Abstract:
A circuit and method for reducing artifacts in decoded color video and images are disclosed. The circuit includes a buffer for receiving an input pixel in a first color-space, and a detector for determining after transformation into a second color-space, if at least one component of the transformed pixel would fall outside a predetermined range. The determination may be made by comparing components of the input pixel, to corresponding ranges in the first color-space. Upon determining that at least one component of the transformed pixel would be outside a corresponding predetermined bound in the second color-space, the detector causes the circuit to output a pixel in the first color-space, with at least one predetermined component. The output of the circuit may subsequently be converted to the second color-space by an external color-space converter and displayed onto a color display. The method reduces visible artifacts caused by clipping during color-space conversion.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to digital image processing, and more particularly to reduction of visual artifacts in color images and video arising from transmission errors or storage media defects. 
     BACKGROUND OF THE INVENTION 
     Current digital technologies are widely used in the production, transmission, storage and playback of images and video. Digital processing of images and video offers numerous advantages over analog video, including improved quality, efficient transmission using compression, a variety of storage media, and the convenient organization of content. As a result, images and video are now largely distributed digitally using mediums such as digital versatile discs (DVDs). In addition to DVDs, higher resolution formats such as high definition DVD (HD-DVD) and Blu-ray have become increasingly popular formats for movie distribution over the last few years. 
     In networked environments such as the Internet or local area networks, digital content can be easily downloaded to a client device (for example, a client computer&#39;s hard disk) from content servers. The trend toward digital distribution of multimedia content has thus been helped by the explosive growth of the Internet as a medium of communication over the last number of years. The ability to generate and store digital content inexpensively has in turn helped expand the reach of the Internet. 
     Video and image data are often compressed prior to being written onto storage media such as hard disks, flash memory, and DVD to reduce storage requirements; or prior to transmission to save transmission bandwidth. At a receiver, encoded video or image data is decoded and sent to a display device. Typical decoders include DVD players, HD-DVD players, Blu-ray players, portable digital video players, personal computers equipped with video player software and the like. 
     Part of the reason for the increasingly widespread adoption of digital transmission and storage of video is the ability to use error control codes such as forward error correction (FEC) codes, cyclic redundancy checks (CRC) and the like, to detect and sometimes correct corrupted data. Received data may be corrupted as a result of transmission errors or due to storage media defects. 
     Error control coding involves the controlled introduction of redundancy in the transmitted (or stored) data stream at a transmitter, in such a manner that allows a receiver to detect and sometimes correct erroneously received data. However, the use of error correcting codes adds to the bandwidth requirement of transmitted data (or equivalently to storage), which is undesirable. Using robust error correcting codes also increases the processing overhead and complexity of implementation of the transmitter and receiver. Therefore in most applications—including video streaming applications or digital video broadcasting—the error control codes used do not permit all transmission errors to be corrected. Consequently, some transmission errors do occur. Unfortunately, in image and video transmission, some of these errors may sometimes result in noticeable artifacts that are displeasing to the eye. Obviously, noise on the transmission channel increases the likelihood of bit errors in the received video stream. 
     When errors are detected in received images and video, the receiver typically attempts to correct the errors, or at least reduce their undesirable effects. However, this often may not always lead to a subjectively acceptable outcome. For example, in color image or video transmission, color images are typically transmitted and received as pixels with color components (Y, Cb, Cr) in the YCbCr color-space representing the luma Y and chroma Cb, Cr. At the receiver, these components are converted to their equivalents in the RGB color-space which is typically used by digital displays. 
     For a receiver that uses 8-bit per color component in RGB space, each color component (R, G, B) ranges from 0 to 255. In the presence of transmission errors however, received YCbCr components may map to RGB components that are invalid—(i.e., with one or more color components are outside the permissible bounds). In this case, erroneous values are often truncated to the nearest acceptable value for the color component. Unfortunately however, this often leads to very noticeable artifacts. Very bright colors that standout in an otherwise demure image are very visible and distracting to a viewer and therefore undesirable. 
     Accordingly, an improved method of processing received digital color images is needed to reduce artifacts that result from transmission errors. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention there is provided, a circuit including a buffer for receiving an input pixel in a first color-space, and a detector. The buffer is in communication with the detector. The detector determines if a pixel formed by transforming the input pixel into a second color-space includes at least one component outside a corresponding predetermined bound in the second color-space. The circuit outputs an output pixel in the first color-space with at least one predetermined component upon determining that the transformed pixel would include at least one component outside its corresponding predetermined bound in the second color-space. 
     In accordance with another aspect of the present invention there is provided, a display adapter including a circuit and a color-space converter. The circuit includes a buffer for receiving an input pixel in a first color-space, and a detector. The buffer is in communication with the detector. The detector determines if a pixel formed by transforming the input pixel into a second color-space includes at least one component outside a corresponding predetermined bound in the second color-space. The circuit outputs an output pixel in the first color-space with at least one predetermined component upon determining that the transformed pixel would include at least one component outside its corresponding predetermined bound in the second color-space. The color-space converter is in communication with the circuit. The color-space converter receives the output pixel in the first color-space from the circuit, and outputs a corresponding pixel in the second color-space. 
