field coding unit  400  preferably. The sub-field code words are forwarded to a memory unit  600 . The control unit  900  also controls reading and writing from and to this memory unit. For plasma display addressing, the sub-field code words are read out of the memory device and all the code words for one line are collected in order to create a single very long code word, which can be used for the line-wise PDP addressing. This is carried out in the serial to parallel conversion unit  700 . The control unit  900  generates all scan and sustain pulses for PDP control. It receives vertical and horizontal synchronising signals for reference timing. 
   The invention can be used in particular in PDPs. Plasma displays are currently used in consumer electronics, e.g. for TV sets and also for a monitor for computers. However, use of the invention is also appropriate for matrix displays where the light generation is also controlled with small pulses in sub-fields, i.e. where the PWM principle is used for controlling light generation.