| CPC H04N 19/61 (2014.11) [H04N 19/126 (2014.11); H04N 19/159 (2014.11); H04N 19/176 (2014.11); H04N 19/18 (2014.11)] | 8 Claims |
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1. An image decoding apparatus that decodes, from a bit stream, an image using a plurality of blocks including a first block of a P×Q array of pixels (P and Q are integers), the image decoding apparatus comprising:
a decoding unit configured to decode data corresponding to quantized transform coefficients from the bitstream;
an inverse quantization unit configured to derive transform coefficients from the quantized transform coefficients by using a quantization matrix, the transform coefficients representing frequency components, wherein a quantization parameter is used to derive the transform coefficients in addition to the quantization matrix;
an inverse transform unit configured to derive prediction residuals from the transform coefficients by performing inverse transform processing on the transform coefficients; and
a reconstruction unit configured to derive a predicted image by using a prediction method in which an intra prediction method and an inter prediction method are combined, and to reconstruct image data by using the predicted image and the prediction residuals,
wherein, in a case where a target block is the first block,
(i) the inverse quantization unit is configured to derive an N×M array of transform coefficients from an N×M array of quantized transform coefficients by using a quantization matrix of an N×M array of elements, wherein N is an integer satisfying N<P, and M is an integer satisfying M<Q, and
(ii) the inverse transform unit is configured to derive an N×Q array of intermediate values by performing multiplication of the N×M array of transform coefficients and a matrix of a M×Q array, and to derive a P×Q array of prediction residuals for the first block by performing multiplication of a matrix of a P×N array and the N×Q array of intermediate values, and
wherein, in a case where a target block is a second block smaller than the first block,
(i) the inverse quantization unit is configured to derive an array of transform coefficients for the second block from an array of quantized transform coefficients for the second block by using a quantization matrix for the second block, wherein a size of the array of transform coefficients for the second block, a size of the array of quantized transform coefficients for the second block, and a size of the quantization matrix for the second block are the same as a size of the second block, and
(ii) the inverse transform unit is configured to derive an array of prediction residuals a size of which is the same as the size of the second block by performing multiplication using at least the array of transform coefficients for the second block and a matrix a size of which is the same as the size of the second block.
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