ffmpeg codec conversion; can't configure encod

Question

I am simply trying to convert a vp9 webm I have into a vp8 webm. this is the command I'm using.

ffmpeg -i in.webm -c:v vp8 out.webm

the vp8 encoder returns a strange error

Input #0, matroska,webm, from 'in.webm':
  Metadata:
    encoder         : google
  Duration: 00:02:34.60, start: 0.000000, bitrate: 404 kb/s
    Stream #0:0(eng): Video: vp9 (Profile 0), yuv420p(tv, bt709/unknown/unknown), 640x360, SAR 1:1 DAR 16:9, 30 fps, 30 tbr, 1k tbn, 1k tbc (default)
Stream mapping:
  Stream #0:0 -> #0:0 (vp9 (native) -> vp8 (vp8_v4l2m2m))
Press [q] to stop, [?] for help
[vp8_v4l2m2m @ 0x56195db9f9f0] Could not find a valid device
[vp8_v4l2m2m @ 0x56195db9f9f0] can't configure encoder
Error initializing output stream 0:0 -- Error while opening encoder for output stream #0:0 - maybe incorrect parameters such as bit_rate, rate, width or height
Conversion failed!

In case I get any questions about whether I have the right encoders and decoders installed here is the output of ffmpeg -codecs | grep "vp[8-9]":

 DEV.L. vp8                  On2 VP8 (decoders: vp8 vp8_v4l2m2m vp8_cuvid ) (encoders: vp8_v4l2m2m )
 D.V.L. vp9                  Google VP9 (decoders: vp9 vp9_v4l2m2m vp9_cuvid )

Answer 1

Your ffmpeg build has a Video4Linux wrapper available for a hardware VP8 encoder, but no actual hardware VP8 encoder available.

For software encoding of VP8/VP9, you'll need libvpx enabled and linked. You can get a static git binary from https://johnvansickle.com/ffmpeg/