The invention relates to a method that allows random access to picture blocks in a video picture, for example a television picture.
In image processing, it is often necessary to have recourse to picture areas of contiguous or adjacent pixels in order to subject the areas to a comparison of the picture contents or a filter function. Video pictures are generally composed of a sequence of successive pixels. The data of the pixels are transmitted in such a way that a picture line starts to be built up only when the preceding picture line is completely present. In the case of television pictures transmitted using the line interlacing method, a first line (for example the second line) of a first field is displayed before the second line of this field (in that case the fourth line in the above example) is built up.
The fact that the video picture is built up line by line in this way results in that pixels have to be buffer-stored if pixels of a picture column are intended to be accessed directly in succession or simultaneously. The next pixel with respect to a specific pixel in the vertical direction of the picture is available only after a delay in the case of the sequential data transmission described. In the case of buffer-storage, arbitrary access to picture blocks, that is to say a group of pixels in horizontal and vertical proximity is possible.
Dynamic random access memories (DRAMs) are suitable for buffer-storage since they have a high memory density relative to the chip area. In dynamic semiconductor memories, the memory cells are usually disposed in a plurality of memory array block units. In the event of an access to a specific memory cell, the memory array block unit containing this cell must first be activated. If two pixels which are intended to be accessed virtually simultaneously lie in different memory array block units, then first one unit and then the other must be activated in order to read out the respective pixel. At a high pixel transmission rate, real-time image processing is then not possible. Access to two pixels which are disposed in two different memory array block units is complicated particularly when these pixels have to be multiply accessed for an image processing operation, which is often the case.
It is accordingly an object of the invention to provide a method for random access to picture blocks in video pictures which overcomes the above-mentioned disadvantages of the prior art methods of this general type, which can be carried out with only little additional outlay in respect of memory.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for random access to picture blocks in a video picture composed of lines composed of pixels. The method includes the steps of:
storing a first excerpt of a sub-picture in a first area of an access memory, the first excerpt is dimensioned such that it encompasses a main picture block having x pixels in a horizontal direction of the sub-picture and also at least one adjacent picture block adjoining it in the horizontal direction and having equivalent dimensions;
reading out one of the main picture block and the at least one adjacent picture block from the access memory;
storing a portion of the sub-picture which adjoins the first excerpt horizontally in a second area of the access memory while a picture block is being read out, the portion being chosen such that it contains x pixels in the horizontal direction and, in a vertical direction, as many of the pixels as the first excerpt has in the vertical direction; and
storing a second excerpt of the sub-picture having equivalent dimensions as the first excerpt in the first area of the access memory, the second excerpt being shifted horizontally relative to the first excerpt such that it completely contains the portion of the sub-picture stored in the second area.
The invention has the advantage that, with the access memory having a small size in relation to the number of pixels in the video picture, image processing, for example of television pictures, is possible in real time. With regard to the access time, more stringent requirements are imposed on the access memory than on the picture memory. On account of the comparatively low capacity of the access memory, the method according to the invention can nevertheless be carried out cost-effectively. Each individual pixel can be accessed within the first area of the access memory.
A further advantage is that the ratio of the data rate at the output of the access memory and that at its input is determined by the size of the main picture blocks. Consequently, the ratio of the data rate to the size of the access memory can be adapted to the given circumstances.
In a preferred embodiment, the second area of the access memory is three times as large as that of the main picture block. The first area of the access memory preferably contains the main picture block and also eight adjacent picture blocks adjoining it and having the same size as the main picture block. Once the image processing has been performed for the main picture block, the adjacent picture block adjoining the right-hand side of the previous main picture block becomes the new main picture block. At the end of the line of the video picture, the new main picture block emerges from the adjacent picture block that adjoins the bottom side of the main picture block with its position at the start of the line. The main picture block typically migrates in the horizontal direction from left to right across the video picture. If the main picture block has reached the end of the video picture, it is offset vertically downward by the height of the main picture block and returned to the left-hand edge of the video picture in this line.
It has proved to be expedient for the access memory to be embodied such that a picture having 48 pixels in the horizontal direction and 24 pixels in the vertical direction can be stored in its first area and a picture having 16 pixels in the horizontal direction and likewise 24 pixels in the vertical direction can be stored in its second area.
Static semiconductor memories (SRAMs) are especially suitable as embodiments of the access memory. They allow short access times. Their large area requirement relative to the memory capacity is of no consequence since only a fraction of the pixels of the video picture is stored in the access memory.
In accordance with an added mode of the invention, there is the step of providing the first excerpt with at least one further adjacent picture block adjoining the main picture block in the vertical direction and having equivalent dimensions as the main picture block. At least some of the pixels are read out from the second excerpt from the access memory. Subsequently, a third excerpt of the sub-picture is stored in the first area of the access memory. The third excerpt being shifted vertically relative to the first excerpt such that the main picture block still lies within the first excerpt.
In accordance with an additional mode of the invention, there is the step of forming the main picture block to be rectangular and the main picture block is surrounded by eight adjoining adjacent picture blocks.
In accordance with another mode of the invention, there is the step of setting the first area of the access memory to have 48 pixels in the horizontal direction and 24 pixels in the vertical direction and the second area is set with 16xc3x9724 pixels.
In accordance with a further mode of the invention, there is the step of embodying the access memory as a static random access memory (SPAM).
In accordance with a concomitant feature of the invention, the video picture is a field of a line-interlaced television picture.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for random access to picture blocks in video pictures, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.