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How to decode a MJPEG file

Posted on 2012-09-04
Medium Priority
Last Modified: 2013-11-08
I need help to understand how one specific MJPEG file is put together. The file(s) are produced by a Synology Disk Station. The disk station is connected to Axis M1054 cameras which produce a MJPEG stream. I believe that the disk station is actually just saving images from the camera directly to the disk, but at the same time, it probably adds encapsulation to the file. I want to build a program that can browse quickly in the files produced by the disk, so I need information on how to read the file. I tried to read the file by VirtualDub and it was capable to browse through it super fast, so that's why I believe it is a MJPEG file. I will put it out on a .torrent soon, cause it is very big.

Are there any programs that will show me which bytes are for what, like Wireshark is doing for a network packet?
Question by:lefodnes

Author Comment

ID: 38365921
I analyzed the file in MediaInfo, and I'm attaching the result from it. I'm not allowed to upload a torrent file here, so it is accessible from http://lef.no/mjpeg_test.avi.torrent
LVL 70

Expert Comment

ID: 38366313
I find wiki is good for a starter explains the inner workings
Motion JPEG
The mjpeg programs are a set of tools that can do recording of videos and playback, simple cut-and-paste editing and the MPEG compression of audio and video. Command line tools

Author Comment

ID: 38367275
From what I can tell, wiki gives no information about the inner workings of Motion JPEG, except the workings of a http stream. Just pro/cons, tools that can edit video, which is not what I am after. I am not looking for simple editing software (MJPEG-Tools).

I'm looking for information on the protocol, so that I will be able to decode AVI/MJPEG from the byte level of a file.
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LVL 31

Expert Comment

ID: 38372189
Hi lefodnes,

did you already take a look at ffmpeg? With this tool you should be able to extract single frames from the MJPEG. You can find it here: http://ffmpeg.org/

If you want to write your own program to extract frames I think you could use libav which is a free library which supports audio/video en-/decoding: http://libav.org/


Author Comment

ID: 38372329
Hi Zoppo,
Thank you for the link to libav.org. I will have a look at it.
My goal is to make something that reads the file in an optimized way, since the file is stored on a Synology on a LAN. I'd like to download only certain parts of the file on the fly, and build this file locally as a cache. This's why I need to understand the bits and bytes of the file. In my search I will also be looking closer at the RIFF/AVI format, since this MJPEG is contained in an AVI.
As I mentioned earlier, I do not need a program or library that can show or edit the file as video or images. I need a program from which I can learn how to read and browse quickly inside an AVI / MJPEG file.
LVL 83

Assisted Solution

leakim971 earned 400 total points
ID: 38372463
LVL 25

Accepted Solution

kode99 earned 1600 total points
ID: 38374925
The Synology unit stores the mjpeg in avi format but the 'frames' or jpg files are easily extracted.

All you need to do is scan the file for the start marker for the jpg pictures,  it will be a hex sequence 0xFF 0xD8.  This is the start of a jpg picture.  Then just scan ahead for the next start marker and everything before it is the jpg file - including the start marker.


0xFF 0xD8 ...... picture data ...  0xFF 0xD8 .... next picture data ... etc

You can check this out with a hex editor so you can look at the raw data in the files.  A hex editor is the equivalent to wireshark but for files.  

Heres a free editor that works good,  

Search out the marker and you can check it out by cutting out a frame and pasting it into a new file.  I actually verified this on a file from a synology unit just to be sure they did not re-encode anything.

Jpeg uses the 0xFF <2nd hex value> as the start for headers and encoding information.  The 0xFF 0xD8 is always the start of the whole picture though.

When you look at jpg files with the hex editor you will likely see other 0xFF sections at the start with other information.  For example I'm looking at a jpg file and it shows me a date time and that the file was created with Adobe Photoshop.  If the Axis camera puts in a date and time you would be able to extract that as well.

Author Closing Comment

ID: 38375150
Hi kode99 and leakim971,

Thank you both for providing good information.

kode99, I appreciate that you actually verified the file format and described this in a good way.

leakim971, Thank you for the links. I have not read them all yet, but you both provided very good information for me to continue working with this project.

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