     In accordance with yet another aspect of the present invention there is provided, a method of processing an input pixel including: receiving the input pixel in a first color-space; determining if at least one component of a pixel formed by transforming the input pixel into a second color-space falls outside a corresponding predetermined bound; and if so providing an output pixel in the first color-space with at least one predetermined component. 
     Other aspects and features of the present invention will become apparent to those of ordinary skill 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 
       In the figures which illustrate by way of example only, embodiments of the present invention, 
         FIG. 1  is a simplified block diagram of a conventional video receiver; 
         FIG. 2  is logical diagram of the RGB color cube; 
         FIG. 3  is a logical diagram of a subset of values in the YCbCr color cube that remain valid in the RGB color cube of  FIG. 2 ; 
         FIG. 4  is a schematic block diagram of a video receiver device exemplary of an embodiment of the present invention; 
         FIG. 5  is an enlarged schematic diagram of an in-loop processing unit in the video receiver device of  FIG. 4 ; and 
         FIG. 6  is an enlarged schematic diagram of another embodiment of a detector in the in in-loop processing unit of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a simplified block diagram of a conventional video receiver  100  capable of decoding and processing a compressed digital video stream. Receiver  100  includes a decoder  102  and a video processor  104 . 
     Decoder  102  includes an entropy decoder or variable length decoder (VLD)  108 , an inverse quantization block  110 , an inverse transform block  112 , a motion compensation block  114 , and a de-blocker  118 . Video processor  104  includes processing sub-blocks such as a scaling unit  120 , a de-interlace block  122 , color converter  124  and a video output interface  126 . Video output interface  126  is interconnected with display  106 . 
     Decoder  102  and video processor  104  are in communication with a block of memory  116  which may be used to provide a frame buffer. Output interface  126  may be a random access memory digital to analog converter (RAMDAC), digital visual interface (DVI) interface, a high definition multimedia interface (HDMI) interface or the like. Display  106 , can be one of a television, computer monitor, liquid crystal display (LCD), a projector or the like. 
     Decoder  102  receives an encoded/compressed video stream, decodes it into pixel values and outputs decoded pixel data. The received input video stream may be compliant to an MPEG-2 format, H.264 (MPEG-4 Part 10) format, VC-1 (SMPTE 421M) format or the like. The input video stream may be received from a digital satellite receiver, or cable television set-top box, a local video archive, a flash memory, a DVD, an optical disc such as HD-DVD or Blu-ray disc, or the like. 
     Video processor  104  receives the decoded pixel data from decoder  102 , processes the received data and provides a video image to an interconnected display  106 . 
     Scaling unit  120 , de-interlace block  122 , and color converter  124  are functional blocks that may be implemented as dedicated integrated circuits, or as firmware code executing on a microcontroller or a similar combination of hardware and software. 
     Decoded video data may be transferred from decoder  102  to video processor  104  using data lines  130  or memory  116 . An internal bus is used to transfer data from one sub-block to another with in decoder  102 , and video processor  104  respectively. 
     The received video stream is entropy decoded by VLD  108 . The output of VLD  108  is then inverse quantized using inverse quantization block  110  and an inverse transform (e.g., inverse discrete cosine transform) is carried out using inverse transform block  112 . After appropriate motion compensation in MC block  114  and removal of blocking artifacts in de-blocker  118 , decoded pixels are then output to video processor  104 . 
     Video processor  104  may perform a variety of video post processing functions such as scaling, de-interlacing, and color-space conversion before outputting a final image to display  106 . 
     As noted above, some data corruption may occur during transmission and these errors may sometimes result in noticeable artifacts. For example, invalid values may be output by decoder  102  as a result of corrupted input values. Invalid values may include pixel color components that are outside of valid ranges. At the encoder, input pixel color values of raw video are all within a predetermined bound or range, typically 0-255 for red, green and blue values. These RGB values are first transformed to YUV or YCbCr color-space and encoded using standard blocks for quantizing, transforming and entropy coding (variable length coding) to produce a compressed bit stream. 
       FIG. 2  depicts a color cube  200  in the RGB color-space. The color components may be gamma corrected R′G′B′ values. Each color is represented by its red component plotted along axis  202 , its green component along axis  204  and its blue component shown along axis  206 . Thus each color may be represented by a point (r′, g′, b′) in the three dimensional color cube  200 . For example the color black is located at (0,0,0); while the color white is at (255,255,255). All points along diagonal line  208  represent grey valued colors ranging from black to white. 
     The YCbCr color-space on the other hand, is a scaled and offset version of the YUV color-space. Y is defined to have a nominal 8-bit range of 16-235; Cb and Cr are defined to have a nominal range of 16-240. The YUV color-space is used by PAL (Phase Alternation Line), NTSC (National Television System Committee), and SECAM (Sequential Color with Memory) composite color video standards. Detailed discussions of the relationship between YCbCr, YUV and R′G′B′ color-spaces can be found in Jack, Keith. 2005.  Video Demystified: A handbook for the digital engineer  4th ed. Oxford: Elsevier, the contents of which are hereby incorporated by reference. 
     Conversions from YUV to gamma corrected R′G′B′ values may be carried out using the following equations.
 
