G. Intra-picture prediction
6) Reference sample substitution
The neighboring reference samples are not available at the slice or tile boundaries. In addition, when a loss-resilience feature known as constrained intra prediction is enabled, the neighboring reference samples inside any inter-coded PB are also considered not available in order to avoid letting potentially-corrupted prior decoded picture data propagate errors into the prediction signal. While only Intra_DC prediction mode is allowed for such cases in H.264/MPEG-4 AVC, HEVC allows the use of other intra prediction modes after substituting the non-available reference sample values with the neighboring available reference sample values. 邻近参考像素点在片或者片边界是无效的。此外，当强制帧内预测的丢失恢复功能开启后，为了避免由前面的解码图像造成差错扩散，所有帧间预测PB的相邻参考像素也是无效的；在H.264/MPEG-4 AVC中只有Intra_DC模式可以允许用这功能，HEVC在用邻近有效参考像素值来取代无效参考像素值后用其他帧内模式进行预测。7) Mode coding
HEVC supports a total of 33 Intra_Angular prediction modes as well as Intra_Planar and Intra_DC prediction modes for luma intra prediction of all block sizes. Due to the increased number of directions, HEVC considers three “most probable modes” (MPMs) when coding the luma intra prediction mode predictively,rather than the one most probable mode considered in H.264/MPEG-4 AVC.
Among the three most probable modes, the first two are initialized by the luma intra prediction modes of the above and left PBs if those PBs are available and are coded using an intra prediction mode. Any unavailable prediction mode is considered to be Intra_DC. The PB above the luma CTB is always considered to be unavailable, in order to avoid the need to store a line buffer of neighboring luma intra prediction modes.
HEVC亮度帧内预测的所有大小块都支持33种方向预测模式以及Intra_Planar and Intra_DC模式。由于增加的预测方向比较多
，HEVC对亮度帧内预测提出了三种最有可能的模式（MPMS），这个比H.264/MPEG-4 AVC当中一种最可能的模式要有效的多。
在MPM中，如果上方以及左侧PB有效且是用帧内预测模式，则前两种则采用上方和左方的PB的亮度帧内预测模式来初始化。任意无效的预测模式都视为Intra_DC.为了避免存储相邻亮度预测模式的一行buffer，亮度CTB之上的PB都认为是无效的
When the first two most probable modes are not equal, the third most probable mode is set equal to Intra_Planar, Intra_DC or Intra_Angular[ 26 ] (vertical), according to which of these modes, in this order, is not a duplicate of one the first two modes. When the first two most probable modes are the same, if this first mode has the value Intra_Planar or Intra_DC, the second and third most probable modes are assigned as Intra_Planar, Intra_DC, or Intra_Angular[ 26 ], according to which of these modes, in this order, are not duplicates. When he first two most probable modes are the same and the first mode has an Intra_Angular value, the second and third most probable modes are chosen as the two angular prediction modes that are closest to the angle (i.e. the value of k) of the first.
当前两种可能模式不相等的情况下，第三种可能模式会设定为 Intra_Planar, Intra_DC or Intra_Angular[ 26 ]（垂直）模式中不等于前两种模式的其中一种。
当前两种模式相同的情况下，如果第一种模式是the value Intra_Planar or Intra_DC，第二种和第三种模式将根据Intra_Planar, Intra_DC, or Intra_Angular[ 26 ]在顺序上不重复的原则设定其中的两个。当前两个最可能模式是相等时，且第一个模式的值为Intra_Angular， 则将第二个和第三个最可能模式设为和第一个模式在方向最近的预测方向值；
In the case that the current luma intra prediction mode is one of three MPMs, only the MPM index is transmitted to the decoder. Otherwise, the index of the current luma intra prediction mode excluding the three MPMs is transmitted to the decoder by using a 5-bit fixed length code. For chroma intra prediction, HEVC allows the encoder to select one of five modes: Intra_Planar, Intra_Angular[ 26 ] (vertical), Intra_Angular[ 10 ] (horizontal), Intra_DC, and Intra_Derived. The Intra_Derived mode specifies that the chroma intra prediction uses the same angular direction as the luma intra prediction. With this scheme, all angular modes specified for luma in HEVC can, in principle, also be used in the chroma intra prediction, and a good trade-off is achieved between prediction accuracy and the signaling overhead. The selected chroma intra prediction mode is coded directly (without using an MPM prediction mechanism).
如果当前的亮度预测模式是三个最可能模式(MPM)中的一种，则只有MPM的索引号被传输给解码器；此外， 除了三个最可能模式外，当前亮度预测模式的索引也要使用5比特的定长码字传输给解码器；
对于色度帧内预测模式，HEVC允许其选择下面五种模式：
Intra_Planar,
Intra_Angular[26](垂直),
Intra_Angular[10](水平),
Intra_DC,
Intra_Derived,
其中，Intra_Derived是将色度预测模式设定为对应亮度块的帧内预测同一个方向；对于这样一种设计， 在原则上，HEVC支持的所有亮度方向预测模式在色度上也同样支持；这样的设计很好地取得了预测精度和信号传输开销间的平衡；被选择的色度预测模式是直接编码的(不用MPM);
