[FFmpeg-user] understanding DTS PTS

[Alexandre Ferrieux]

Andy Sheen wrote:
> 
> Bouke wrote:
>> ----- Original Message ----- 
>> From: "Alexandre Ferrieux" <alexandre.ferrieux at orange-ftgroup.com>
>> To: "FFmpeg user questions and RTFMs" <ffmpeg-user at mplayerhq.hu>
>> Sent: Wednesday, March 04, 2009 9:25 AM
>> Subject: Re: [FFmpeg-user] understanding DTS PTS
>>
>>
>>> Marc Mason wrote:
>>>> John wrote:
>>>>
>>>>> I am going through the ffmpeg tutorials and am having trouble making
>>>>> sense of this statement in tutorial 5.
>>>>>
>>>>> <quote>
>>>>> However, ffmpeg reorders the packets so that the DTS of the packet
>>>>> being processed by avcodec_decode_video() will always be the same as
>>>>> the PTS of the frame it returns.
>>>>> </quote>
>>>>>
>>>>> What reordering is it talking about? From the example
>>>>>
>>>>> PTS: 1 4 2 3
>>>>> DTS: 1 2 3 4
>>>>> Stream: I P B B
>>>>>
>>>>> The frames are being read in I P B B. When P is being processed by
>>>>> avcodec_decode_video() its DTS is 2. P's PTS cannot be 2. What frame
>>>>> reordering is ffmpeg supposed to be doing? Thanks...
>>>> Suppose we have 4 frames, and we decide to encode them the following 
>>>> way: F1=I F2=B F3=B F4=P
>>>>
>>>> To encode F2 and F3, we need F4 (B frames).
>>>>
>>>> The presentation order (PTS) is F1, F2, F3, F4.
>>>>
>>>> But since the decoder needs F4 to decode F2 and F3, we can send them in 
>>>> a different order : F1, F4, F2, F3 (the decoding order).
>>>>
>>>> The decoder will receive frames in the following order.
>>>>
>>>> F1 DTS=1 PTS=1
>>>> F4 DTS=2 PTS=4
>>>> F2 DTS=3 PTS=2
>>>> F3 DTS=4 PTS=3
>>> OK, but what purpose does the DTS serve then ?
>>>
>>> -Alex
>> Tell the decoder when to display the frames (thus, in the correct order)
>>
> 
> Err.. DTS == DECODE time stamp, PTS == PRESENTATION time stamp. The DTS 
> is used by the decoder to decide when to decode the frame, and PTS is 
> used to decide when to put it up on screen.


Isn't "when to decode" equal to "as soon as available" ?
In what respect does DTS differ from simply the order of arrival ?

-Alex