[FFmpeg-user] considering image sequence+audio for video preservation

[Peter B.]

Dear Ibrahim,

I've now read your message completely :)

I find your idea of normalizing everything to RGB an interesting approach.
I'm not a mathematician, but I believe using the Reversible Color
Transform algorithm to convert between RGB and YCbCr, should be lossless
- and as the name suggests "reversible".

Yet, it doesn't feel right to do this...

As Kieran O'Leary already said:

[quote]
It would seem to me that one of the significant properties of your
source is the colour space, and altering it for your long term
preservation file seems a bit risky and unnecessary?
[/quote]


Other observations I've made when I was researching image sequences as
alternative to codec+container were:

1) Non-RGB in image formats greatly narrows down the choice of formats
2) DPX might be a good option (since intended for moving image), but
brings in other issues...
3) If this is your only copy and space is an issue, compression choices
(and proper support across your current and future workflow) might be an
issue
4) Depending on your source, some properties cannot be properly
maintained (as cehoyos already mentioned Variable Frame Rate (VFR), for
example)
5) Transcoding access/production copies from the archive copy always
requires some way of handling image sequence + audio
6) Sample- and frame-exact audio/video synchronization might be an issue
(tool dependent)


And as I've already mentioned in my previous mail:
The speed-loss due to filesystem overhead is tremendous.

See for yourself:
Try to transcode e.g. 1 minute PAL (=1500 frames = 1500 files) from:
* uncompressed in AVI
* uncompressed BMP image sequence

The data size is almost the same, but the performance difference
(especially over network filesystems) was very noticable in all my tests :(


Yet, if you can afford to have 2 different versions of your archive copy
(e.g. codec+container and image_sequence+audio), you would indeed
increase the chances of being able to access your content in the future.



Kind regards,
Pb