no more hslv format ?

[Owen Smith]

That doesn't make any difference to the H.264 encoding. At best if you get lucky every odd frame will be marked as being interpolated from the preceding even frame and then no actual changes within the frame. But all it takes is for the encoder to slip by a frame somewhere or to encode an I frame (a complete standalone frame) in the even or odd half that you are trying to discard every one of and then you are screwed.

And if the 50fps file genuinely does have every other frame saying "I'm identical to the previous frame" the stripping these won't save much on the file size. The fact that people complain the 50fps files are double the size of the same resolution 25fps files implies to me this isn't how it has been done. Even if the BBC have only created motion interpolated frames from a 25fps source, they are still encoded in the video stream and you still can't casually toss alternate frames without screwing up the I-P-B frame interpolation.

-- 
Owen Smith <owen.smith at cantab.net>
Cambridge, UK

> On 2 May 2018, at 19:43, Peter S Kirk <peter.kirk at isauk.biz> wrote:
> 
> Many posts back it was mentioned they are not true 50fps, instead each 
> frame from a 25fps is duplicated merely to allow BBC to boast about 50fps 
> streaming.
> 
> On 2 May 2018 at 19:07, Owen Smith Owen Smith <owen.smith at cantab.net> 
> wrote:
> 
>> What do you mean this isn't a lossy transcoding? How can ffmpeg go from 50fps to 25fps without losing anything? The
>> frames are not all complete frames, software can't just throw alternate frames away. Well it could,  but the only way
>> to do that is a full H.264 decode, then discard alternate frames, then a full H.264 encode again which is going to
>> involve loss.
>> 
>> Most frames are not fully present in the original stream, they are interpolated from previous and subsequent frames.
>> You can't throw any of those away, because other frames are interpolated from them. It would need to be a very special
>> original encode which had all even frames only interpolated from other even frames and ditto for odd frames to allow
>> alternate frames to be discarded. And a special encode like that would bloat the file size substantially, almost
>> doubling it I would expect.
> 
> 
> 
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