 R′=Y+ 1.140 V  
 
 G′=Y− 0.395 U− 0.581 V  
 
 B′=Y+ 2.032 U  
 
     Similarly, conversions from YCbCr to gamma corrected R′G′B′ values may be carried out using the following equations (with Y, Cb, Cr having nominal 8-bit ranges of 16-235, 16-235, 16-235 respectively).
 
 R′=Y+ 1.371( Cr− 128)  [1]
 
 G′=Y− 0.698( Cr− 128)−0.336( Cb− 128)  [2]
 
 B′=Y+ 1.732( Cb− 128)  [3]
 
     Equations [1]-[3] are approximations and slightly different coefficients may be used for different applications depending on the display device, gamma correction, the video source, and the like. For example, the equations below may be used for some display terminals.
 
 R′= 1.164( Y− 16)+1.596( Cr− 128)  [5]
 
 G′= 1.164( Y− 16)−0.813( Cr− 128)−0.391( Cb− 128)  [6]
 
 B′= 1.164( Y− 16)+2.018( Cb− 128)  [7]
 
     Not all possible YCbCr input values map to valid R′G′B′ values within the defined range (0-255 for each of R′, G′ and B′). This may be easily seen when examining the RGB color cube  200 ′ within the context of the YCbCr color-space as depicted in  FIG. 3 . As shown, there are many values in the YCbCr color-space  300  that lie outside the RGB cube  200 ′. 
     In the presence of transmission errors, or due to defects in physical media such as DVDs or optical discs, or other sources of error, invalid YCbCr color values may be output by decoder  102  of conventional receiver  100  (of  FIG. 1 ). As noted, each YCbCr value is obtained from an R′G′B′ color value. Each R′G′B′ color includes defined ranges for R′, G′ and B′—for example, 0-255 when using 8 bits. Thus, if a YCbCr value is transformed to RGB color-space using equations [1]-[3], then the resulting R′, G′ and B′ values should be with in the defined range (e.g., 0-255). If any of the resulting R′, G′ or B′ values are invalid—that is, they fall outside the defined range—then the received YCbCr value is likely corrupted. In other words, if the received video bit stream is corrupted, then decoded YCbCr values may be outside of color cube  200 ′. 
     In conventional receivers such as receiver  100 , color converter  124  which converts color components from a non-RGB color-space to an RGB color-space, uses a simple logic to limit or clip the R′G′B′ output to be within the defined range. For example, in RGB displays that use 8-bits per color component, each color component may only range from 0 to 255. During color-space conversion, color converter  124  substitutes 0 when a negative value is calculated for a given color component, while for a computed color component that is greater than 255 is truncated to 255 by color converter  124 . Unfortunately, this often leads to very noticeable bright pink or bright green artifacts. For example, when Cb and Cr are negative or zero, the computed R, B components are also negative (and hence typically truncated to 0) while the G component is positive, which leads to a green artifact. Similarly when Cb and Cr are above 255, a pink artifact may be observed after color-space conversion and truncation. 
     To prevent such artifacts, video receivers exemplary of embodiments of the present invention may include a different logic to translate non-RGB (e.g. YCbCr) colors that do not map to predetermined bounds or valid ranges in the RGB color-space. 
     Accordingly,  FIG. 4  depicts a schematic block diagram of a video receiver  400  exemplary of an embodiment of the present invention. Video receiver  400  accepts, decodes, and processes a compressed digital video stream, and outputs decoded images to an interconnected display  106 . 
     Receiver  400  may include a decoder  402  and a video processor  404 . Decoder  402  may further include a variable length decoder (VLD)  408 , an inverse quantization (IQ) block  410 , an inverse transform block  412 , a motion compensation (MC) block  414  and an in-loop processing unit  406 . A microcontroller  430  in communication with decoder  402  may form part of receiver  400 . Video processor  404  may include a scaling unit  420 , a de-interlace block  422 , color converter  424  and a video output interface  426 . Decoder  402  and video processor  404  may be in communication with memory  416  which may be used to provide a frame buffer. 
