[FFmpeg-user] vaapi: Impossible to convert between the formats

[André Hänsel]

I'm trying to transcode an MPEG4 avi to MP4 with VAAPI, but I'm getting an
error:

Impossible to convert between the formats supported by the filter
'Parsed_null_0' and the filter 'auto_scaler_0'
Error reinitializing filters!
Failed to inject frame into filter network: Function not implemented

The failing command is:
ffmpeg -hwaccel vaapi -vaapi_device /dev/dri/renderD128
-hwaccel_output_format vaapi -i test.avi -c:v h264_vaapi out.mp4

Other combinations work just fine, like hardware decoding that same video:
ffmpeg -hwaccel vaapi -vaapi_device /dev/dri/renderD128 -i test.avi -c:v
libx264 out.mp4

Hardware transcoding an MPEG2 input video works fine as well:
ffmpeg -hwaccel vaapi -vaapi_device /dev/dri/renderD128
-hwaccel_output_format vaapi -i test.mpg -c:v h264_vaapi out.mp4

Hardware transcoding an H264 video works, too:
ffmpeg -hwaccel vaapi -vaapi_device /dev/dri/renderD128
-hwaccel_output_format vaapi -i test.mp4 -c:v h264_vaapi out.mp4

How should I go about debugging this further?