Past encoding for moving pictures is mainly executed in the YUV format. The YUV format is a color space format that is formed by a luminance component Y and two color difference components U and V. That is, in past encoding for moving pictures, input image data is transformed into the YUV format to be encoded in a case in which the input image data has the RGB format. Incidentally, requests for desiring to execute encoding image signals from imaging apparatuses (input sources) such as digital cameras while maintaining color space formats of the input sources have recently increased. Therefore, in the H.265/HEVC standard which is a latest moving picture encoding standard, a mode in which input moving picture data is encoded while maintaining the RGB format in a case in which the input moving picture data has the RGB format has been added. In this encoding mode, a signal with the RGB format is encoded in accordance with the same scheme as a signal with the YUV format.
This encoding method is realized with screen content coding (SCC) which is an extension of HEVC. In encoding for a moving picture to which the SCC is applied, in a case in which a moving picture with the RGB format is input, the moving picture is encoded assuming that a G component (green component) corresponds to the Y component of the YUV format.
In the SCC, a technology called adaptive color transform (ACT) is defined as an encoding tool. In the ACT, a path through which the RGB format is transformed into the YUV format is added before a prediction error signal is subjected to orthogonal transform. Then, either the RGB format or the YUV format is determined to execute orthogonal transform and quantization for each subblock set in a processing block. In a case in which the ACT is turned on, a format is transformed into the YUV format and orthogonal transform and quantization are executed. In a case in which the ACT is turned off, orthogonal transform and quantization are executed while maintaining the RGB format. In the ACT, a path through which a subblock with the YUV format in a prediction error signal restored by executing inverse quantization and inverse orthogonal transform on a quantized value (transform coefficient) is transformed into the RGB format is added, and a decoded image in which all of the subblocks have the RGB format is generated. In the HEVC standard, a processing block is called a coding unit (CU) and a subblcok in orthogonal transform and quantization is called a transform unit (TU). For each subblock included in the processing block, transform between the RGB format and the YUV format is defined as complete reversible transform so that a problem does not occur even when the ACT is switched to be turned on and off.
As an example of a technology of the related art, there is known “HEVC Screen Content Coding Extension (SCC)”, [online], [searched on Oct. 7, 2015], the Internet <URL: https://hevc.hhi.fraunhofer.de/scc>.