Video predictive coding apparatus and method

Abstract

A predictive coding in a video coding system, which can enhance the coding efficiency by predictively coding DC coefficients of a block to be coded using DC gradients of a plurality of previously coded neighboring blocks. The predictive coefficient is selected according to the difference between the quantized DC gradients (coefficients) of a plurality of neighboring blocks of the block to be coded, and the DC coefficient of the block to be coded is predictively coded by the selected predictive coefficient, thereby enhancing the coding efficiency.

1 . A method for coding a video signal, comprising: 
 sampling a video signal into a plurality of blocks;    transforming the video signal of each block into a two dimensional array of discrete cosine transform (DCT) coefficients having a DC coefficient and a plurality of AC coefficients; and    predicting a predictive DC coefficient of a block from either a left block or an upper block based on comparison of a vertical gradient and a horizontal gradient, the vertical gradient from a DC coefficient of a left upper block to a DC coefficient of the left block, and the horizontal gradient from the DC coefficient of the left upper block to a DC coefficient of the upper block.    
   
   
       2 . The method of  claim 1 , wherein the step of predicting a predictive DC coefficient comprises: 
 comparing the vertical gradient with the horizontal gradient;    selecting the DC coefficient of the upper block as the predictive DC coefficient of the block if the horizontal gradient is larger than the vertical gradient; and    selecting the DC coefficient of the left block as the predictive DC coefficient of the block if the horizontal gradient is not larger than the vertical gradient.    
   
   
       3 . The method of  claim 1 , further comprising: 
 encoding a DC residual, the DC residual being a differential value between a transformed DC coefficient of the block and the predictive DC coefficient of the block.    
   
   
       4 . The method of  claim 1 , further comprising: 
 quantizing the DCT coefficients.    
   
   
       5 . The method of  claim 1 , wherein the DC coefficient of the left block or the DC coefficient of the upper block is set to a predetermined value.  
   
   
       6 . The method of  claim 5 , wherein the predetermined value is a function of a quantization step size and a bits per pixel value, and equals 2 (bits     —     per     —     pixel+2) /(quantization step size).  
   
   
       7 . The method of  claim 1 , wherein the step of predicting a predictive DC coefficient is performed for a macroblock having a plurality of blocks.  
   
   
       8 . An apparatus to code a video signal, comprising: 
 a sampling portion that samples a video signal into a plurality of blocks;    a discrete cosine transform (DCT) portion that transforms the video signal of each block into a two dimensional array of DCT coefficients having a DC coefficient and a plurality of AC coefficients; and    a predictive block selector that predicts a predictive DC coefficient of a block from either a left block or an upper block, the predictive block selector comprising:    a horizontal gradient calculator that determines a horizontal gradient of DC coefficients from a DC coefficient of a left upper block to a DC coefficient of the upper block,    a vertical gradient calculator that determines a vertical gradient of DC coefficients from the DC coefficient of the left upper block to a DC coefficient of the left block,    a comparator that compares the horizontal gradient with the vertical gradient, and    a DC predictor that determines a predictive DC coefficient of the block based on the comparison of the vertical gradient and the horizontal gradient.    
   
   
       9 . The apparatus of  claim 8 , wherein the DC predictor selects the DC coefficient of the upper block as the predictive DC coefficient of the block if the horizontal gradient is larger than the vertical gradient, and selects the DC coefficient of the left block as the predictive DC coefficient of the block if the horizontal gradient is not larger than the vertical gradient.  
   
   
       10 . The apparatus of  claim 8 , further comprising: 
 a coder that encodes a DC residual, the DC residual being a differential value between a transformed DC coefficient of the block and the predictive DC coefficient of the block.    
   
   
       11 . The apparatus of  claim 8 , further comprising: 
 a quantizer that quantizes the DCT coefficients with a quantization step size.    
   
