hevc 中IDR图像类型为何有 IDR_N_LP和IDR_W_RADL
IDR_N_LP表示没有leading picture
IDR_W_RADL表示可能有RADL 图像（associated random access decodable leading (RADL) pictures）
The reason for having two different IDR picture types is to enable system The reason for having two different IDR picture types is to enable system
layers to know at random access whether the IDR picture is the first picture to output layers to know at random access whether the IDR picture is the first picture to output
or not. The POC value of an IDR picture is always equal to zero. Thus, the leading or not. The POC value of an IDR picture is always equal to zero. Thus, the leading
pictures associated with an IDR picture, if any, all have negative POC values. pictures associated with an IDR picture, if any, all have negative POC values.
之所以有这两种类型，原因在于可以使得系统层知道IDR是否是第一个输出的图像。IDR图像的POC值总是为0，所以如果有关联的RADL图像，则其POC值为负值。 之所以有这两种类型，原因在于可以使得系统层知道IDR是否是第一个输出的图像。IDR图像的POC值总是为0，所以如果有关联的RADL图像，则其POC值为负值。
IDR_W_RADL的一种情况是，实际显示顺序处在后面的帧编码为IDR帧，然后再编码显示顺序时间上处在前面的帧，并且可以参考刚才的IDR帧，那么此帧就可以作为RADL帧。 IDR_W_RADL的一种情况是，实际显示顺序处在后面的帧编码为IDR帧，然后再编码显示顺序时间上处在前面的帧，并且可以参考刚才的IDR帧，那么此帧就可以作为RADL帧。
