Abstract:
A method and apparatus are provided for motion compensated video interpolation. Each field or frame in a sequence of video images is subdivided into a plurality of blocks and a motion vector field is derived using block matching for a current video field using data matched to a previous video field or frame. A first time instance is determined at which an interpolated block is to be displayed and a second time instance is determined at which a corresponding interpolated block is to be created. Video data for each block is interpolated at its second time instance for each block and is then output for display at the first time instance.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to motion compensated video interpolation systems, and in particular to methods and apparatus associated with reducing the visibility of artefacts in motion compensated video interpolation. 
       BACKGROUND TO THE INVENTION 
       [0002]    Video sequences typically comprise a series of non interlaced frames of video data, or a series of interlaced fields of video data. Interlaced sequences are produced by fields which carry data on alternate lines of a display, such that a first field will carry data for alternate lines, and a second field will carry data for the missing lines. The fields are thus spaced both temporally and spatially. Every alternate field in a sequence will carry data at the same spatial locations. A pair of interlaced field may be referred to as a frame and a non-interlaced field may also be referred to as a frame. Embodiment of this invention will be described in terms of a non-interlaced system, but the invention is equally applicable to systems processing either interlaced or non-interlaced video data. 
         [0003]    Identification of motion in video sequences is well known and results in a motion vector for each pixel, or group of pixels that describes the motion between one frame and another. A well-known motion estimation method based on block matching techniques will be used to illustrate the invention, although other types of motion estimator would be equally suitable. Block based motion estimation methods generally consider two consecutive frames from a video sequence and subdivide them into multiple regions known as blocks or macroblocks. In a motion search procedure, each block is compared with pixel data from various candidate locations in the previous frame. The relative position of the best match gives a vector that describes the motion in the scene at that block position. Collectively, the set of motion vectors at each block position in a frame is known as the motion vector field for that frame. Note that use of the term “vector field” should not be confused with the use of “field” or “video field” to describe the data in an interlaced video sequence. 
         [0004]      FIG. 1  illustrates a typical example of a block matching motion estimator. In all figures, including  FIG. 1 , motion vectors are shown with the head of the arrow at the centre of the block to which the vector corresponds. The frames are divided into blocks, and an object  101  in the previous frame has moved to position  102  in the current frame. The previous position of the object is shown superimposed on the current frame as  103 . Motion estimation is performed for blocks rather than for objects, where a block of pixels in the current frame is matched with a block sized pixel area in the previous frame which is not necessarily block aligned. For example, block  104  is partially overlapped by the moving object  102 , and has contents as illustrated at  105 . Motion estimation for block  104 , if it performs well, will find the pixel data area  106  in the previous frame, which can also be seen to contain the pixels illustrated in  105 , i.e. a good match has been found. Superimposed back onto the current frame, the matching pixel data area is at  107 . The motion vector associated with block  104  is therefore as illustrated by arrow  108 . 
         [0005]    Many block based motion estimators select their output motion vector by testing a set of motion vector candidates with a method such as a sum of absolute differences (SAD) or mean of squared differences (MSD), to identify motion vectors which give the lowest error block matches.  FIG. 2  illustrates the candidate evaluation process for the block  201  in the current frame which has pixel contents shown in  211 . In this simple example system, three motion vector candidates  206 ,  207  and  208  are considered which correspond to candidate pixel data areas at locations  202 ,  203  and  204  in the previous frame. The pixel contents of these pixel data areas can be seen in  212 ,  213  and  214  respectively. It is apparent that the pixel data at location  202  provides the best match for block  201  and should therefore be selected as the best match/lowest difference candidate. Superimposed back onto the current frame, the matching pixel data area is at  205  and the associated motion vector is  206 . 
