[Ffmpeg-devel] av_seek_frame() units

Steve Willis ffmpeg
Thu Jul 7 00:35:47 CEST 2005

Gary Corcoran wrote:
> Steve Willis wrote:
>> Nathan Kurz wrote:
>>> On Wed, Jul 06, 2005 at 12:04:44PM -0700, Ian Gowen wrote:
>>>> On 7/6/05, Andy Parkins <andyparkins at gmail.com> wrote:
>>>>> I'm not sure why you would need to do this.  What source for 
>>>>> frame_number are
>>>>> you using - it's not in any of the AV structures.
>>>> I'm working on a video editing application. If the user wants to
>>>> preview frame 320, I
>>>> need it to seek to frame 320, not the nearest keyframe or a frame in
>>>> the general
>>>> vicinity of frame 320.
>>> Not to be too discouraging, but you should also know that seek is
>>> badly broken for many formats.  It's quite possible that you are doing
>>> things correctly, but are being bitten by bugs.  If you search  the
>>> archives from (I think) the end of last year you'll find a fairly
>>> elaborate test program I wrote to test which formats work.  I had
>>> submitted it as a regression test, but as it pointed out new bugs
>>> rather than regressions it wasn't included in the test suite.
>>> Nathan Kurz
>>> nate at verse.com
>>> _______________________________________________
>>> ffmpeg-devel mailing list
>>> ffmpeg-devel at mplayerhq.hu
>>> http://mplayerhq.hu/mailman/listinfo/ffmpeg-devel
>> It sounds like Ian and I are both working on applications that need 
>> precise frame positioning. In light of what Nathan said above, can 
>> anyone offer recommendations for the best way to ensure that the user 
>> is looking at the frame he or she thinks they should be looking at? Is 
>> there a reliable way to detect when seek has gone wrong ("I expected 
>> to be looking at the frame for time 13.125 seconds, but I actually got 
>> the frame for time X...error!")?
>> In response to my original question a couple of weeks ago, someone 
>> suggested using a format like MJPEG that treats all frames as 
>> keyframes. If there is not a more general solution for precicely 
>> positioning arbitrarily-formatted streams, could someone give me an 
>> expert opinion on the best format to transcode all input videos into? 
>> The videos I am dealing with are printed from high speed film (1000 
>> fps) for scientific analysis frame-by-frame. I'm looking for a 
>> reasonable balance between decoding speed and size. Image quality is 
>> of course important for seeing detail in each frame of interest. Most 
>> important is that the frame shown is actually correct for the time in 
>> question to match up with external sensor data, so a format that does 
>> not use keyframes would also be important.
> Couldn't you convert it to MPEG, specifying that only I-frames (keyframes)
> be used?  Then the "seek only directly works to I-frames" feature wouldn't
> be a problem...

I don't think MPEG supports arbitrary frame rates. The FAQ on the ffmpeg 
Web site mentions that only a small number of frame rates are allowed. 
The videos are printed from film. They are usually in the ballpark of 
1000 fps. No player can realistically play them at this speed, but they 
work very well for frame-by-frame analysis. One other requirement for a 
format I should mention is the ability to encode an arbitrary number of 
video streams. Some of these videos contain multiple video tracks with 
different angles of the same event. I need a format that lets me 
preserve this in a single file.

Thanks for your suggestion, and please, let me know if I'm wrong about 
MPEG and framerates!


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