Appendix-Intra-Block-Copy.md

Intra Block Copy

1. Description of the algorithm

In intra frames, intra block copy (IBC) allows for the prediction of a given Intra coded block to be a copy of another intra coded block in the same frame (i.e. from the reconstructed part of the current frame). The copied block is specified using a displacement vector (DV). Only integer DVs are allowed since force_integer_mv will be equal to 1 for intra frames. Bilinear interpolation is considered for chroma prediction in the case of odd DVs. IBC is applicable only to key frames and intra-only non-key frames. When IBC is active, all in-loop filtering is disabled for the whole frame, including deblocking filter, CDEF and restoration filter. The prediction is generated from the reconstructed intra pixels, where the latter would not have been processed by the in-loop filters since they are disabled. The predicted block is considered an inter predicted block using a single reference frame (the current intra frame), and where the DV is full-pel. Only single-reference mode is allowed.

A 256 pixels wide area just before the block being predicted is excluded from the block copy intra search area, i.e. the valid region of the frame consists of already reconstructed blocks that are 256 pixels away (in a raster scan order) from the current block. Figure 1 below illustrates the allowed search area.

Figure 1. Diagram showing blocks not allowed in the search area.

The IBC feature is useful in encoding screen content that involves repeated patterns, text and texture in the same frame.

2. Implementation of the algorithm

Main function calls

A diagram of the main function calls associated with the IBC algorithm is shown in Figure 2 below. The functions are shown according to the depth of the function call.

Figure 2. Main function calls associated with the IBC algorithm.

Candidate Injection

In the function inject_intra_bc_candidates , up to 2 intra candidates are injected as IBC modes. These modes are considered DC_PRED modes when coding the block mode information in the bit stream. Simple translation is used and no fractional DVs are allowed for this case. For Chroma, bi-linear interpolation is used to produce predicted pixels. The two candidates are determined through the intra_bc_search function call, which is discussed next.

DV Search

The function intra_bc_search performs a search within the current picture (already decoded reconstructed area). The search is a combination of a classic Diamond search followed by Hash search (CRC is used as Hash metric). The search is only performed in full pel resolution as sub-pel displacements are not allowed in the IBC tool in AV1.

The decoded reconstructed area is divided into two search areas: Top and Left. As explained above, due to HW constraints, not all of Top reconstructed area is used to derive DV vectors. To support wavefront like SW based processing, more constraints are added to only consider the valid SBs in such scenario.

More detailed steps involved in the DV search are listed below:

1. Set the reference frame to INTRA_FRAME.

2. Get nearest and near MVs from MV stack for the specified reference frame. See (svt_av1_find_best_ref_mvs_from_stack)

3. Set dv_ref to either nearest or near.

4. Constrain the dv_ref mv to be at least a block size away from the current block, and also to point at least 256 samples away to the left in the x direction when too close to the tile top boundary. (av1_find_ref_dv)

5. Two types of searches could be performed: Search above the current block (IBC_MOTION_ABOVE) only or search above and to the left of the current block (IBC_MOTION_ABOVE and IBC_MOTION_LEFT), depending on the setting of ibc_mode. Up to two dv candidates could be generated.

6. Limits on mv sizes are computed and refined (svt_av1_set_mv_search_range).

7. Perform full-pel diamond search followed by hash search (svt_av1_full_pixel_search).

8. Make sure returned mv is within the specified mv bounds (mv_check_bounds)

9. Make sure the returned mv meets HW and SW constraints (av1_is_dv_valid)

Control macros/flags

The Intra block copy algorithm is enabled only when screen content is detected. Alternatively, there is a manual mode where the user can configure the encoder to enable or disable IBC via –scm 0/1 in the encoder command line instruction. With -scm 1, IBC will be used regardless of the value of the SC detector. Command line option -intrabc-mode allows to specify intraBC mode as per table below.

The control flags associated with the IBC feature are listed in Table 1 below.

Table 1. Control flags for IBC.

Flag	Level(Sequence/Picture)	Description
-scm	Sequence	Command line token. 0: No IBC, 1: IBC ON 2:Auto mode (detector based)
-intrabc-mode	Configuration	To specify intraBC mode on command line. 0: OFF, 1-3: IBC ON with complexity as below, -1: Default behavior
ibc_mode	Picture	Controls the complexity-quality tradeoff of the feature. 0: OFF, 1: Slow mode, 2: Faster, 3: Fastest
allow_intrabc	Picture	For intra pictures and for encoder presets 0 to 5, allow_intrabc setting is the same as the sc_content_detected setting; otherwise, it is set to 0.

3. Optimization of the algorithm

The complexity-quality tradeoffs in the IBC feature are controller using the ibc_mode flag. The description of the flag settings is given in Table 2 below.

Table 2. Description of the ibc_mode settings.

ibc_mode	Description
0	OFF
1	Slow mode.
2	Fast mode. A more relaxed threshold is used to enable an early exit in the DV search for Nx4 or 4xN blocks.
3	Faster mode. Only Above search area is considered in the search algorithm

The settings for ibc_mode are a function of the encoder preset and are given in Table 3 below.

Table 3. ibc_mode as a function of the encoder preset.

Encoder preset (enc_mode)	ibc_mode
0 to 2	1
3 to 8	2

4. Signaling

The main signal that is sent in the bitsream to enable IBC is allow_intrabc that is sent in the frame header. Note that IBC is only allowed for Intra coded frames. At the sequence header screen content tools have to be enabled in order to use IBC at the frame level.