[FFmpeg-user] swScaler: Unknown format is not supported as input pixel format, Cannot get resampling context

[Burlen Loring]

Hi,

I am unable to convert a sequence of png files to mp4 with the following 
command

ffmpeg -loop_output 0 -qscale 1 -r 10 -b 9600 -i test4b.%04d.png test4b.mp4

swScaler: Unknown format is not supported as input pixel format
Cannot get resampling context

never have had a problem with this before, and I use this all the time. 
The pngs seem to be standard rgb files with 8 bit per channel. I tried 
-pix_fmt=rgb8, and the command ran with tons of Error while decoding 
stream #0.0 errors.

How can I get past this error?