         [0006]    Different systems have different requirements of the motion estimation. In a video encoder, the requirement is to form the most compact representation of a frame, by reference to a previous frame from the sequence. The requirement is generally to find motion vectors which give the lowest error block matches, and while the resulting motion vectors are usually representative of the actual motion of objects in the scene, there is no requirement that this is always the case. In other applications, such as de-interlacing or frame rate conversion, it is more important that the motion vectors represent the true motion of the scene, even if other distortions in the video mean that the block matches do not always give the lowest error. By applying appropriate constraints to the candidate motion vectors during motion search, the results can be guided towards “lowest error” or “true motion” as necessary. 
         [0007]    Frame rate conversion systems are also well known. In such a system, a sequence of input fields or frames at an input frame rate is changed to a different output frame rate. Conventionally this is done by repeating and/or dropping frames until the desired output frame rate is achieved. An alternative method that results in smoother output motion is called motion compensated frame interpolation. In a motion compensated frame interpolation system, motion estimation is used to determine motion vectors that represent the true motion of objects in a scene and these motion vectors are used to create additional frames in which moving objects are represented at the correct intermediate positions. We describe the temporal position of a frame, with respect to the original frames of the input sequence, as the time instance of the frame. For a frame rate doubling application, where an interpolated frame is required at the temporal midpoint between two input frames at t=−1 and t=0, the time instance of the interpolated frame is at t=−0.5. 
         [0008]    Motion compensated frame interpolation systems require the creation of intermediate frames between the two input frames. The temporal position at which each interpolated frame must be output is known from the input and output frame rates and is called the “ideal time instance” of the frame. In a conventional motion compensated frame interpolation system, each interpolated frame is created using the ideal time instance to determine the intermediate positions of objects in the scene. 
         [0009]    A block diagram of a frame rate conversation system is shown in  FIG. 15 . The system comprises a video input  1501  which goes to a memory  1502  to store the input frame history and also to a motion estimation unit  1503  which performs motion estimation by comparing the current input frame from the video input  1501  with a previous frame from the memory  1502 . Motion vectors are sent to an interpolation unit  1504  which constructs an interpolated frame from the input frames and provides a video output  1507 . Knowing the motion vectors allows the interpolation unit  1504  to place pixels such that objects in the interpolated frame appear in the appropriate positions according to the trajectory of their motion. A timing control unit  1505  calculates the ideal time instances for the interpolated output frames. An image analysis unit  1506  may also analyse the input frame data to detect events such as scene changes and cross-fades where motion estimation is known to struggle. In these situations it is preferable to set the ideal time instances to t=−1 and thereby repeat the original input frame. 
         [0010]      FIG. 3  illustrates an example frame rate conversion system where three input frames are used to create an output sequence with double the number of output frames. Output  1  can be seen to be a direct copy of the frame provided as Input  1 . Output  2  must be created at the temporal midpoint between the Input  1  time instance and the Input  2  time instance. Output  3  can be seen to be a direct copy of the frame provided as Input  2 . Output  4  must be created at the temporal midpoint between the Input  2  time instance and the Input  3  time instance. Output  5  can be seen to be a direct copy of the frame provided as Input  3 . 
         [0011]      FIG. 4  illustrates an example frame rate conversion system where 3 input frames are used to create an output sequence with 2.5× the number of output frames. Output  1  can be seen to be a direct copy of the frame provided as Input  1 . Output  2  must be created at a time instance ⅖ of the way between Input  1  and Input  2 . Output  3  must be created at a time instance ⅘ of the way between Input  1  and Input  2 . Input  2  is not shown in the output sequence. Output  4  must be created at a time instance ⅕ of the way between Input  2  and Input  3 . Output  5  must be created at a time instance ⅗ of the way between Input  2  and Input  3 . Output  6  can be seen to be a direct copy of the frame provided as Input  3 . 
