[x264-devel] Different output with i5-2500 and i3-4XXX with the same x264 version and source file

[Kim Long-Ding]

Hello everyone,
 
i have the same ffmpeg binary and .mkv (with mpeg2 video stream) source file on two different computers. One computer has a i5-2500 cpu and the other has a i3-4XXX cpu. I would like to encode the mpeg2 stream to H.264 with the following command:
 
>> ffmpeg.exe -i test.mkv -c:v libx264 -c:a copy -c:s copy -map 0 -preset slow -tune film -crf 19 test-encoded.mkv
 
On the i5-2500 computer the resulting file size is: 1.283.574.741 bytes
On the i3-4XXX computer the resulting file size is: 1.283.575.076 bytes
 
Comparing the output on both machines with ffmpeg's md5 filter
 
>> ffmpeg.exe -i test-encoded.mkv -c copy -f md5 -loglevel warning -
 
On the i5-2500: MD5=3db1e5139d01b40e73c13d4e531329f2
On the i3-4XXX: MD5=dcec58f562a25b7a403f9e0cd71991de
 
The only difference in the ffmpeg/x264 output (see attachments) are in the used cpu instruction sets
 
i5-2500: MMX2 SSE2Fast SSSE3 SSE4.2 AVX
i3-4XXX: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 AVX2 LZCNT BMI2
 
Should new CPU abilities actually only improve the performance, but not the output?
 
Used ffmpeg.exe: https://ffmpeg.zeranoe.com/builds/win64/static/ffmpeg-20170503-a75ef15-win64-static.zip
 
-- 
Kind regards
 
Kim
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