     Decoder  402  and video processor  404  may contain combinatorial and sequential circuitry, numerous local memory blocks, first-in-first-out (FIFO) memory structures, registers, and the like. Output interface  426  may provide output signals compliant to video graphics array (VGA), super VGA (SVGA), digital visual interface (DVI), high definition multimedia interface (HDMI) or other display interface standards. Display  106  may be a cathode ray tube (CRT) monitor, LCD, a projector, a television set, a flat panel display or the like. 
     Scaling unit  420 , de-interlace block  422 , and color converter  424  may be substantially similar to their counterparts in  FIG. 1  and may be implemented in the form of dedicated circuits, firmware code executing on a microcontroller  428 , or some other suitable combination of hardware and software. 
     A bus  428  may interconnect the various blocks and sub-blocks within receiver  400 . Decoded video data may be transferred from decoder  402  to video processor  404  using bus  428 , memory  416 , or dedicated signal lines  432 . Microcontroller  430  may program registers in sub-blocks such as inverse transform block  412 , motion compensation block  414  and an in-loop processing unit  406  using bus  428 . 
       FIG. 5  depicts an enlarged schematic diagram of in-loop processing unit  406  illustrating additional details. In-loop processing unit  406  may include filtering block  434 , memory unit  440 , an invalid color detector  442 , and control register  448 . Memory unit  440  may further include an incoming data input interface  436 , data buffer  438  and output interface  444 . Memory unit  440  may also include a flag register  450 . Input interface  436  and output interface  444  may each include FIFO structures. Flag register  450  may have 2 m  status bits or flags (e.g. 2 6 =64 flags) and may be in communication with a bus  456 . Detector  442  may include a color-space conversion block  460  interconnected to a number of comparators  462 A,  462 B,  462 C,  462 D,  462 E,  462 F, (individually and collectively  462 ). Detector  442  may be capable of writing to at least some of the 2 m  status bits in register  450  using bus  456 . To address 2 m  bits (e.g. 64 bits) in register  450 , bus  456  may have m address line (i.e., 6 address lines), at least one data line and one or more control lines. 
     In operation, decoder  402  may also receive a compressed video stream compliant to a known standard such as MPEG-2, H.264 (MPEG-4 Part 10), VC-1 (SMPTE 421M). Again, the encoded input video stream may be received from a digital satellite receiver, or cable television set-top box, a local video archive, a flash memory, a DVD, an optical disc such as HD-DVD or Blu-ray disc, or the like. 
     The received video stream is entropy decoded by VLD  408 . The output of VLD  108  is then inverse quantized using inverse quantization block  410  and an inverse transform may be carried out in inverse transform block  412 . The inverse transform may be the inverse discrete cosine transform (IDCT). The output of inverse transform block  412  may be received by MC block  414  which may carry out required motion compensation processing. Output pixels from MC block  414  may be received by in-loop processing unit  406  directly; or alternately may be placed memory  416  from which they may be read into in-loop processing unit  406 . 
     Video processor  404  may perform substantially the same functions as its counter part in  FIG. 1  (video processor  104 ), including scaling, de-interlacing, color-space conversion and the like. 
     In-loop processing unit  406  contains filtering block  434  which may be used to remove blocking artifacts that are often observed when a block-oriented transform (such as DCT) is used by the encoding scheme to produce compressed video stream. An input bus  452  may be used to transfer data from MC block  414  to in-loop processing unit  406 . 
     Detector  442  may tap input bus  452  and perform detection of pixel color values that are outside RGB cube  200 ′ in  FIG. 3  and therefore would not map to valid an R′G′B′ value. For example, in an exemplary embodiment using 8-bits for each color component, detector  442  may signal output interface  444  by writing an error indicator bit to flag register  450  unless the conditions:
 
