Changes between Version 1 and Version 2 of framemd5 Intro and HowTo


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Timestamp:
Jul 20, 2013, 2:03:02 AM (3 years ago)
Author:
llogan
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fix link

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  • framemd5 Intro and HowTo

    v1 v2  
    33Within digital preservation environments, the generation and verification of checksums against digital files can aid the confirmation or denial of digital authenticity over time. A checksum mismatch is an alert that a file under care has changed from a prior state; potentially triggering retrieval of backups, review of hardware, or migration of content. Generally, if a given checksum algorithm is applied to a file, then as long as the same checksum can be regenerated from the file then the data is verified, else a mismatched checksum reveals a digital change. Further details such as the whereabouts, extent, or significance of the change in data are not revealed by the checksum mismatch but only that the data examined now is not the same as the data examined before. 
    44 
    5 The FFmpeg [http://ffmpeg.org/ffmpeg-formats.html#framemd5] framemd5 format and [http://ffmpeg.org/ffmpeg-formats.html#framecrc framecrc] format as used to decode input audiovisual data to produce one checksum per frame. These formats facilitate testing functions such as verifying that an adjusted decoder maintains intended results or that an FFmpeg decoder decodes a stream to the same data as another decoder. 
     5The FFmpeg [http://ffmpeg.org/ffmpeg-formats.html#framemd5 framemd5] format and [http://ffmpeg.org/ffmpeg-formats.html#framecrc framecrc] format as used to decode input audiovisual data to produce one checksum per frame. These formats facilitate testing functions such as verifying that an adjusted decoder maintains intended results or that an FFmpeg decoder decodes a stream to the same data as another decoder. 
    66 
    77By producing checksums on a more granular level, such as per frame, it is more feasible to assess the extent or location of digital change in the event of a checksum mismatch. By decoding a file and processing the decoded data to generate a framemd5 document, each decoded audio and video frame is documented according to its timestamp, digital size, and MD5 checksum. For the first three frames of video, the framemd5 output could be: 
     
    6464This output reflects the default handling of framemd5 where each frame is decoded (to rawvideo for video or pcm_s16le for audio) and then the checksum is generated from that decoded data. 
    6565 
    66 The following command adds '-c copy' which causes the framemd5 to generate checksums of the data as it is stored. 
     66The following command adds `-c copy` which causes the framemd5 to generate checksums of the data as it is stored. 
    6767{{{ 
    6868ffmpeg -i MOVIE.mov -c copy -f framemd5 MOVIE.framemd5 
     
    104104}}} 
    105105 
    106 Where this command will transcode the source video using libx264 before transcoding; 
     106Where this command will transcode the source video using libx264 before transcoding: 
    107107{{{ 
    108108ffmpeg -i MOVIE.mov -c:v libx264  -f framemd5 MOVIE.framemd5