         [0012]    Various schemes exist for creating each block of pixels in an interpolated output frame. Where motion estimation has identified a perfect match between areas of pixel data in the input frames, interpolation may be as simple as copying pixels from one of the input frames to a position in the interpolated frame that is determined by the motion vector and the time instance of the interpolated frame. Where the match identified by motion estimation is not perfect, it is necessary to generate the block of pixels in the interpolated output frame from some combination of the pixel data from the input frames. Detailed description of appropriate methods is beyond the scope of this application, but will be known to those skilled in the art. A simple method suitable for illustrating the concept of this invention is the use of a weighted blend between the motion compensated pixel data from the two adjacent input frames. 
         [0013]      FIG. 5  illustrates a weighted blend interpolation system. Each pixel value in an interpolated pixel area can be considered to be an interpolation between the pixel values from the corresponding locations in the pixel data areas at the motion compensated positions in the adjacent input frames. In  FIG. 5 , an interpolation between a black pixel (A) from the previous input frame and a white pixel (B) from the current input frame is required. The interpolated pixel at an intermediate time instance between the input frames can be created by an appropriate weighted blend between the black pixel and the white pixel. A suitable weighting function is to use the fractional time instance of the interpolated frame relative to the time instances of the input frames. For example, if the interpolated frame&#39;s time instance is located one quarter of the temporal distance between the previous input frame (containing pixel A) and the current input frame (containing pixel B), the interpolated pixel should use a 75% contribution from pixel A and a 25% contribution from pixel B, i.e. a ¾:¼ blend between the pixel A and pixel B. If the interpolated pixel was created at the midpoint between the two frames, an equal contribution from each pixel would be used, i.e. a ½:½ blend between the pixel A and pixel B. 
         [0014]    The quality of interpolated frames produced by motion compensated frame interpolation is largely dependent upon the performance of the motion estimator. In situations where the motion estimator performs badly, and produces non-true motion vectors, interpolation using these poor vectors causes artefacts. Poor motion vectors may arise due to complex or erratic motion, objects being occluded/revealed between input frames, aliasing, or objects being scaled or rotated so that they cannot be described by a simple translation of pixels from one area of the screen to another. 
         [0015]      FIG. 6  illustrates a conventional motion compensated frame interpolation system that creates output frames at double the input frame rate. In this example, the previous input frame&#39;s time instance is t=−1 and the current input frame&#39;s time instance is t=0, i.e. 100% of the input frame time interval. The motion vector that describes the motion of the object from position  601  to position  602  during this interval is shown as  603 . Other vectors in the motion vector field have been omitted for clarity. As interpolated frames must be output at the time instance halfway between the input frame time instances position, i.e. at 50% of the input frame time interval, the ideal time instance is known to be t=−0.5 and the object&#39;s position at that time instance can be interpolated at a position half way along the motion vector, as shown at  604 . The pixel data required to represent the object in the block sized pixel area  614  is created by interpolating between the block sized pixel area  611  (contents shown as  611 -A) and the block of pixels  612  (contents shown as  612 -A). The 50% blend result determined by the ideal time instance, between  611 -A and  612 -A is shown as  614 -A. This method works well when the motion vector describing the movement of the object over an input frame interval accurately describes the true motion of an object. The resulting objects are smoothly interpolated with linear position between their positions in the previous input frame to their positions in the current input frame. 
         [0016]      FIG. 7  illustrates a conventional motion compensated frame interpolation system that creates output frames at double the input frame rate in a case where motion estimation has performed poorly. In this example, the previous input frame&#39;s time instance is t=−1 and the current input frame&#39;s time instance is t=0. The motion vector that describes the true motion of the object from position  701  to position  702  during this interval could not be accurately determined by the motion estimator (for some reason). In place of the true motion vector, the motion estimator introduced the non-true motion vectors shown as  703  and  704 . Again, other vectors in the motion vector field have been omitted for clarity. As interpolated frames must be output at the time instance halfway between the input frames the ideal time instance is t=−0.5. The object will therefore be interpolated at positions half way along the motion vectors  703  and  704 . Interpolating along the motion vector  703 , a block sized pixel area  715  is created by interpolating between the block sized pixel area  711  (containing the object pixels as shown in  711 -A) and the block of pixels  722  (containing the background pixels as shown in  722 -A). The result of the 50% blend between block sized pixel areas  711 -A and  722 -A is shown as  715 -A as a ½:½ blend between the object and the background pixels. Interpolating along the motion vector  704 , a block sized pixel area  716  is created by interpolating between the block sized pixel area  721  (containing the background pixels shown in  721 -A) and the block of pixels  712  (containing the object pixels shown in  712 -A). The result of the 50% blend between block sized pixel areas  721 -A and  712 -A is shown as  716 -A as a ½:½ blend between the object and the background pixels. The interpolated frame now includes two interpolated versions of the original object, with a different opacity to the object in the input frames, and located in the wrong position. Interpolation using non-true motion vectors therefore causes significant visual artefacts in interpolated frames. 
