Patent Application
20050068465
The present invention relates to an image processing device arranged to perform a motion compensation technique on received input signals, comprising an adapter for adapting the input signals to create images, a motion compensator for compensating the images for motion, a motion estimator for estimating motion vectors using the input signals, the motion vectors being used by the motion compensator.
To increase the number of grey levels in Plasma Display Panels (PDP's), the so-called “subfield driven method” may be used. An image frame is shown in a number of successive periods called subfields. During a subfield, an amount of light is emitted which is dependent on the weight of the subfield. Each subfield has a different weight. A desired intensity level for a pixel in the image is realized by controlling the specific subfields. The human eye sees the sum of the intensity levels of the enabled subfields within a field (i.e. an image) due to the integrating character of the human eye. In this way a subfield driven method using for example 8 subfields can display a maximum of 28 halftone levels. A well-known problem of PDPs using the subfield method is the occurrence of motion artefacts like false contouring and blur. To decrease motion artefacts, PDP systems may use motion compensation, as, e.g., described in copending Philips application having application number EP 01202410.5 [internal Philips reference number PHNL 010407], not published before the filing date of the present application.
Apart from motion compensation, in PDPs motion estimation may be used, e.g., in situations where received 50 Hz images are to be converted into 100 Hz images. Then, between any two received consecutive images one additional image need be calculated. To that end, the images are divided into blocks of a predetermined number of pixels, e.g., 8×8 pixels. For every block, a motion estimator determines a speed and direction of movement, with which the block is moving on a screen displaying the images, resulting in motion vectors. The additional images are then calculated using the motion vectors per block as determined and displayed between two received images.
However, when combining the “subfield driven method” with the motion estimation for PDPs, motion estimation vectors for all subfields per frame need to be calculated. If a frame comprises, e.g., 8 subfields instead of one, seven more motion vectors per frame need to be calculated.
Assuming that the human eye moves with a moving object on the screen, motion compensation may result in one or more subfields of a frame being displayed on different pixels as determined by the estimated motion vectors per subfield. Then, the human eye will receive all image data related to the same frame and correctly integrate all subfields to see the proper grey level related to the frame concerned.
Thus before displaying an image, the (luminances of) subfields are moved in space and time, using the motion vectors, to make the human eye experience the correct luminance at the correct pixel on the screen. In this way motion artefacts are reduced considerably.
However, motion vector estimation is not fee from errors. For instance, a block of 8×8 pixels may be part of a moving large object having one color. Then, adjacent to this block there are several “equal” blocks and it may be very difficult for the motion estimator to estimate the motion of the block concerned since it may be difficult to identify the block between its adjacent blocks that look the same and may have the same motion vectors. Then, it may happen that a motion estimator estimates a block to have a certain speed whereas the speed is in reality much lower. Applying a motion compensation in such a case may result in a decrease of the image quality. The reverse may also be true, i.e., an estimated speed is much lower than the real speed. Then, motion compensation also results in a decrease of image quality, however, less than in the first situation. Errors in the motion estimation may result in (unexpected) decrease of image quality.
In addition, at situations when the human eye is not tracking a moving object correctly, the calculated motion vectors differ from the motion of the eye. This also results in lower perceived quality.
It is an object of the present invention to provide an arrangement and a method for driving a display screen using motion compensation whereby motion artefacts are reduced when either the estimated motion vectors are incorrect or the human eye is not tracking the motion of an object on the screen.
To obtain the object, the invention as defined at the outset is characterized in that the image processing device further comprises an adjustor for adjusting the motion vectors before feeding them to the motion compensator, the adjustor being arranged to multiply the motion vectors by a reduction factor, the reduction factor being a positive value less than one.
In such a device the chance of having a too high decrease in image quality due to a wrong motion estimation is significantly reduced while still having most advantages of motion compensation known from the prior art.
It is observed that U.S. Pat. No. 5,175,618 discloses a compression method for moving picture signals, where motion vectors are estimated and reduced before using them. This reduction relates to the conversion between motion vectors for a frame to motion vectors for a field.
Furthermore, the invention relates to a display arrangement, comprising an image processing device as defined above, and a display for receiving output signals from the motion compensator. The invention also relates to a method for driving an image processing device comprising:
The invention also relates to a computer program product to be loaded by a digital image processing device, the computer program product providing the device with the capacity of:
Finally, the invention relates to a data carrier provided with such a computer program product.
