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N-vop

an N-vop is a frame that doesn't exist, they are used as place holders in avi-files.

S-vop

Sprite对象是针对背景对象的特点提出的。通常情况下背景对象自身没有任何运动,而图像序列中的背景变化是由于摄像机的运动和前景物体的运动所造成的,通过图像的镶嵌技术把整个序列的背景图像拼接成一个大的完全的背景图像,这个图像就叫Sprite图像. Sprite图像只需要编码传输一次并存储在解码端,在随后的图像只需要传输摄像机相对于背景的运动参数,就可以从Sprite上恢复所有的图像的背景,在视频会议和视频电话等场景固定的应用中,背景的全景图可以事先传输过去保存在解码端。实现这种编码必须要有两个前提条件：1.前景对象和背景图像要能很好的分割开；2.要无痕迹的从一段视频或一些图像中拼接出Sprite图像。

Packed Bistream

The differece is "packing": first b-frame is packed together (in an avi) with its future reference (a p-frame most likely). Future reference is transmitted first because it has to be decoded first, but the frame you actually want to *see* is the b-frame. So, packing delivers the b-frame immiedietly, so it can also be decoded and shown on time.

Without packing, decoder gets future reference, but has to wait for the b-frame before it can display anything. This creates a one-frame lag, which makes such avi difficult to seek in (for example) virtualdub.

Packed Bitstream can be detected from the divx userdata tag, the tag takes the form "DivX%dB%d%c", version, build, packed. 

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