Intra-Picture Prediction in HEVC - High Efficiency Video Coding (HEVC) - page 94

a b

p [0][-1] p [2 N -1][-1]

p [-1][-1] E F G H D E F G H H H H H

D D

C C

Block to be
predicted

Block to be
predicted

B B

p [-1][ N -1]

A A

A
A
A
A

p [-1][2 N -1]

Fig. 4.3 An example of reference sample substitution process. Non-available reference samples
are marked as grey :( a ) reference samples before the substitution process ( b ) reference samples
after the substitution process

4.2.1 Reference Sample Substitution

Some or all of the reference samples may not be available for prediction due
to several reasons. For example, samples outside of the picture, slice or tile are
considered unavailable for prediction. In addition, when constrained intra prediction
is enabled, reference samples belonging to inter-predicted PUs are omitted in order
to avoid error propagation from potentially erroneously received and reconstructed
prior pictures. As opposed to H.264/AVC which allows only DC prediction to
be used in these cases, HEVC allows the use of all its prediction modes after
substituting the non-available reference samples.
For the extreme case with none of the reference samples available, all the
reference samples are substituted by a nominal average sample value for a given
bit depth (e.g., 128 for 8-bit data). If there is at least one reference sample marked
as available for intra prediction, the unavailable reference samples are substituted
by using the available reference samples. The unavailable reference samples are
substituted by scanning the reference samples in clock-wise direction and using the
latest available sample value for the unavailable ones. More specifically, the process
is defined as follows:

1. When p [ 1][2 N 1] is not available, it is substituted by the first encoun-
tered available reference sample when scanning the samples in the order of
p [ 1][2 N 2], :::, p [ 1][ 1], followed by p [0][ 1], :::, p [2 N 1][ 1].
2. All non-available reference samples of p [ 1][ y ] with y D 2 N 2 ::: 1are
substituted by the reference sample below p [ 1][ y C 1].
3. All non-available reference samples of p[ x ][ 1] with x D 0 :::2 N 1 are substi-
tuted by the reference sample left p [ x 1][ 1].

Figure 4.3 shows an example of reference sample substitution.