ffmpeg PNG - MP4 - Error opening input files: invalid argument

Question

I am following along with this visualization project converting PNG files in an MP4.

The only change from the code on the example is that my time stamp starts with 1.

ffmpeg -r 20 -b 20M -i example%01d.png output.mp4

And this is what returns

ffmpeg version N-53055-g7b43120 Copyright (c) 2000-2013 the FFmpeg developers
built on May 14 2013 20:43:53 with llvm-gcc 4.2.1 (LLVM build 2336.11.00)
configuration: --disable-yasm
libavutil      52. 31.100 / 52. 31.100
libavcodec     55.  9.100 / 55.  9.100
libavformat    55.  7.100 / 55.  7.100
libavdevice    55.  0.100 / 55.  0.100
libavfilter     3. 67.100 /  3. 67.100
libswscale      2.  3.100 /  2.  3.100
libswresample   0. 17.102 /  0. 17.102
**Option b (video bitrate (please use -b:v)) cannot be applied to input file
example%01d.png -- you are trying to apply an input option to an output file or
vice versa. Move this option before the file it belongs to.
Error parsing options for input file example%01d.png.
Error opening input files: Invalid argument**

Solution

If you look at your terminal output it says the FFmpeg parameters are wrong, because you are passing an incorrect parameter to the input file. The bitrate parameter must be applied to your output file, since you are encoding using that bitrate. Moving it to the right position will do the trick.

I tested it out, this works:

ffmpeg -r 20 -i example%01d.png -b:v 20M output.mp4