[PD] Re: [GEM-dev] Codecs in Gem (again!) & ffmpeg

[chris clepper]

On Aug 30, 2005, at 10:05 PM, B. Bogart wrote:

> Thanks for the suggestions.
>
> I'm not sure about using a mpeg for the following reasons:
>
> #1. was does the interframe compression thing work, in other words how
> does it compare when seeking to a quicktime with photo-jpg codec?

You want to avoid interframe compression at all costs.  In order to  
grab a random frame in an interframe codec the nearest previous  
keyframe must be found and all interframes between it and the  
requested frame are decoded.  You could get lucky and hit a keyframe  
or be very unlucky and hit the frame right before one.

Interframe  compression techniques are used almost exclusively for  
final content distribution (web, DVD, ATSC).  Acquisition and  
production codec never use this with the exception of HDV (which is  
MPEG2).

> Aside: What id the different between mjpeg and photojpeg as a video
> codec?

mjpeg is motion-jpeg which is for interlaced two field video.  Each  
field (50 or 60 per second) is a compressed with jpeg techniques.

> ffmpeg reports my sequence of jpegs as "mjpeg"...

It is mistaken if your sequence is single progressive frames, which  
it almost certainly is.

cgc