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I am unable to re-encode an H.264 video that I downloaded from Youtube back to its original size. The MediaInfo is pasted below. The size of the video stream is 20 MB.

I decoded the video into raw YUV (using ffmpeg and JM separately, ensuring that I get an identical file). Then I re-encoded it using the x264 library with a variety of configs. The encoded H.264 videos that I generated are around 40 MB in size.

This is the commandline that I used: x264 -o out.h264 --input-res 1280x720 --preset slow autl.yuv

Any ideas on what options I might be missing that YouTube would have used to encode the video?

Mediainfo:

General
Complete name                            : autl.h264
Format                                   : AVC
Format/Info                              : Advanced Video Codec
File size                                : 19.9 MiB
Writing library                          : x264 core 155 r2901 7d0ff22

Video
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : [email protected]
Format settings                          : CABAC / 3 Ref Frames
Format settings, CABAC                   : Yes
Format settings, Reference frames        : 3 frames
Width                                    : 1 280 pixels
Height                                   : 720 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Variable
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Writing library                          : x264 core 155 r2901 7d0ff22
Color range                              : Limited
Color primaries                          : BT.709
Transfer characteristics                 : BT.709
Matrix coefficients                      : BT.709

1 Answer 1

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I was able to produce the same size as Youtube by varying the qp option for x264. The value that worked to approximately the same size was qp=20.

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