Picture motion measurement

Abstract: A region from two frames of a television picture is subjected to Fourier transform in processing stages (3 and 4) one via a picture memory (2), and the resulting transforms compared in a comparator (5) whose output is subjected to inverse Fourier transform in a processing stage (6). The output of comparator (5) is also applied to a sample rate modifier (7) in which this output is enlarged prior to being subjected to inverse Fourier transform in a further processing stage (8). The outputs of processing stages (6 and 8) are respectively low and higher correlation functions of one pixel and one-quarter pixel resolution respectively. A given number, say 20, of sample points of greatest magnitude in the low resolution correlation function are selected in a largest vector selector (9) whose coordinates are used in a peak locator (10) which locates the peaks in the higher resolution correlation function associated with the selected samples to produce coordinates for these peaks to quarter pixel accuracy. A limited number of these, say 10, can be selected by a vector limiter (11) if used with a system having limited data capacity for motion vectors.

Description: The present invention relates to a method of picture motion measurement and in particular to a method in which two pictures are correlated to determine correlation as a function of selective displacement. The invention also relates to apparatus for use with such a method. A method of the above type is described in GB-A-2,188,510 (corresponding to WO-A-8705769) which is particularly concerned with the generation of motion vectors for television pictures. In the field of High Definition Television (HDTV) such motion vectors are conveyed with a television signal and are used to produce at a receiver an enhanced display either by assisting in producing additional lines to those in the received signal or in producing additional fields between those in the received signal. In order to improve the resolution at the display it is preferable if the location of the motion vectors can be identified to sub-pixel accuracy. It is an object of the present invention to provide a method of the above type and apparatus in which the locations and hence resulting motion vectors can be identified to sub-sample accuracy. To this end, the invention according to a first aspect provides a method of picture motion measurement as defined in claim 1. Such a method has the advantage that it is much easier to identify the position of samples of greatest magnitude in a lower resolution correlation function before identifying the locations of the peaks in the higher resolution correlation function. An apparatus for picture motion measurement according to a second aspect of the invention is defined in claim 6. Advantageous embodiments are defined in the dependent claims. The method as additionally characterised in claim 2 is particularly important when there are more peaks in the correlation function representing motion vectors than it is possible to transmit due to system capacity. Where the method is used with a high definition television system it may be further characterised as defined in claim 3. The above and other features of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of apparatus for use with the invention, and Figure 2 is a correlation surface of the same region from adjacent frames of a television picture. Figure 1 shows a block diagram of apparatus which may be used for performing the method of the present invention. Such apparatus would in general be located with the signal processing equipment at the transmission side in the case of television signals but it is also possible, though less economic, for such apparatus to be located in television receiving apparatus. Figure 1 the reference 1 indicates an input terminal to which a luminance signal is applied either frame by frame or region by region. This luminance signal is applied to a picture memory 2 in which the luminance signal is delayed by a frame period which delayed luminance output is applied to a processing stage 3 where it is subjected to a two dimensional Fourier transform. The luminance signal at input terminal 1 is also applied directly to a second processing stage 4 in which the undelayed luminance is similarly subjected to a two dimensional Fourier transform. The resulting transformed information is applied to a phase subtractor 5 which establishes the difference in phase between its two inputs and applies this to a third processing stage 6 whose output is the inverse Fourier transform. This output is a correlation function as illustrated by the typical correlation surface as shown in Figure 2 though in practice it would not appear as a continuous correlation surface but as a series of discrete samples of given magnitudes spaced one sample interval apart. The output of processor stage 6 is a low resolution correlation function where the sample interval is one pixel. It is assumed that the correlation surface of Figure 2 is derived from a region of a television picture which is 32 pixels by 32 lines and thus the displacemtnt in the X and Y directions of Figure 2 run from -16 to +16 sample positions. Zero displacement represents lack of movement and the peaks located away from the zero displacement position indicate varying degrees of movement in the region. The output of phase subtractor 5 is also applied to a sample modifier 7 which enlarges this output and this may be achieved by by the technique known as "padding with zeros". The resulting enlarged output from modifier 7 is applied to a fourth processing stage 8 whose output is the inverse Fourier transform of the enlarged signal from modifier 7 and is an enlarged correlation function which would be similar to the correlation surface of Figure 2 with the peaks more accurately defined. As for processing stage 6, the correlation function produced by processing stage 8 is not a continuous surface but a series of discrete samples of given magnitudes though in this case these samples are spaced one quarter of a pixel apart and is thus a higher resolution correlation function than that from processing stage 6. The low resolution correlation function from processing stage 6 is applied to a largest samples locator 9 which selects a given number, say 20, of the samples of greatest magnitude from the low resolution correlation function and identifies the locations of these selected samples. The locations for these 20 samples are applied to a peak locator 10 which also receives the higher reslution correlation function from the processing stage 8. Peak locator 10 identifies the corresponding samples in the higher resolution correlation function using the coordinates provided by the sample locator 9 and using the positions for these 20 samples searches for the locations of the higher resolution peaks associated with these corresponding samples. This can be achieved by making a "steepest ascent" search in which starting from a selected corresponding sample point the gradients between its adjacent quarter pixel interval samples are determined until the steepest gradient is established. This is repeated until the surrounding quarter interval sample points are lower in magnitude when the peak is located to quarter pixel accuracy. The coordinates for the peaks so found and their magnitudes may be applied to a vector limiter 11 in which up to a limited number, say 10, of the coordinates of the peaks of greatest magnitude are selected from those produced by the peak locator 10, this being preferable with television systems having a limited data capacity for the transmission of motion vectors. These coordinates thus form the limited number of motion vectors for a region which are applied to an output 12. Although in the above the method of the invention is described as being carried out by apparatus units which have specified functions, it will be realised that the method may be performed by a microcomputer under dedicated software control. In addition, the correlation functions may be produced by a method other than that described such as optimal matching of the corresponding regions. From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of apparatus and component parts thereof and which may be used instead of or in addition to features already described herein.