         [0017]      FIG. 8  illustrates how motion compensated frame interpolation can be used to generate two intermediate frames between a pair of input frames, at time instances other than the midpoint between the two input frames. This is suitable for an application such as 2.5× frame rate conversion. Object  801  in the previous frame has moved to position  811  in the current frame. The previous object position is superimposed on the interpolated frames as  821  and  831 , and the current position of the object is superimposed on the interpolated frames as  824  and  834 . Motion estimation has performed well, and the motion vector describing the motion of the object between input frames is shown as  826  and  836 . In order to increase the frame rate it is necessary to create interpolated frames at fractional points between the time instance of the previous input frame and the time instance of the current input frame. In the example shown of the first span in a 2.5× frame rate conversion, it is necessary to create interpolated frames at ⅖ and ⅘ of the time instance span (i.e. t=−0.6 and t=−0.2). Assuming linear interpolation of object position it can be expected that in the first interpolated frame, the object will have moved ⅖ of the distance along the path defined by the motion vector  826  at the ⅖ time instance between the input frames. The interpolated block sized area of pixels at position  823  in the first interpolated frame can therefore be created by interpolation between the block sized pixel area  802  and the pixels in block  812  with a blend of ⅗ from the previous input frame and ⅖ from the current input frame. Similarly, assuming linear interpolation of object position, it can be expected that in the second interpolated frame the object will have moved ⅘ of the distance along the path defined by the motion vector  836  at the ⅘ time instance between the input frames. The interpolated block sized area of pixels at position  833  in the second interpolated frame can therefore be created by interpolation between the block sized pixel area  802  and the pixels in block  812  with a blend of ⅕ from the previous input frame and ⅘ from the current input frame. 
         [0018]    To improve visual quality of motion compensated interpolation systems it is desirable to minimize the visibility of artefacts that arise in regions of poor motion estimation. 
       SUMMARY OF THE INVENTION 
       [0019]    Preferred embodiments of the present invention provide apparatus and methods associated with reducing the visibility of artefacts in motion compensated frame interpolation by varying the temporal position used in the creation of the interpolated frame relative to the temporal position at which it is displayed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Preferred embodiments of the invention will now be described in detail by way of reference to the accompanying drawings in which: 
           [0021]      FIG. 1  illustrates a typical prior art block matching motion estimator; 
           [0022]      FIG. 2  illustrates a prior art comparison between candidate motion vectors; 
           [0023]      FIG. 3  illustrates the required interpolation time instances for a 2× frame rate conversion; 
           [0024]      FIG. 4  illustrates the required interpolation time instances for a 2.5× frame rate conversion; 
           [0025]      FIG. 5  illustrates an example prior art pixel interpolation system using weighted blending between source pixels. 
           [0026]      FIG. 6  illustrates prior art object interpolation for a 2× frame rate conversion using true motion vectors. 
           [0027]      FIG. 7  illustrates prior art object interpolation for a 2× frame rate conversion using non-true motion vectors. 
           [0028]      FIG. 8  illustrates prior art object interpolation for 2.5× frame rate conversion using true motion vectors. 