   
       12 . A method for decoding a video signal comprising a plurality of blocks, the method comprising: 
 determining a horizontal gradient from a reconstructed DC coefficient of a left upper block to a reconstructed DC coefficient of a upper block    determining a vertical gradient from the reconstructed DC coefficient of the left upper block to a reconstructed DC coefficient of a left block; and    predicting a predictive DC coefficient of a block from either the upper block or the left block.    
   
   
       13 . The method of  claim 12 , wherein the step of predicting a predictive DC coefficient comprises: 
 comparing the vertical gradient with the horizontal gradient;    selecting the reconstructed DC coefficient of the upper block as the predictive DC coefficient of the block if the horizontal gradient is larger than the vertical gradient; and    selecting the reconstructed DC coefficient of the left block as the predictive DC coefficient of the block if the horizontal gradient is not larger than the vertical gradient.    
   
   
       14 . The method of  claim 12 , further comprising: 
 reconstructing a reconstructed DC coefficient of the block by adding the predictive DC coefficient of the block with a coded DC residual, the coded DC residual being a differential value between the reconstructed DC coefficient of the block and the predictive DC coefficient.    
   
   
       15 . The method of  claim 14 , further comprising: 
 inverse quantizing the reconstructed DC coefficient according to a quantization step size.    
   
   
       16 . The method of  claim 12 , wherein the reconstructed DC coefficient of the left block or the reconstructed DC coefficient of the upper block is set to a predetermined value.  
   
   
       17 . The method of  claim 16 , wherein the predetermined value is a function of a bits per pixel value, and equals 2 (bits     —     per     —     pixel+2) .  
   
   
       18 . The method of  claim 16 , wherein the step of predicting a predictive DC coefficient is performed for a macroblock having a plurality of blocks.  
   
   
       19 . An apparatus to decode a video signal comprising a plurality of blocks, the apparatus comprising: 
 a horizontal gradient calculator that determines a horizontal gradient of DC coefficients from a DC coefficient of a left upper block to a DC coefficient of an upper block,    a vertical gradient calculator that determines a vertical gradient of DC coefficients from the DC coefficient of the left upper block to a DC coefficient of the left block;    a comparator that compares the horizontal gradient with the vertical gradient, and    a DC predictor that determines a predictive DC coefficient of a block based on the comparison of the horizontal gradient and the vertical gradient.    
   
   
       20 . The apparatus of  claim 19 , wherein the DC predictor selects the DC coefficient of the upper block as the predictive DC coefficient of the block if the horizontal gradient is larger than the vertical gradient and selects the DC coefficient of the left block as the predictive DC coefficient of the block if the horizontal gradient is not larger than the vertical gradient.  
   
   
       21 . The apparatus of  claim 19 , further comprising: 
 a decoder that reconstructs a reconstructed DC coefficient of the block by adding the predictive DC coefficient of the block with a coded DC residual, the DC residual being a differential value between the reconstructed DC coefficient of the block and the predictive DC coefficient of the block.    
   
   
       22 . The apparatus of  claim 19 , further comprising: 
 an inverse quantizer that de-quantizes the reconstructed DC coefficient according to a quantization step size.    
   
   
       23 . A method for forming a bitstream representing video information, comprising: 
 sampling a video signal into a plurality of blocks;    transforming the video signal of the blocks into a two dimensional array of discrete cosine transform (DCT) coefficients having a DC coefficient and a plurality of AC coefficients;    calculating a vertical gradient from a DC coefficient of a left upper block to a DC coefficient of a left block;    calculating a horizontal gradient from a DC coefficient of the left upper block to a DC coefficient of an upper block;    comparing the vertical gradient with the horizontal gradient;    selecting the DC coefficient of the upper block as the predictive DC coefficient of a block if the horizontal gradient is larger than the vertical gradient;    selecting the DC coefficient of the left block as the predictive DC coefficient of the block if the horizontal gradient is not larger than the vertical gradient; and    encoding a DC residual, the DC residual being a differential value between a transformed DC coefficient of the block and the predictive DC coefficient of the block.    
   
   
       24 . A bitstream representing video information formed by the method of  claim 23.