【LibJPEG专题系列二十】动态图像的M-JPEG标准

席萌0209

                       Section - [20] Isn't there an M-JPEG
                              standard for motion pictures?

As was stated in section 1, JPEG is only for still images.  Nonetheless, you will frequently see references
to "motion JPEG" or "M-JPEG" for video. *There is no such standard*.  Various vendors have applied
JPEG to individual frames of a video sequence, and have called the result "M-JPEG". Unfortunately, in
the absence of any recognized standard, they've each done it differently.  The resulting files are usually
not compatible across different vendors.


MPEG is the recognized standard for motion picture compression.  It uses many of the same techniques
as JPEG, but adds inter-frame compression to exploit the similarities that usually exist between
successive frames. Because of this, MPEG typically compresses a video sequence by about a factor of
three more than "M-JPEG" methods can for similar quality. The disadvantages of MPEG are
(1) it requires far more computation to generate the compressed sequence (since detecting visual
similarities is hard for a computer), and
(2) it's difficult to edit an MPEG sequence on a frame-by-frame basis (since each frame is intimately tied
to the ones around it).  This latter problem has made "M-JPEG" methods rather popular for video editing
products.


It's a shame that there isn't a recognized M-JPEG standard.  But there isn't, so if you buy a product
identified as "M-JPEG", be aware that you are probably locking yourself into that one vendor.


Recently, both Microsoft and Apple have started pushing (different :-() "standard" M-JPEG
formats.  It remains to be seen whether either of these efforts will have much impact on the current
chaos.  Both companies were spectacularly unsuccessful in getting anyone else to adopt their ideas
about still-image JPEG file formats, so I wouldn't assume that anything good will happen this time
either...


See the MPEG FAQ for more information about MPEG.

席萌0209

                      Section 20—动态图像的M-JPEG标准


正如第一节所述，JPEG只适用于静态图像。尽管如此，还是会看到视频中“动态JPEG”或“M-JPEG”的
引用。当然，不存在这样的标准。各个供应商已经将JPEG应用到视频序列的各个帧，并称其结果为“M-
JPEG”。在没有任何公认的标准下，他们已经渐行渐远。因而，所生成的文件通常不能兼容于各个供应商。


MPEG是公认的用于动态图像压缩的标准。它使用了很多与JPEG相同的技术，但同时又增加了帧间压缩来利用
存在于连续帧之间的相似点。正因为如此，MPEG通常要通过三倍于“M-JPEG”的方法来压缩视频序列以
获取相似的质量。MPEG的劣势在于：
（1）需要更多的计算来产生压缩的序列（因为检测视觉上的相似点对于计算机来说不是件容易事。）
（2）在逐帧的基础上很难编辑一个MPEG序列。（因为每一帧都和其周围的帧紧密联系在一起。）
这后一个问题的存在已使得“M-JPEG”的方法在视频剪辑产品中非常流行。

没有一个公认的M-JPEG标准是很尴尬。正因为没有，所以如果你购买了一个认定为“M-JPEG”的产品，你
需要清楚，这就将你捆绑了这一产品的供应商。


近日，微软和苹果公司已经开始推出（不同的）“标准的”M-JPEG格式。这些努力是否对当前的混乱产生冲
击还有待观察。在让他人采纳自己关于静态图像的JPEG文件格式的想法上，两家公司均不成功。所以我认为
目前不会有更好的情况发生。

关于MPEG的更多信息，请查看MPEG的FAQ（常见问题解答）。