Pass 1
ffmpeg -i input.avi -codec:v libx264 -profile:v high -preset slow -b:v 500k -maxrate 500k -bufsize 1000k -vf scale=-1:720 -threads 0 -pass 1 -an -f mp4 /dev/null
Pass 2
ffmpeg -i input.avi -codec:v libx264 -profile:v high -preset slow -b:v 500k -maxrate 500k -bufsize 1000k -vf scale=-1:720 -threads 0 -pass 2 -an -f mp4 output.mp4