Method and apparatus for coding/decoding image

An apparatus for encoding a video, the apparatus comprising:

• a bitstream decoder configured to extract information on a size of a transform unit and generate a quantized frequency transform block from a bitstream;
• an inverse quantization unit configured to reconstruct a frequency transform block by inversely quantizing the quantized frequency transform block;
• an inverse transformer configured to reconstruct a residual block by inversely transforming the frequency transform block, the inverse transformer selecting an inverse transform scheme among a plurality of inverse transform schemes in consideration of the size of the transform unit;
• a predictor configured to generate a predicted block in a prediction mode selected among an intra prediction mode and an inter prediction mode;
• and
• an adder configured to reconstruct a current block by adding a first pixel corresponding to the residual block and a second pixel corresponding to the predicted block,
• wherein the inverse transformer considers a first transform scheme only when the size of the transform unit corresponds to a transform unit size of 4×4 among a plurality of available transform unit sizes,
• wherein the transform unit is divided from a coding unit in a quad tree, and the bitstream decoder decodes a split transform flag, as one of the information on the size of the transform unit, for at least one depth of the quad tree,
• wherein the first transform scheme corresponds to a transform scheme in which each of four output values {X′(0), X′(1), X′(2), X′(3)} is generated by using at least two of four input values {X(0), X(1), X(2), X(3)} multiplied by coefficients corresponding to each of the input values,
• wherein the transform scheme in which each of four output values {X′(0), X′(1), X′(2), X′(3)} is generated corresponds to a transform in which,
• a first group of the coefficients used for generating the first output value X′(0) includes a coefficient for the X(0), a coefficient for the X(1) and a coefficient for the X(2), all having positive signs,
• a second group of the coefficients used for generating the second output value X′(1) includes a coefficient for the X(0) and a coefficient for the X(1), both having positive signs,
• a third group of the coefficients used for generating the third output value X′(2) includes a coefficient for the X(1) and a coefficient for the X(2), both having negative signs, and
• a fourth group of the coefficients used for generating the fourth output value X′(3) included a coefficient for the X(1), having a negative sign, and a coefficient for the X(2), having a positive sign.