Render a 50fps project at 25fps

[Evert Vorster]

Hi there, Cyril.

I can describe the bug to you, and then you can decide whether you want to
take your chances with it.
Kdenlive uses frames to refer to certain points in the video, as well as
the associated clips.

Say you have a clip of you playing fetch with a dog that is 2000 frames
long, and encoded at 50fps.

If you read that clip into a project that is only 25fps, kdenlive would
then do some behind the scenes math and present the clip to you as only
1000 frames, with every second frame dropped.
This is to preserve the original length of the clip, and make editing
easier. It's all good and well, and the right thing to do.

The problem comes in when you start your project with one framerate, and
loaded your clips, and then change to another framerate.
The clips in the project are re-read and updated to have the right amount
of frames for the new framerate, but the starting end ending points of the
clips on the timeline are not.

To put this into the example of the clip with the dog playing fetch....
Let's assume that at 50fps from frame 1500 to 1800 the dog is catching the
ball. You select this range, and put it on the timeline.
At the lower framerate, the catching of the ball should happen at frame 750
to 900, right? The timeline never updates that start and end frame are
never updated.
In this case it would refer to frames that does not even exist in the clip
at the new framerate. In all cases it changes the _content_ of the clips,
so at best you will be looking at another part of the clip in the final
render than the part you selected.

I highly recommend against changing a project's frame rate at this point in
time.

Here is the latest bug in the bug reporter:
https://bugs.kde.org/show_bug.cgi?id=368962

Kind regards,
-Evert-


On 24 October 2017 at 12:37, harald.albrecht <harald.albrecht at gmx.net>
wrote:

> Yes, and no. As Evert pointed out, there's currently a bug in the timeline
> handling that sometimes causes issues (but not always as far as I
> understand, but I may be wrong here). The safest approach is as Evert
> suggested to render at 50fps and then use ffmpeg for post-processing to
> drop every second frame.
>
> Best regards,
> Harald
>
>
> -------- Ursprüngliche Nachricht --------
> Von: Cyril Duveau <linux4cyril at free.fr>
> Datum: 24.10.17 10:12 (GMT+01:00)
> An: kdenlive at kde.org
> Betreff: Re: Render a 50fps project at 25fps
>
> Hello Harald,
>
> Thank you for your detailed response.
> Just to be sure, I can do that change while all my files are loaded and
> I am ready to render?
> Once I have the settings at 25fps, could I go back to 50fps the same
> way (HD 1080p 50 fps)?
>
> I want to perform tests to see if it is really worth rendering at 50fps
> or if 25 is enough...
>
> Thanks & Regards,
> Cyril
>
> Le lundi 23 octobre 2017 à 21:08 +0200, Harald Albrecht a écrit :
> > Cyril,
> > you don't mention the other project settings, so I need to guess to
> > some extend; I'm assuming FullHD for the moment. In the Project >
> > Project Settings... dialog, set the "Fps" drop down to 25 so you
> > don't need to sift through all the many profiles. In the list, open
> > the "Full HD 1080", so you see (assuming that no filter is set for
> > scanning) two entries: HD 1080i 25 fps, as well as HD 1080p 25 fps.
> > Select the second one (HD 1080p 25 fps), click "Ok". Done.
> >
> > Best regards,
> > Harald
> >
> > Am 23.10.2017 um 12:38 schrieb Cyril Duveau:
> > > Hi,
> > >
> > > I would like to produce a video at 25fps from footage at 50fps.
> > > The project settings are for 50fps, that's the rendering profile
> > > for
> > > exporting that I don't know where/how to specify the fps.
> > >
> > > I found this but I can't find how to adapt it to the new system:
> > > https://forum.kde.org/viewtopic.php?f=272&t=124869#p329129
> > > Can you help?
> > >
> > > Thanks,
> > > Cyril
> >
>



-- 
Evert Vorster
Isometrix Acquistion Superchief
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.kde.org/pipermail/kdenlive/attachments/20171024/b2e02484/attachment-0001.html>