Abstract:
A programmable spatial filter system  30  for a video signal includes a position extraction block  33  arranged to extract a position in an image of a picture element to be spatially filtered. A programmable mask generator  34  receives output from the position extraction block and generates a selectable filter mask dependent on the extracted position. A programmable spatial filter  31  filters the image using the selected filter mask from the programmable mask generator  34.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to image processing and in particular to processing of image border portions for improved image compression performance, using a programmable spatial filter system.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is well known that the human visual system (HVS) has a reduced sensitivity to spatial resolution toward a periphery of a field of vision of a human eye. This is due to a variation in a density of rods and cones across a retina.  
         [0003]     Furthermore, many television receivers use glass cathode ray tubes (CRTs) to display an image using electron beam impingent upon a phosphor screen. It has long been the practice initially to set up these receivers to scan over the edge of the screen to minimize an effect of aging of circuitry of the display which causes a reduction in deflection of the beam.  
         [0004]     Both these considerations have lead to a practice of quantizing the periphery of an image more coarsely, or harshly, than a central region during video compression for television transmission, since the peripheral portion of the image is rarely actually displayed to the viewer due to over scan, and in any case the viewer&#39;s resolution has less acuity at the periphery of the viewing field of vision.  
         [0005]     However, this practice leads to blocking, a noticeable artifact readily detected by the HVS, not only in the region harshly quantized but also into the central region, because motion prediction for spatial redundancy uses parts of this periphery in predicting the central parts of the image.  
         [0006]     Another factor affecting this practice is a gradual change from use of CRT displays towards use of alternative display technologies such as plasma and LCD displays. These displays allow a complete transmitted image to be viewed since the display matrix is of a fixed resolution and size and no reduction in beam deflection, and therefore scan size, occurs during a life of the display.  
         [0007]     There is therefore a desire to gain advantage from properties of the HVS, while preventing harsh, unwanted artifacts on modern screens.  
         [0008]     GB 0609154.0 discloses an image pre-processing stage in which a degree of filtering is linked to occupancy of an encoder output buffer, immediately prior to image compression. This linkage causes a reduction in spatial bandwidth as the buffer level rises in order to assist in keeping a system stable and within its operating margins.  
         [0009]     In this former disclosure, a degree of filtering may vary across the image proportionally to a distance from a centre of a screen. However, preferably a central portion would benefit from no processing while the border portion can be filtered rather more harshly. In addition, no allowance for source image content is made which limits the success of the system in removing detail without introducing unwanted, noticeable artifacts.  
         [0010]     It is an object of the present invention at least to ameliorate the aforesaid disadvantages in the prior art.  
       SUMMARY OF THE INVENTION  
       [0011]     According to a first aspect of the invention, there is provided a programmable spatial filter system for a video signal comprising position extraction means arranged to extract a position in an image of a picture element to be spatially filtered; source image content detector means; programmable mask generator means arranged to receive output from the position extraction means and from the source image content detector means and to generate a filter mask dependent on the extracted position and on source image content in a border portion of the image; and programmable spatial filtering means arranged to filter the image using the filter mask input from the programmable mask generator means.  
         [0012]     Conveniently, the programmable mask generation means further comprises user selection means arranged for a user to select a filter mask.  
         [0013]     Advantageously, the filter mask is arranged to filter the image to a greater extent in border portions of an image than in a central portion of the image.  
         [0014]     Advantageously, the filter mask is arranged to filter the image in a transition portion between the border portions and the central portion of the image to an extent decreasing from the border portions to the central portion.  
         [0015]     Conveniently, a transition in a degree of filtering from the border portions to the central portion is non-linear.  
         [0016]     Advantageously, the source image content detector means comprises graphics detector means arranged to detect whether the picture element comprises a graphics picture element and to output a first resultant signal to the programmable mask generator, wherein the programmable mask generator is arranged to modify generation of the filter mask dependent on the first resultant signal.  
         [0017]     Advantageously, the source image content detector means comprises skin tone detector means arranged to detect whether the picture element comprises skin tones and to output a second resultant signal to the programmable mask generator, wherein the programmable mask generator is arranged to modify generation of the filter mask dependent on the second resultant signal.  
         [0018]     According to a second aspect of the invention, there is provided a method of spatially filtering an image represented by a video signal comprising the steps of: inputting a picture element of the video signal; extracting a position of the picture element within the image; detecting source image content in a border portion of the image; generating a filter mask for spatially filtering picture elements of the image dependent on the position of the picture element within the image and on source image content in the border portion of the image; using the filter mask spatially to filter the image; and outputting a video signal representing the filtered image.  
         [0019]     Conveniently, generating a filter mask further comprises a user selecting a filter mask.  
         [0020]     Advantageously, the method comprises filtering an image to a greater extent in border portions of the image than in a central portion of the image.  