           [0029]      FIG. 9  illustrates the required interpolation time instances and working time instances for a 2× frame rate conversion with a working scale factor of ½. 
           [0030]      FIG. 10  illustrates the required interpolation time instances and working time instances for a 4× frame rate conversion with a working scale factor of ½ using various methods of time instance shifting. 
           [0031]      FIG. 11  illustrates object interpolation for a 2× frame rate conversion using non-true motion vectors with the varying temporal interpolation method applied. 
           [0032]      FIG. 12  shows a comparison of frame interpolation artefacts before and after the application of the varying temporal interpolation method. 
           [0033]      FIG. 13  illustrates an example determination of the working scale factor based on the performance of the motion estimator. 
           [0034]      FIG. 14  illustrates an example determination of the working scale factor based on the performance of the motion estimator and a clamping scale factor. 
           [0035]      FIG. 15  is the block diagram of a conventional motion compensated frame rate conversion system. 
           [0036]      FIG. 16  is the block diagram of a motion compensated frame rate conversion system modified to include varying temporal interpolation. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0037]    Embodiments of this invention provide a modification to motion compensated frame interpolation systems that is capable of reducing the visibility of artefacts in interpolated frames. In a conventional motion compensated frame interpolation system, the time instance at which an output frame is output in the sequence is used to determine the position of the objects in the interpolated scene. It is proposed that while an interpolated frame should still be output at the time instance at which it is required (the “ideal time”), a second “working time instance” may be used to assist with calculation of the position of objects in the screen. Conveniently, the working time instance may be defined as the ideal time instance modified by a “working scale factor”. This brings the time instance at which the frame is interpolated closer to that of an input frame. This should reduce the effect of any erratic motion vectors and provide an output with fewer artefacts which can then be used at the ideal time. 
         [0038]      FIG. 9  illustrates an example frame rate conversion system where three input frames are used to create an output sequence with the output frame rate being twice the input frame rate. Output  1  can be seen to be a direct copy of the frame provided as Input  1 . Output  2  must be output at the ideal time instance t=−0.5 which is half way between the Input  1  time instance and the Input  2  time instance. However, the working time instance for interpolation of Output  2  is scaled back from the ideal time instance by a working scale factor of 0.5. If we assume, for example, a scaling function that determines a working time instance by multiplying the ideal time instance by the working scale factor, this results in the output frame being interpolated as if it was at a time instance one quarter of the way between the Input  1  time instance and the Input  2  time instance. The working time instance for Output  2  is therefore t=−0.75. Output  3  can be seen to be a direct copy of the frame provided as Input  2 . Output  4  must be output at the ideal time instance t=0.5 which is half way between the Input  2  time instance and the Input  3  time instance. The working time instance for interpolation of Output  4  is scaled back from the ideal time instance by a working scale factor of 0.5. This results in the output frame being interpolated as if it was at a time instance one quarter of the way between the Input  2  time instance and the Input  3  time instance. The working time instance for Output  4  is therefore t=0.25. Output  5  can be seen to be a direct copy of the frame provided as Input  3 . 
         [0039]    In the above example, the working scale factor is always used to shift the working time instance towards a previous input frame&#39;s time instance. It is also possible to apply the method to shift the working time instance towards a current input frame&#39;s time instance or to shift the working time instance towards a nearest input frame&#39;s time instance. 
         [0040]      FIG. 10  illustrates how a working scale factor of 0.5 may be used to shift the working time instances towards the time instance of the input frames using various methods of time instance shifting. The example frame rate conversion system shown interpolates between two input frames (at t=−1 and t=0) to create an output sequence with four times the number of output frames. The first and last output frames can be seen to be direct copies of the frames provided as Input  1  and Input  2  respectively and are displayed at the input frame time instances. The ideal time instances of the interpolated output frames are at t=−0.75, t=−0.5 and t=−0.25. With a working scale factor of 0.5 and time instances shifted towards the previous input frame, the working time instances can be calculated by multiplying the ideal time instance by the working scale factor. This gives working time instances sequentially located at temporal positions t=−0.825, t=−0.75, and t=−0.625. The largest temporal distance between an input frame time instance and an interpolated frame time instance is 0.375 of the input frame period and the largest interval between consecutive working time instances is 0.625 of the input frame period. 