Claims: A method of picture motion measurement for the production of motion vectors in which two pictures are correlated to determine correlation as a function of selective displacement, said method being characterised by the steps of: i) correlating (1-6) two pictures to determine a first resolution correlation as a function of displacement thereby to determine first sample correlation values to a first resolution, ii) correlating (1-5, 7, 8) said two pictures on the basis of an enlarged number of samples to determine a second resolution correlation as a function of displacement thereby to determine second sample correlation values to a higher resolution than said first resolution, iii) determining (9) the positions of a given number of sample points of greatest magnitude in said first resolution correlation function, and iv) applying (10) the positions found for said given number of sample points to said higher resolution correlation function to identify the corresponding sample points in said higher resolution correlation function, and to determine the locations of peaks associated with the said number of sample points in said higher resolution correlation function which locations are defined to sub-sample interval accuracy at said higher resolution. A method as claimed in Claim 1, characterised in that said method additionally comprises selecting (11) from the locations of the peaks so found up to a limited number, which limited number is less than said given number, of said locations whose coordinates form said motion vectors. A method as claimed in Claim 1 or 2, characterised in that sample points in said first resolution correlation function have an interval of one pixel whilst sample points in said higher resolution correlation function have an interval of one quarter of a pixel. A method as claimed in Claim 1, 2 or 3, characterised in that the locations of the peaks in said higher resolution correlation function are found by examining the gradients between adjacent higher resolution sample points starting from the said given number of corresponding sample points. A method as claimed in Claim 1, characterised in that said step (i) of correlating (1-6) said two pictures to determine said first sample correlation values includes the steps of fourier transforming (3, 4) a first picture and said first picture delayed over a picture period to furnish fourier transformed pictures, phase subtracting (5) said fourier transformed pictures to furnish a phase difference picture, and inverse fourier transforming (6) said phase difference picture to furnish said first sample correlation values, and in that said step (ii) of correlating (1-5, 7, 8) said two pictures on the basis of said enlarged number of samples to determine said second sample correlation values includes the steps of fourier transforming (3, 4) said first picture and said first picture delayed over a picture period to furnish said fourier transformed pictures, phase subtracting (5) said fourier transformed pictures to furnish said phase difference picture, sample modifying (7) said phase difference picture to furnish a phase difference picture at said enlarged number of samples, and inverse fourier transforming (8) said phase difference picture at said enlarged number of samples to furnish said second sample correlation values. Apparatus for picture motion measurement in which two pictures are correlated to determine correlation as a function of selective displacement, characterised in that said apparatus comprises: means (1-6) for correlating two pictures to determine first resolution correlation as a function of displacement thereby to determine first sample correlation values to a first resolution, means (1-5, 7, 8) for correlating said two pictures on the basis of an enlarged number of samples to determine a second resolution correlation as a function of displacement thereby to determine second sample correlation values to a higher resolution than said first resolution, means (9) for determining the positions of a given number of sample points of greatest magnitude in said first resolution correlation function, and means (10) for applying the positions found for said given number of sample points to said higher resolution correlation function to identify the corresponding sample points in said higher resolution correlation function, to determine the locations of peaks associated with the said number of sample points in said higher resolution correlation function which locations are defined to sub-sample interval accuracy at said higher resolution. Apparatus as claimed in Claim 6, characterised in that it further comprises means (11) for selecting from the locations of the peaks so formed up to a limited number, which limited number is less than said given number, of said locations whose coordinates form the motion vectors. Apparatus as claimed in Claim 6, characterised in that said means (1-6) for correlating said two pictures to determine said first sample correlation values include means (3, 4) for fourier transforming a first picture and said first picture delayed over a picture period to furnish fourier transformed pictures, means (5) for phase subtracting said fourier transformed pictures to furnish a phase difference picture, and means (6) for inverse fourier transforming said phase difference picture to furnish said first sample correlation values, and in that said means (1-5, 7, 8) for correlating said two pictures on the basis of said enlarged number of samples to determine said second sample correlation values include said means (3, 4) for fourier transforming said first picture and said first picture delayed over a picture period to furnish said fourier transformed pictures, said means (5) for phase subtracting said fourier transformed pictures to furnish said phase difference picture, means (7) for sample modifying said phase difference picture to furnish a phase difference picture at said enlarged number of samples, and means (8) for inverse fourier transforming said phase difference picture at said enlarged number of samples to furnish said second sample correlation values.

Inventors: FERNANDO GERARD MARIUS XAVIER, FERNANDO, GERARD MARIUS XAVIER
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS UK LTD, PHILIPS ELECTRONICS N.V., PHILIPS ELECTRONICS UK LIMITED
Application year: 1989
Main classification: H04N5
IPC codes: G06T7, H04N5, H04N7, H04N17