Below, the invention will be explained with reference to some drawings, which are intended for illustration purposes only and not to limit the scope of protection as defined in the accompanying claims.
In
The invention will first be explained with reference to
In
However, when the motion estimator 5 has generated a larger or lower estimated motion vector speed vemv the SMSE value increases since the estimated motion vector speed vemv is wrong and the motion compensation as performed by adjustor 6 is based on a wrong motion vector speed The larger the difference between the tracking speed vtrack and the estimated motion vector speed vemv, the larger is the SMSE.
A further observation of
When vtrack=2 pixels/frame and vemv=2 pixels/field, SMSE=4.0. However, when vemv increases to about 2.75 pixels/field SMSE does not substantially increase and remains 4.0. Thus, when an estimated motion vector speed vemv would be multiplied by a factor R of 0.7-0.8 to render a reduced estimated motion vector R.vemv, when the estimated motion vector vemv is 2.75 pixels/field whereas the real motion vector speed should be 2.0 pixels/field (assumed to be equal to vtrack), and this reduced estimated motion vector R. vemv would be used by the motion compensator 3 to perform motion compensation, SMSE would not change.
Moreover, starting with a correct estimated motion vector speed vemv=2.0 pixels/field and then multiplying by R=0.7-0.8, would render an increase in SMSE. However, the increase is no more than about 10% and still very acceptable. Therefore, in accordance with the main idea of the invention, the estimated motion vector speed vemv is reduced by a factor R, where 0<R<1. This can be implemented by the adjustor 6 in
It is observed that for other values of vtrack than 2 pixels/frame (and, thus, for other motion vector speeds of blocks tracked by the human eye) similar figures as
However, in a more sophisticated embodiment, a quality factor F(q) depending on an estimated quality factor F(q) depending on an estimated quality level q for each motion vector can be taken into account. Then, for each motion vector, the motion estimator estimates a quality level. This can be done using any (known) technique to estimate a quality level for a motion vector. When using block matching, this can be done by examining the difference between a current block of pixels in a current image and a block of pixels in the last image before the current image that looks most similar to the current block of pixels. Blocks of pixels having the best match relate most probably to the same block. One can use, e.g., a “sum of absolute difference” (called “SAD”) as an error value for the match. SAD is known as such and can be calculated fast. The smaller the value of SAD, the more reliable is the motion estimation. The quality level q of the motion estimation may be derived from such an SAD. However, alternatively, quality level q may be calculated for objects within an image or for an entire image. Moreover, quality level q may depend on the number of subfields used per subfield or the values of the subfields. E.g., certain subfield values result in lower quality levels q than others depending on the number of bit changes to be made when the value changes only slightly. In a preferred embodiment, the quality levels q are sent to the adjustor 6. In the adjustor 6, the motion vectors are adjusted by way of multiplying them by the reduction factor R and by the quality factor F(q). The quality F(q) is a function that increases with the quality level of a motion vector. The function is limited between zero and one. So, if the quality of an estimated motion vector in poor, the factor F(q) will be low, and if the quality of an estimated motion vector is perfect, the factor F(q) will be approximately equal to one. Investigations have shown that R.F(q)=0.3 may be a preferred lower limit for R.F(q) when q is very low.
It will be evident to persons skilled in the art that, instead of multiplying each motion vector with R.F(q), where q depends on the motion vector concerned, each motion vector may be multiplied by one function taking both R and q into account. It will be understood that the arrangement of
Moreover, all of the boxes shown in digital image processing device 1 may be implemented as one computer with a memory storing proper instructions and data to perform the desired functions. As persons skilled in the art know, such a memory may comprise one or more of the following units: RAM, ROM, EEPROM, hard disc, etc.
Thus, where in the claims reference is made to units as shown in
It is observed that the invention can advantageously in all kinds of digital image systems where a subfield driven method is combined with a motion compensation technique, and is, therefore, not restricted to the field of PDPs.
Furthermore it is observed that the invention can be applied in other kinds of systems, where the motion compensation is executed for other reasons. For example, the invention may be used for scan rate conversion where interpolated images have to be calculated between the input images in order to increase the frame rate.
| Number | Date | Country | Kind |
|---|---|---|---|
| 012050480 | Dec 2001 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB02/05165 | 12/5/2002 | WO |