         [0041]    Instead of simply being calculated as the product of time instance and working scale factor, the working time instance can be calculated so that the working time instances are shifted towards the input frame that is nearest to the interpolated frame&#39;s ideal time instance. The working time instances can now be seen to be sequentially located at temporal positions t=−0.875, t=−0.75, and t=−0.125. The largest temporal distance between an input frame time instance and an interpolated frame time instance is now reduced to 0.25 of the input frame period while the largest interval between consecutive working time instances remains at 0.625. As will be discussed, a reduced largest temporal distance between an input frame time instance and an interpolated frame time instance results in less visible artefacts in the interpolated frame. Also, by maintaining the same interval between consecutive working time instances the perceived smoothness of the output video sequence is unaffected. 
         [0042]      FIG. 11  shows the example previously illustrated in  FIG. 7  with the introduction of a working scale factor of 0.5 to shift the working time instances towards the previous frame. In this example, the previous input frame&#39;s time instance is t=−1 and the current input frame&#39;s time instance is t=0. The motion vector describing the true motion of an object from position  1101  to position  1102  during this interval could not be accurately determined by the motion estimator (for some reason). In place of the true motion vector, the motion estimator generated the non-true motion vectors shown as  1103  and  1104 . Other vectors in the motion vector field have been omitted for clarity. An ideal time instance of t=−0.5 and a working scale factor of 0.5 give rise to a working time instance of t=−0.75 for the interpolated frame. In order to interpolate at this working time instance, the object must be interpolated at a position one quarter of the way along the motion vectors  1103  and  1104 . Interpolating along the motion vector  1103 , a block sized pixel area  1115  is created by interpolating between the block sized pixel area  1111  (containing the object pixels shown in  1111 -A) and the block of pixels  1122  (containing the background pixels shown in  1122 -A). The blended result between  1111 -A and  1122 -A is determined by the working time instance to use 25% from the previous input frame and 75% from the current input frame. The result is shown as  1115 -A as a ¾:¼ blend between the object and the background pixels. Interpolating along the motion vector  1104 , a block sized pixel area  1116  is created by interpolating between the block sized pixel area  1121  (containing the background pixels shown in  1121 -A) and the block of pixels  1112  (containing the object pixels shown in  1112 -A). The blended result between  1121 -A and  1112 -A is determined by the working time instance to use 75% from the previous input frame and 25% from the current input frame. This result is shown as  1116 -A as a ¼:¾ blend between the object and the background pixels. The interpolated frame now includes two interpolated versions of the original object, with different opacity to the object in the input frames, and located in the wrong position. Interpolation using non-true motion vectors therefore still causes visual artefacts in interpolated frames. However, the artefacts that arise after a working scale factor is applied to change the relative visibility of the object at different time instances are significantly less visible than the artefacts introduced by interpolating at the ideal time instance. 
         [0043]    Conveniently, the introduction of a working scale factor that shifts the working time instance closer to the input frames biases the interpolated frame to look more like the input frame that it is shifted towards. In areas where poor motion estimation occurs this is often a better alternative than interpolating at the ideal time instance, since the quality of the output image will be improved. 
         [0044]      FIG. 12  illustrates the differences in artefact visibility using conventional motion compensated interpolation methods and using the modified method described here. In this example, the output frame rate is 3× the input frame rate, requiring two interpolated frames at ideal time instances ⅓ and ⅔ of the time instance between source frames.  FIG. 12-A  shows the input frames.  FIG. 12-B  shows the perfect result where a true motion vector is used to interpolate the object at the ideal time instances. 
