TECHNICAL FIELD

The embodiments of the present invention relate to a prediction image generation apparatus, a video decoding apparatus, a video coding apparatus, and a prediction image generation method.

BACKGROUND ART

A video coding apparatus which generates coded data by coding a video, and a video decoding apparatus which generates decoded images by decoding the coded data are used for efficient transmission or recording of videos.

For example, specific video coding schemes include H.264/AVC and High-Efficiency Video Coding (HEVC) schemes, and the like.

In such a video coding scheme, images (pictures) constituting a video are managed in a hierarchical structure including slices obtained by splitting an image, coding tree units (CTUs) obtained by splitting a slice, units of coding (coding units; which will be referred to as CUs) obtained by splitting a coding tree unit, and transform units (TUs) obtained by splitting a coding unit, and are coded/decoded for each CU.

In such a video coding scheme, usually, a prediction image is generated based on a local decoded image that is obtained by coding/decoding an input image (a source image), and prediction error components (which may be referred to also as “difference images” or “residual images”) obtained by subtracting the prediction image from the input image are coded. Generation methods of prediction images include an inter-picture prediction (an inter-prediction) and an intra-picture prediction (intra prediction). Non Patent Literature (NPL) 1 is exemplified as a recent technique for video coding and decoding.

Furthermore, in recent video coding and a decoding technique, a Cross-Component Linear Model (CCLM) prediction for generating a prediction image of a chroma image from a luma image is known. In the CCLM prediction, a linear prediction parameter is derived using a decoded image adjacent to a target block, and chroma of the target block is predicted from the linear prediction model (CCLM model) (NPL 2).

CITATION LIST

Non Patent Literature

NPL 1: “Versatile Video Coding (Draft 3)”, JVET-L1001, Joint Video Exploration Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 2018-11-0817:06:06NPL 2: “CE3-5.1: On cross-component linear model simplification”, JVET-L0191, Joint Video Exploration Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 2018 Oct. 3

SUMMARY OF INVENTION

Technical Problem

As described above, in the CCLM processing, the linear prediction parameter is derived, and the prediction image is generated using the linear prediction model. For the derivation of the linear prediction parameter, an integer operation and table reference are used, but there is a problem in that the amount of memory to be used of this table is large.

Furthermore, in a case of deriving a prediction value using a product of a gradient term of the linear prediction parameter (CCLM prediction parameter a) and a pixel value, there is a problem in the method of NPL 1 that since the bit width of the CCLM prediction parameter a increases, the complexity of the product is large. In addition, NPL 1 also uses a product for derivation of the gradient term (CCLM prediction parameter a) of the linear prediction parameter and an offset term (CCLM prediction parameter b) of the linear prediction parameter, but this product is also the product of values having a large bit width and thus is complex. Note that the product of values having a large bit width increases the hardware scale.

Solution to Problem

In order to solve the above-described problems, a CCLM prediction unit according to an aspect of the present invention is a CCLM prediction unit configured to generate a prediction image by a CCLM prediction, the CCLM prediction unit including: a CCLM prediction parameter derivation unit configured to derive CCLM prediction parameters (a, b) by using a luma difference value, a chroma difference value, and a table; and a CCLM prediction filter unit configured to generate a chroma prediction image by using a luma reference image and the CCLM prediction parameters (a, b), in which the CCLM prediction parameter derivation unit derives the CCLM prediction parameter a by performing right shift of a value obtained by multiplying a value of an inverse number table referred to by using the luma difference value and the chroma difference value by a first shift value shift_a, and the CCLM prediction filter unit derives the chroma prediction image by performing right shift of a product of the parameter a and luma by a second prescribed shift value shiftA.

Advantageous Effects of Invention

According to an aspect of the present invention, in a CCLM prediction, an effect in which multiplication with a linear prediction parameter is simplified is obtained.

