[FFmpeg-user] (no subject)

[MohammadH Sedaaghi]

 When using ffmpeg, a strange behavior is observed. Suppose the name of the video file is ref.mp4
Its duration is 1 sec with a frame rate of 30 fps.
When the images and wave file is extracted using:
"
del *.jpg
del *.wav
ffmpeg -i ref.mp4 -r 30 Img%d.jpg Wave.wav
"
30 .jpg images and 1 .wav file is generated.
To test the correctness of ffmpeg if in turn we run the following command:
"
ffmpeg -i Wave.wav -f image2 -i Img%d.jpg -r 30 out.mp4
"
it produces out.mp4
But when trying the following commands:
"
del *.jpg
del *.wav
ffmpeg -i out.mp4 -r 30 Img%d.jpg Wave.wav
"
it produces 36 .jpg images and 1 bigger .wav file compared to the first .wav file.

My question is:
Why should ffmpeg increase the size of .mp4 files, or in other words, the number of still images?

If the option -r 25 is omitted, the same error still exists.