Unable to change video fps

[JustSimplyKyle]

I wanna change my video fps without slowing down, essentially just dropping frames.
base codec is vp9.
I tried using ffmpeg_filter_args to do. -r 30, the encoder complains about frame mismatch

I then changed extra split to 300 instead of the default 600. Same exact problem.
Then I tried -filter:v fps=fps=30, same exact error.
Then I tried using vp9's fps filter, --fps=30/1, it works this time, but it slows down the footage by 50%(cause my original video is 60fps)
Even by combining vp9 fps filter + ffmpeg filter, it all comes down to the same problem.

[silverbacknet]

That's not something Av1an is set up to do, since it completely changes its expectations. What you can do instead is, load the video via a VapourSynth script that performs the decimation before it reaches Av1an instead.

[redzic]

Yeah, pretty much just echoing what @silverbacknet said here.

--ffmpeg filters each chunk separately, and the filtering is not set up in a way such that it expects filters to change the number of frames in a chunk, i.e. it does not filter the entire video first and then encode it.

A vapoursynth that changes the video fps should work though.

[JustSimplyKyle]

Oh ok. I assume this can do it?

import vapoursynth as vs
import functools
import math

target_fps_num = 30
target_fps_den = 1
core = vs.core
source_clip = core.ffms2.Source(source='abr213hrH2.mkv')
def frame_adjuster(n, clip, target_fps_num, target_fps_den):
    real_n = math.floor(n / (target_fps_num / target_fps_den * clip.fps_den / clip.fps_num))
    one_frame_clip = clip[real_n] * (len(clip) + 100)
    return one_frame_clip

attribute_clip = core.std.BlankClip(source_clip, length=math.floor(len(source_clip) * target_fps_num / target_fps_den * source_clip.fps_den / source_clip.fps_num), fpsnum=target_fps_num, fpsden=target_fps_den)
adjusted_clip = core.std.FrameEval(attribute_clip, functools.partial(frame_adjuster, clip=source_clip, target_fps_num=target_fps_num, target_fps_den=target_fps_den))
adjusted_clip.set_output()

[lastrosade]

Changing the framerate from vapoursynth does in fact work.

Tested with

import vapoursynth as vs
import mvsfunc
import lasfunc

src_file = r"H:\test\av1an_fps\testclip_117.mp4"
lwi_file = r"H:\test\av1an_fps\testclip_117.lwi"

k_r = lasfunc.util.fmtc_kernel_kwargs("robidoux")
k_d = lasfunc.util.fmtc_kernel_kwargs("didée")

interpolation_downto_num = 30000
interpolation_downto_den = 1001

# --------------------------------------------------------------------

clip = vs.core.lsmas.LWLibavSource(source=src_file, cachefile=lwi_file)
clip = mvsfunc.ToYUV(clip, css="444", depth=16, **k_r)
clip = lasfunc.mv_motion_interpolation(clip, fpsnum=interpolation_downto_num, fpsden=interpolation_downto_den, preset="fast", block=False)
clip = mvsfunc.ToYUV(clip, matrix="bt709", css="420", depth=10, **k_d)

clip.set_output()

[silverbacknet]

Thanks for the completed script! This would actually be a decent addition to Av1an down the road, since it's not an uncommon situation.