Using ffmpeg to halve frame rate

[Nick Payne]

On 16/04/2018 6:59 AM, Nick Payne wrote:
> On 16/04/2018 2:18 AM, RS wrote:
>> A couple of years ago there were problems with HLS and there was
>> speculation that we might have 50 fps HVFhd and DVFhd as the only HD
>> modes.  At that time I wondered if it would be feasible to drop every
>> other frame to reduce the frame rte to 25 fps.  I thought because of
>> the H.264 delta encoding it would probably mean transcoding and that
>> it would be too slow.
>>
>> On a 42" screen for the programmes that I usually watch I cannot see
>> any difference between a HD picture at 50 fps and one at 25 fps, so 50
>> fps for me is just a waste of bandwidth and storage space, so I have
>> been looking at it again.
> I experimented a while ago with running some GiP downloads through
> Handbrake to convert the video to the x265 codec. That reduced the file
> size to about 1/3 of the original download, with no subjective
> difference in video quality that I could see when viewing on our TV. A
> one hour 1280x720 25fps program that downloads as a 1Gb file was reduced
> in size to ~350Mb. On my PC, the re-encode runs at about 50fps for
> 1280x720, so ~2x real time for 25fps and ~real time for 50fps. For
> smaller video dimension such as 960x540, the re-encode speed was around
> 85-90FPS.
I just ran another test on some 1280x720 50fps and 1280x720 25fps
downloads from the last UK snooker championship. As before, the 25fps
downloads were reduced to about 1/3 previous size by Handbrake (eg a
4h58m match that was 5.03Gb came down to 1.55Gb). However, the 50fps
downloads were reduced by far more with exactly the same settings in
Handbrake - for example, a 4h44m match that was a 10.2Gb download from
GiP came down to 1.48Gb, and a 4h14m 8.93Gb file came down to 1.30Gb.
This leads me to wonder if the 50fps downloads are from a 25fps original
and just have each frame duplicated, as after running through Handbrake,
the file sizes just seem proportional to the length of the program, and
whether 50fps or 25fps has no effect on the file size.

Nick