         [0021]     Advantageously, the method comprises filtering the image in a transition portion between the border portions and the central portion of the image to an extent decreasing from the border portions to the central portion.  
         [0022]     Conveniently, a transition in a degree of filtering from the border portions to the central portion is non-linear.  
         [0023]     Advantageously, detecting source image content in a border portion of the image comprises detecting whether the picture element comprises a graphics picture element and outputting a first resultant signal to the programmable mask generator, and modifying generation of the filter mask dependent on the first resultant signal.  
         [0024]     Advantageously, detecting source image content in a border portion of the image comprises detecting whether the picture element comprises skin tones and outputting a second resultant signal to the programmable mask generator, and modifying generation of the filter mask dependent on the second resultant signal.  
         [0025]     According to a third aspect of the invention, there is provided a computer readable medium comprising computer executable software code, the code being for spatially filtering an image represented by a video signal comprising the steps of: inputting a picture element of the video signal; extracting a position of the picture element within the image; detecting source image content in a border portion of the image; generating a filter mask for spatially filtering picture elements of the image dependent on the position of the picture element within the image and on source image content in the border portion of the image; using the filter mask spatially to filter the image; and outputting a video signal representing the filtered image.  
         [0026]     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying Figures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0028]      FIG. 1  is a graphical representation of a graded filter profile applied across an image, plotting a degree of filtering as ordinates against a spatial dimension across the image as abscissa;  
         [0029]      FIG. 2   a  is a first exemplary image resulting from the profile of  FIG. 1  applied in two dimensions;  
         [0030]      FIG. 2   b  is a second exemplary image resulting from the profile of  FIG. 1  applied in two dimensions;  
         [0031]      FIG. 3  is a block diagram of a first embodiment of a spatial filtering system according to the invention;  
         [0032]      FIG. 4  is a block diagram of a second embodiment of a spatial filtering system according to the invention; and  
         [0033]      FIG. 5  is a flow chart of a method of spatially filtering an image according to the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0034]     Throughout the description, identical reference numerals are used to identify like parts.  
         [0035]     A degree to which images can be compressed is proportional to a complexity and level of detail in the image. By applying strong spatial filtering to a periphery of an image sequence, detail can be selectively removed to reduce a number of symbols required to represent that region, without adversely affecting the perceived overall image quality.  
         [0036]     However, this filtering could lead to a noticeable boundary region where the image pre-processing would stand out as a softened halo around the central region. To obviate this effect a graded profile  10  is applied to the processing as illustrated in  FIG. 1 .  
         [0037]     In  FIG. 1 , points P a  and P f  represent limits of the image either vertically or horizontally. P b  and P e  are inner limits of maximum processing up to which a degree of pre-processing is highest, F max . These points are usually set to be equidistant from their respective limit points, P a  and P f , but are not necessarily so. More details in this respect are provided hereinafter in respect of embodiments of the invention.  
         [0038]     Points P b  and P e are not necessarily inside the outer limits, P a  and P f , as shown, but may be coincident with these points. In that situation, a non-linear profile of transition may be beneficial.  
         [0039]     Points P c  and P d  bound a central portion of the image where border pre-processing falls to a minimum level F min , which may, or may not; represent an unfiltered image, since some degree of spatial filtering across the whole image may be desirable.  
         [0040]     Transition regions  12 ,  13  from P b  to P c  and from P d  to P e , respectively, show a progression from one degree of filtering to another. P b  to P c  shows a linear transition  12  that is generally chosen when a transition rate, defined by Equation 1 below, is less than an arbitrary threshold chosen to make the transition as unnoticeable as possible. Equation 1 is an expression of a rate of change of bandwidth reduction across a transition.  
             ∇     =         F   max     -     F   min           P   h     -     P   c                   Equation   ⁢           ⁢   1             
 
         [0041]     The transition  13  from P d  to P e  shows a non-linear transition from F max  to F min . This non-linear technique is chosen when the transition rate is high or would be particularly appropriate if either P b  or P c  were coincident with the outer limit points.  
         [0042]      FIGS. 2   a  and  2   b  show exemplary images in a schematic manner of the degree of filtering applied across the image when the profile of  FIG. 1  is applied in two dimensions.  
         [0043]     The degree of filtering is mapped to the luminance of each picture element or pel in the image such that high processing is represented by bright pels and low filtering by dark ones.  
         [0044]     It can be seen schematically from  FIGS. 2   a  and  2   b  that a highly filtered, border  21 ,  221  exists around the periphery and a lesser-filtered, region  23 ,  223  exists in the central section. In practice, moving inwards, the transition  22 ,  222  between the border portion and the central portion in some embodiments has a graduated decrease in intensity of filtering.  
         [0045]     Further, since receptors of the HVS are distributed in a radial profile from the centre of the retina, further advantage is gained by rounding the edges of the mask as shown in  FIG. 2   b . However, care must be exercised not to make the profile too rounded since active interest in the picture can move towards the diagonals which are highly filtered.  
