Using ffmpeg to halve frame rate

[RS]

On 16/04/18 09:27, Nick Payne wrote:
> On 16/04/2018 5:21 PM, Steve Dodd wrote:
>> On Sun, Apr 15, 2018 at 9:59 PM, Nick Payne <njhp94 at gmail.com> wrote:
>>
>>> 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.
>> Figures - most of the comparison work done on H.265 v. H.264 says you
>> should get same subjective quality for half the bitrate.
>>
>>> 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.
>> Do you know what x265 quality setting you used? I'm also experimenting
>> now it seems HLSHD is being turned off (series 2 of Salamander has HVF
>> only so far).. RF17 seems to be spitting about half the bitrate of the
>> BBC h.264 output, and running at ~40fps which I can live with (Core
>> i5-6500). Noticeable quality difference still when looking at stills,
>> but have tried watching it moving yet..
> For re-encoding 1280x720 downloads to x265, on the Video tab in
> Handbrake I have the encoder preset at "Faster" and the RF quality at 23.
> 
I tried using ffmpeg and the libx265 codec with default quality 
(CRF=28?) on HVFhd.  I only did half a minute because it was so slow. 
It is convenient to do it on this machine, but it has only got a 2.3GHZ 
Core 2 duo.  I set -r=25 to reduce the frame rate to 25fps by dropping 
alternate frames.  It confirmed that dropping frames worked, and I did 
not notice any problems as a result.  It only processed 2 frames a 
second, so an hour's recording would take 12 hours!  I'll have to try it 
on my latest laptop which has a core i5-7200U.

Best wishes
Richard