Lossless
You'll have to use a truly lossless format if you require "a quality of output video same as input video". Example for ffv1 using the hstack filter instead of movie+pad+overlay:
ffmpeg -i left.avi -i right.avi -filter_complex hstack -c:v ffv1 output.avi
FFmpeg supports several additional lossless compressed formats such as:
- lossless H.264:
-c:v libx264 -crf 0
- huffyuv:
-c:v huffyuv
- ffvhuff:
-c:v ffvhuff
- UT Video:
-c:v utvideo
- And of course lossless uncompressed formats including rawvideo.
Since it is lossless the output file will be huge, and your player or device will probably not like it.
Visually lossless
Of course what you may actually want is "lossy, yet 'visually lossless', but not a gigantic, huge file like true lossless provides". In that case use:
ffmpeg -i left.avi -i right.avi -filter_complex "hstack,format=yuv420p" -c:v libx264 -crf 18 output.mp4
You didn't mention audio. If you want to merge the audio from each input as well use the amerge filter:
ffmpeg -i left.avi -i right.avi -filter_complex "[0:v][1:v]hstack,format=yuv420p[v];[0:a][1:a]amerge[a]" -map "[v]" -map "[a]" -c:v libx264 -crf 18 -ac 2 output.mp4
See FFmpeg Wiki: H.264 Video Encoding.