         [0046]      FIG. 3  is a block diagram of a first embodiment of a pre-processing system  30  according to the invention. A programmable spatial filter  31  has a video input  32  which also acts as an input to a position extraction block  33  and a source image content detector  36 . The position extraction block  33  has X and Y coordinate outputs to a programmable mask generator  34 . The source image content detector  36  also has an output to the programmable mask generator  34 . The programmable mask generator  34  has a user selection input  341  and an output to a control input of the programmable spatial filter  31 . The programmable spatial filter  31  has a video output  35 .  
         [0047]     Referring to  FIGS. 3 and 5 , in use, an input image enters  51  at the video input  32 . During the active picture, the horizontal and vertical position within the image is extracted  52  by the position extraction block  33  and the corresponding coordinates passed to the mask generator  34 . The source image content detector detects source image content in a border and transition portion of the image  21 ,  22 ;  221 , 222 . The mask generator  34  translates  55  the position within the image, the source image content and a user selection of the mask profile and shape input at the user selection input  341  into a degree of filtering. This value of a degree of filtering is input to the programmable spatial filter and used to control  56  the bandwidth of the image at that position in the image. Filtered video is output  57  from the system at video output  35 .  
         [0048]     A representative collection of pels around a pel under operation, referred to as a window, is usually required to perform spatial filtering. At a very edge of an image, there will not be such a set of pels available. In this case, an average of surrounding pels which are usefully available is selected and used to produce a softened and smoothed border.  
         [0049]     Two situations require attention to obtain optimal performance from the programmable spatial filter system of the invention.  
         [0050]     The first is that the HVS is particularly sensitive to variation of hue and resolution across human skin tones. A loss of resolution on points of a human face would be more noticeable than on other types of detail. This loss of resolution would compromise the overall perceived system performance. Therefore, in a second embodiment of the invention the source image content detector comprises a skin tone detector  47 , as illustrated in  FIG. 4 .  
         [0051]     The skin tone detector overrides the mask generator  44  and can reduce the filtering towards, or to, F min  where skin tone is detected in an image, and particularly in a border portion of the image. If F min  is greater than 0 the filtering may be removed completely, as needed.  
         [0052]     The second issue is that of overlaid computer graphics, tickers and captions. These often contain high detail and sharp transitions of intensity and chrominance and if filtered would be compromised. Thus, in a third embodiment of the invention, shown in  FIG. 4 , the source image content detector comprises a graphics detector  46  to detect such graphics, tickers and captions.  
         [0053]     Therefore, referring to  FIG. 4 , an embodiment of a programmable spatial filter  41  according to the invention has a video input  42  which also acts as an input to a position extraction block  43 , a graphics detector block  46  and a skin tone detector block  47 . The position extraction block  43  has X and Y coordinate outputs to a programmable mask generator  44 , and outputs of the graphics detector block  46  and skin tone detector block  47  also have respective inputs to the programmable mask generator  44 . The programmable mask generator  44  has a user selection input  441  and an output to a control input of the programmable spatial filter  41 . The programmable spatial filter  41  has a video output  45 .  
         [0054]     Referring to  FIGS. 4 and 5 , in use, an input image enters  51  at the video input  42 . During the active picture, the horizontal and vertical position within the image is extracted  52  by the position extraction block  43  and the corresponding coordinates passed to the programmable mask generator  44 . The graphics detector  46  determines  53  whether the portion of the image being processed represents graphics, tickers or captions and outputs a corresponding output to the programmable mask generator  44 , to reduce a degree of filtering where said graphics, tickers or captions are detected. Similarly, the skin tone detector  47  determines  54  whether the portion of the image being processed represents skin tone and outputs a corresponding output to the programmable mask generator  44 , to reduce a degree of spatial filtering where skin tone is detected. The mask generator  44  translates the position within the image and a user selection of the mask profile and shape input at the user selection input  441 , together with the information on whether the portion of image represents graphics or skin tone to select  55  a value of a degree of filtering for generating a filter mask for the image. This value of a degree of filtering is input to the programmable spatial filter and used to control  56  the bandwidth of the image at that position in the image. Filtered video is output  57  from the system at video output  45 .  
         [0055]     It will be understood that the graphics detector and the skin tone detector can be used separately or in combination.  
         [0056]     Alternative embodiments of the invention can be implemented as a computer program product for use with a computer system, the computer program product being, for example, a series of computer instructions stored on a tangible data recording medium, such as a diskette, CD-ROM, ROM, or fixed disk, or embodied in a computer data signal, the signal being transmitted over a tangible medium or a wireless medium, for example microwave or infrared. The series of computer instructions can constitute all or part of the functionality described above, and can also be stored in any memory device, volatile or non-volatile, such as semiconductor, magnetic, optical or other memory device.  
         [0057]     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.