         [0045]      FIG. 12-C  shows a possible result of using incorrect or non-true motion vectors to interpolate objects at the ideal time instances. The incorrect/non true motion vectors are the same as shown in  FIG. 11 . These motion vectors give rise to multiple interpolations of the object in the wrong locations. The interpolations of the object are generated from a blend between the object pixels and the background pixels with an opacity determined by the ideal time instance of the interpolated frames as previously shown in  FIG. 7 . The interpolated object to the left of interpolated frame # 1  is created by a ⅔:⅓ weighted blend between the object pixels and the background pixels due to the ideal time instance of t=⅓. The interpolated object to the right of interpolated frame # 1  is created by a ⅔:⅓ weighted blend between the background pixels and the object pixels due to the ideal time instance of t=⅓. Similarly, the interpolated object to the left of interpolated frame # 2  is created by a ⅓:⅔ weighted blend between the object pixels and the background pixels due to the ideal time instance of t=⅔. The interpolated object to the right of interpolated frame # 1  is created by a ⅓:⅔ weighted blend between the background pixels and the object pixels due to the ideal time instance of t=⅔. These interpolated pixels have a relatively high visibility at their incorrect locations. 
         [0046]      FIG. 12-D  shows the improved result of using the same non-true motion vectors as shown in  FIG. 12-C , in a system using a working scale factor of 0.5 to shift the working time instance of interpolated frames toward the input frame nearest to the interpolated frame&#39;s ideal time instance. The working time instance of interpolated frame # 1  is therefore t=⅙, calculated from the ideal time instance of t=⅓, and the working time instance of interpolated frame # 2  is t=⅚, calculated from the ideal time instance of t=⅔. In this modified system, the interpolated frames show blended interpolations of the object with the blended interpolations closest to the original positions of the object in the source frames being more visible. Conversely, the blended interpolations furthest from the original positions of the object are less visible. The interpolated object to the left of interpolated frame # 1  is created by a ⅚:⅙ weighted blend between the object pixels and the background pixels due to the working time instance of t=⅙. Similarly, the interpolated object to the right of interpolated frame # 1  is created by a ⅚:⅙ weighted blend between the background pixels and the object pixels. Interpolated frame # 2  is created in the same way, using ⅙:⅚ weighted blends. 
         [0047]    While two blended interpolations of the object are still visible in incorrect locations, the working time instance interpolation causes interpolated objects which are close to the location of the original source frame objects (and therefore close to the true motion path of the object) to have increased visibility, while interpolated objects far from the location of the original source frame objects (and therefore far from the true motion path of the object) to have their visibility decreased. 
         [0048]    In the examples so far, a working scale factor has been provided explicitly as a constant which affects all frames and all parts of frames equally. While the embodiments as described so far serve to reduce the visibility of artefacts in regions of poor motion estimator performance, it may also reduce the smoothness of interpolated video in regions where the motion estimator does perform well. In practice significant improvements in the smoothness of interpolated video can still be observed, and a satisfactory compromise between interpolation performance and artefact reduction can generally be found, even with a constant working scale factor. 
         [0049]    For further improvements in performance it is proposed that the working scale factor may also be determined by the performance of the motion estimator either globally or locally within a frame. A wide range of metrics may be used to determine how well a motion estimator is performing. For example, for a global measure of motion estimator performance we may count the number of blocks for which the SAD value of the best match found by the motion estimator was above a certain threshold. Essentially, if the motion estimator is considered to be performing well then a working scale factor close to 1.0 may be used. As the quality of motion estimation decreases, likely due to complex motions, the vector field produced becomes less reliable and a lower working scale factor may be desirable to reduce the visibility of artefacts. The limiting case occurs when a working scale factor of 0.0 is applied and the system will repeat input frames to produce the correct number of frames in the output sequence. This frame repeat case provides a safe, artefact-free state for very poor motion estimation but will lose the benefits afforded by motion compensated frame interpolation. The ability of the proposed scheme to perform a smooth transition from ideal motion compensated frame interpolation to frame repetition is a desirable feature. 