0 ≦Y+ 1.371( Cr− 128)≦255 and
 
0 ≦Y− 0.698( Cr− 128)−0.336( Cb− 128)≦255 and
 
     0≦Y+1.732(Cb−128)≦255 are all satisfied by Y, Cb and Cr. As may be appreciated, the inequalities are derived directly from equations [1]-[3] above. Similar inequalities derived from equations [4]-[6] may also be used. 
     The inequalities can be tested by first using color-space conversion (CSC) block  460  within detector  442 , to produce an intermediate pixel with R′G′B′ components, and then using comparators  462  to determine if each component of the intermediate pixel is within predetermined bounds. CSC block  460  may be implemented using standard adders, multipliers and coefficient registers. Comparator  462 A may be used to test that R′≦R max  (e.g., R max =255). Comparator  462 B may be used to test that 0≦R′ (R′ is computed by block  460  using equation [1]). Similarly, comparator  462 C may be used to test that G′≦G max  (e.g., G max =255). Comparator  462 D may be used to test that 0≦G′ (G′ is computed by block  460  using equation [2]). Lastly, comparator  462 F may be used to test that 0≦B′ (G′ is computed by block  460  using equation [3]) while comparator  462 E may be used to test that B′≦B max  (e.g., B max =255). Detector  442  may write an error indicator to flag register  450  using bus  456  for any pixel that fails to satisfy the above inequalities. Prior to outputting a pixel to video processor  404 , output interface  444  may inspect flag register  450  and if an invalid color indicator bit is set then output interface  444  may replace the invalid pixel with a valid replacement pixel and output the valid pixel. 
     In another exemplary embodiment, the detector need not dynamically compute equations [1]-[3] for each (Y, Cb, Cr) component of a received pixel. Instead, predetermined ranges (Y min , Y max ), (Cb min , Cb max ), (Cr min , Cr max ), corresponding to Y, Cb and Cr may be programmed into control register  448 . 
     Accordingly,  FIG. 6  displays another embodiment of a detector  442 ′ for determining if pixel in a first color-space (e.g. YCbCr), once color converted, would contain a component in a second color-space (e.g. RGB) that exceeds a predetermined bound, by performing a comparison of a pixel component in the first color-space (e.g. Y in YCbCr) to a corresponding range in the same first color-space (e.g., Y min  to Y max ). In other words, detector  442 ′ may compare a component of a pixel in a first color-space to a corresponding range also in the first color-space (e.g. check that Y min ≦Y≦Y max ) to determine if transforming the pixel to a second (e.g. RGB) color-space, would lead to a component (either R, G or B) being outside its corresponding predetermined bound (e.g. 0 to 255) in the second color space. 
     Detector  442 ′ may include a number of comparators  464 A,  464 B,  464 C,  464 D,  464 E,  464 F, (individually and collectively  464 ). Detector  442 ′ has the same input and output interfaces as detector  442 , and thus may be capable of writing to at least some of the 2 m  status bits in register  450  using bus  456 . 
     Detector  442 ′ signals output interface  444  to output a replacement pixel when a component is found to be outside its corresponding range in the YCbCr color-space. Exemplary values that may be commonly used to define these predetermined ranges include: 
     Y min =16, Y max =240, Cb min =Cr min =16, Cb max =Cr max =240; or 
     Y min =8, Y max =248, Cb min =Cr min =8, Cb max =Cr max =248. 
     Other values may of course be used to define the ranges. In addition, in specific embodiments, a single range may be used for both chroma values—that is, a single value CbCr min  in register  448  may be used as both Cb min  and Cr min  and similarly the same value CbCr max  in register  448  may be used as both Cb max  and Cr max . 
     An error condition to trigger a pixel component replacement may be flagged if, for example, Y&lt;Y min  or Y&gt;Y max . Similarly, an error may be flagged when one of the conditions Cb&lt;Cb min ; Cr&lt;Cr min ; Cb&gt;Cb max  or Cr&gt;Cr max  is satisfied. Unlike detector  442  ( FIG. 5 ), detector  442 ′ in  FIG. 6  uses fixed limit values defined in the YCbCr space—i.e., predetermined ranges (Y min , Y max ), (Cb min , Cb max ), (Cr min , Cr max )—which are known to generate invalid color values in the RGB color space. Thus, explicit YCbCr to RGB conversion is not needed in detector  442 ′. 
     The replacement pixel may have color components that produce a grey pixel or a pixel color close to grey, so as not to produce highly visible artifacts. 
