Image encoding apparatus, image encoding method, image decoding apparatus, and image decoding method

An image decoding method that decodes an image by performing prediction processing, comprising:

• a prediction mode selection step, which selects one of a plurality of prediction modes for the prediction processing;
• and
• a prediction step, which performs the prediction processing of the prediction mode selected at the prediction mode selection step,
• wherein the prediction modes selectable at the prediction mode selection step include:
• a first intra prediction mode in which first reference pixels are generated as prediction pixels at a location where no decoded pixel exists employing pixels within a decoded block adjacent to one of left and upper sides of a target block to be decoded, at calculation processing of prediction values of target pixels to be decoded which are disposed between two reference pixels, namely, decoded second reference pixels adjacent to one of the upper and left sides of the target block to be decoded which is a different side from the first reference pixels and the first reference pixels, computation being performed employing values of the second reference pixels and values of the first reference pixels;
• and
• a second intra prediction mode which is different from the first intra prediction mode and in which prediction pixels are generated employing values of the decoded pixels adjacent to the target block to be decoded,
• wherein there is a state in the first intra prediction mode in which the calculation processing of the prediction pixels employing the computation is feasible for any pixels excepting all boundary pixels located at a lower end of the target block to be decoded within the target block to be decoded,
• wherein a unidirectional prediction is performed towards a direction designated by an encoding side based on the decoded pixels employing the values of the pixels within the decoded block at the second intra prediction mode, and
• wherein a processing order of target blocks to be decoded does not change irrespectively of whether the first intra prediction mode or the second intra prediction mode is selected at the prediction mode selection step.