h.265 (hevc)

alignment=au means that each output buffer contains the NALs for a whole 
picture. alignment=nal just means that each output buffer contains 
complete NALs, but those do not need to represent a whole frame

https://www.kancloud.cn/digest/hevc-fred/181914

https://mp.weixin.qq.com/s/r5_gO-I0WM-1sv3_vyVBeg