     In one exemplary embodiment, output interface  444  may replace an invalid pixel containing color components (Y, Cb, Cr), with a grey color pixel having color components (Y,128,128) in the YCbCr color-space, if either one of Cb or Cr values is invalid. This replacement leaves the luma value Y unchanged while the chroma values Cr and Cb are set to 128 each. Conveniently, the replacement output pixel contains the same luma information (Y) as the original input pixel. 
     Equations [1]-[3] indicate that replacing any invalid color with a pixel having components (z, 128,128) for 0≦z≦255 in the YCbCr color-space, produces a valid grey color of the form (z, z, z) in RGB space. Any color of the form (z, z, z) lies along line  208  (in  FIG. 2 ) which represents all points of grey in RGB color cube  200 . As noted above, grey is far less noticeable than a bright pink or bright green artifact that often results from truncating values to 0 or 255. 
     Noting that replacing an invalid color component with (Y,128,128) would produces a valid color only if 0≦Y≦255, output interface  444  may replace invalid pixel with color components (Y, Cb, Cr) with (128,128,128) if the invalid components include Y (that is, if Y&lt;0 or Y&gt;255). If Y is an invalid component, output interface  444  may immediately replace Y by 128 or more generally by 2 n-1  when n bits are used to represent Y. 
     Advantageously, detection of invalid values received via bus  452  by detector  442 , ahead of outputting pixels to video processor  404  allows for convenient replacement of the output pixel&#39;s color components by output interface  444 . 
     In another embodiment, output interface  444  may replace an invalid pixel containing color components (Y, Cb, Cr), with a fixed grey color pixel having color components (X,128,128) in the YCbCr color-space. For 8-bit per color component display, by choosing X so that it is in the range 0≦X≦255, a valid RGB color-space output pixel would be sent to display  106 . Again using equations [1]-[3], it can be easily verified that (X,128,128) in the YCbCr color-space translates to (X, X, X) in the RGB color-space. In one specific exemplary embodiment, X may be fixed to 128 so that the replacement pixel is (128,128,128) in the YCbCr as well as RGB color-spaces. 
     In another embodiment, control register  448  may contain programmable fields for storing replacement color values Y new , Cr new  and Cb new . Microcontroller  430  may program control register  448  with replacement color values Y new , Cr new  and Cb new . When detector  442  indicates to output interface  444  that a current pixel has invalid color components (through bus  456  and flag register  450 ), then output interface  444  may replace the invalid pixel color values (Y, Cb, Cr) with (Y new , Cr new , Cb new ) respectively. Video processor  404  thus would receive the replacement pixel with components (Y new , Cr new , Cb new ) as its input. Y new , Cr new  and Cb new  should be chosen so that they fall within color cube  200 ′ in  FIG. 2  (that is, they can be transformed to a valid color in the RGB color-space without further processing). 
     Advantageously programmable replacement color values allow the replacement colors to be adapted to the input video sequence as needed. Thus when out-of-range colors are detected, even less noticeable replacement colors (than grey colors) may be used instead of predetermined color values. For example, if a pixel is found to be corrupted, it may be replaced by a pixel derived from its neighboring pixels. In particular, the pixel to the left, above and above-left of a corrupted pixel, may be used to compute the replacement pixel. Neighboring pixels may be buffered in buffer  438  and used for computing a replacement pixel. Various methods for computing the replacement pixel from neighboring pixels such as averaging, substitution, filtering, interpolation and the like, are well known to those of ordinary skill in the art. 
     The replacement strategy—that is, whether to use neighboring pixels, replace a color component, use a completely predetermined pixel, etc. may be selectable by appropriately programming the video receiver hardware (via a control register  448 , for example). 