         [0050]      FIG. 13  illustrates a suitable method of determining a working scale factor value from the performance of the motion estimator. In this example the proportion of the vectors in the vector field that are considered to give a poor match between source frame pixels is used to generate a single working scale factor metric for every block in the frame. Between 0% poor vectors in the frame and a user defined threshold A % poor vectors in the frame, the working scale factor is set as 1.0. In this region, the performance of the motion estimator is considered good and therefore the working time instance should not be scaled back from the ideal time instance. In the region from a second user defined threshold B % poor vectors in the frame to 100% poor vectors in the frame, the performance of the motion estimator is considered unacceptable and therefore it is safer to output repeated frames than to use the motion compensated interpolated frames. This is achieved by setting the working scale factor to 0.0. In the region between user defined thresholds A % poor vectors in the frame and B % poor vectors in the frame, the motion estimator is considered to be unreliable and a working scale factor between 1.0 and 0.0 is required. This simple example could readily be modified to use, for example, an alternative motion estimation performance metric, a non-linear ramping function, multiple thresholds with different ramp functions, etc. 
         [0051]    In  FIG. 13 , the working scale factor was varied between 0.0 and 1.0. A useful modification to this system is to specify constant scale factors as clamping values for the upper and lower bounds.  FIG. 14  illustrates an extension to the method of determining a working scale factor from the performance of the motion estimator and from an explicitly defined upper bound clamping scale factor.  1401  shows the function for determining a scale factor based on the performance of a motion estimator as previously shown in  FIG. 13. 1402  shows an explicitly defined upper bound clamping scale factor of approximately 0.5.  1403  shows the final working scale factor function as the product of the two scale factors. Between 0% poor vectors in the frame and a user defined threshold A % poor vectors in the frame, the working scale factor is set to the upper bound clamping scale factor. In the region from a second user defined threshold B % poor vectors in the frame to 100% poor vectors in the frame, the performance of the motion estimator is considered unacceptable and therefore it is safer to output repeated frames than to use the motion compensated interpolated frames. In the region between user defined thresholds A % poor vectors in the frame and B % poor vectors in the frame, the motion estimator is considered to be unreliable and a working scale factor between the upper bound clamping scale factor and the lower bound clamping scale factor is required. The introduction of clamping scale factors to the function has, in this example, been used to shift the working time instances used in interpolation towards the input frame time instances, i.e. the system is more conservative and will have fewer visual artefacts. It may also be desirable to apply a lower bound clamping scale factor to ensure the system always introduces some amount of smoothing to the output frames. 
         [0052]    Conveniently, the working scale factor may be derived from a combination of explicitly defined clamping scale factors and from the performance of the motion estimator. The final working scale factor may be selected as the minimum/maximum/average of the two values or may be calculated by an arithmetic function. Suitable arithmetic functions may include, but are not limited to, the product or the sum of the two scale factors. 
         [0053]    A block diagram of a frame rate conversation system modified according to this invention is shown in  FIG. 16 . The system comprises a video input  1601  which goes to a memory  1602  to store the input frame history and also to a motion estimation unit  1603  which performs motion estimation by comparing the current input frame from the video input  1601  with a previous frame from the memory  1602 . Motion vectors are sent to an interpolation unit  1604  which constructs an interpolated frame from the input frames and provides a video output  1607 . An ideal timing control unit  1605  calculates the ideal time instances for the interpolated output frames. An image and error analysis unit  1606  may monitor the performance of the motion estimator for use in the working scale factor calculation to determine how well it is performing either on a block by block basis or a whole picture basis. An output dependant on the motion estimation is provided to a working scale factor calculation  1608 . This uses the received signal to determine a working scale factor which is then used by the working time instance control unit  1609  to provide an adjusted interpolation time to be used by the interpolation unit  1604 . The unit  1608  may determine the working scale factor between clamped upper and lower values.