Image decoding apparatus and image coding apparatus that involves bi-prediction

An image decoding apparatus for performing image decoding processing with reference to a prediction image, the image decoding apparatus comprising:

• a parameter decoding circuit configured to decode a reference picture indices refldxL0 and refldxL1 for each coding unit (CU) from coded data, and decode a plurality of weight information for each slice header from the coded data;
• and
• a prediction image generation circuit configured to generate a bi-prediction image by using first weight coefficients of a first prediction or second weight coefficients of a second prediction,
• wherein the prediction image generation circuit derives variables LumaWeightL0 and LumaWeightL1 based on the weight information, and derives the first weight coefficients of the first prediction equal to LumaWeightL0[refldxL0] and LumaWeightL1[refldxL1],
• the prediction image generation circuit derives a Generalized bi-prediction index gbiIndex for each CU, derives a first one of the second weight coefficients of the second prediction equal to gbwTable[gbiIndex], and derives a second one of second weight coefficients based on the first one of the second weight coefficients, the gbiIndex indicates one of elements of the gbwTable, and the gbwTable is a table not decoded from the coded data,
• in a case that the first prediction is on, the second prediction is on, and the Generalized bi-prediction index is equal to a predetermined value, the prediction image generation circuit generates the bi-prediction image by using the first weight coefficients of the first prediction, and
• in a case that the first prediction is on, the second prediction is on, and the Generalized bi-prediction index is not equal to the predetermined value, the prediction image generation circuit generates the bi-prediction image by using the second weight coefficients of the second prediction.