     The above embodiments are discussed for cases in which color pixels ready for display output are represented by 8-bits per color component. However, the skilled reader would readily appreciate that for general representations with n-bits per color component, the range of valid (r′, g′, b′) values may be determined by the conditions {R min ≦r′≦R max }, {G min ≦g′≦G max } and {B min ≦b′≦B max } in which typically R min =G min =B min =0 and R max =G max =B max =2 n −1. Similarly the ranges (Y min , Y max ), (Cb min , Cb max ), (Cr min , Cr max ) may be set to different values depending on n. 
     Thus for example, instead of using (Y, 128,128) for an invalid Cr or Cb component of an input pixel, for the general n-bit case, output interface  444  may use a replacement color of the form (Y, 2 n-1 , 2 n-1 ) for 0≦Y≦2 n −1 in YCbCr color-space, to produce a grey output pixel of the form (Y,Y,Y) in RGB color-space. 
     In an alternate embodiment, decoding and video processing operations may be combined in a single circuit which outputs R′G′B′ colors. Here, color replacement may take place in the RGB color-space. In this case, computed r′, g′ and b′ values may be temporarily stored in a buffer. If an interconnected display device represents each color component using n-bits, then a temporary buffer may be used to store each color component using m bits (m&gt;n) per color component to allow examination of r′, g′ and b′ without truncating them to n-bit values due to overflow. If at least one of r′, g′, or b′ does not fall with in the range 0 to 2 n −1, replacement color pixel of the form (z, z, z) in RGB color-space with z≈2 n-1  (and 0≦z≦2 n −1) may be used to output a grey color (replacement pixel) directly in RGB color-space. 
     Replacing YCbCr pixels in in-loop processing unit  406  rather than replacing the transformed RGB pixels, may be advantageous as it allows a conventional video processor to be used. A video receiver, exemplary of an embodiment of the present invention, may thus contain a conventional video processor (such as video processor  108 ) interconnected with a video decoder such as decoder  402 . Such a receiver would deliver the benefits of the present invention while still using a conventional video processor. This may be particularly advantageous in applications in which the decoder and the display processor (video processor) are independent from each other. Thus, in typical implementations the pixel replacement may be done within in-loop processing unit  406  while decoded YCbCr pixels are still in a pipeline, rather than at the display processing stage (e.g., in video processor  404 ) in which an extra processing filter would likely be required. 
     Circuits exemplary of embodiments of the present invention may be used in graphics display adapters. A graphics display adapter may include an exemplary circuit such as decoder  402 , in communication with an external color-space converter unit (such as color converter  424 ). The color-space converter accepts its input from the exemplary circuit in YCbCr space and outputs a corresponding pixel for display in R′G′B′ space to a display output interface. Since the exemplary circuit would ensure that its output (color converter&#39;s input) pixel components would map to valid R′G′B′ values (i.e., within predetermined ranges for R′, G′ and B′) artifacts associated with clipping would be avoided. 
     Advantageously, the external color converter unit may be a conventional color converter. That is, the exemplary circuit would provide to a conventional color converter, an input (in YCbCr color-space) that is guaranteed to have its R′G′B components (after color conversions) falling within their corresponding predetermined ranges (e.g., 0 to 255). Conveniently, this allows off the shelf color converter units (e.g., color converter  124 ) to be used, while delivering the benefits of the present invention. 
     Exemplary embodiments of the present invention may be used in conjunction with other error correcting methods implemented in VLD  408 , IQ block  410 , inverse transform block  412  and MC block  414 . As noted, some of the corrupted pixels that are received, may not be detected and corrected in these blocks, and thus it is advantageous to include embodiments of the present invention in video receivers. In addition, some video coding standards may devote a higher proportion of the transmission bandwidth to actual video data and a correspondingly lower proportion to error correcting codes. This may lead to an increased number of received bit errors, which in turn makes the use of embodiments of the present invention in video receivers adapted to receive encoded video streams so encoded, desirable. 
     Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments of carrying out the invention are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.