Cal­cu­lat­ing the tar­get size of H264 videos

From time to time I con­vert videos to H264. When I do this I want to get the best qual­i­ty out of a give file­size. This means I cre­ate VBR videos. The ques­tion here is, how big the tar­get video file shall be.

After search­ing around a lit­tle bit in the net I found a for­mu­la which is sup­posed to give a hint about the tar­get file­size. Nat­u­ral­ly this depends on the encoder (or even the encoder-version), the encoder set­tings and even the video.

The for­mu­la is at least a good hint for my use, so I wrote a script which cal­cu­lates sev­er­al file­size val­ues for a giv­en video (based upon the out­put of medi­ain­fo, which the scripts expects in a file with the file­name as an argu­ment to the script). It cal­cu­lates a CBR and a VBR val­ue for a giv­en video based upon the width, height and dura­tion. It should work on all sys­tem with a POSIX com­pat­i­ble shell.

Exam­ple out­put for a video from my HD-ready cam, orig­i­nal file­size 1.8 GB:

Width: 1280, Height: 720, FPS: 50.000, Time: 1424, Motion: 2
Per sec­ond: 6451200.000 bps / 6300 Kibps
Total CBR: 1148313600 bytes / 1121400 KiB / 1095 MiB
Total VBR: 861235200 bytes / 841050 KiB / 821 MiB
Width: 1280, Height: 720, FPS: 50.000, Time: 1424, Motion: 3
Per sec­ond: 9676800.000 bps / 9450 Kibps
Total CBR: 1722470400 bytes / 1682100 KiB / 1642 MiB
Total VBR: 1291852800 bytes / 1261575 KiB / 1232 MiB
Width: 1280, Height: 720, FPS: 50.000, Time: 1424, Motion: 4
Per sec­ond: 12902400.000 bps / 12600 Kibps
Total CBR: 2296627200 bytes / 2242800 KiB / 2190 MiB
Total VBR: 1722470400 bytes / 1682100 KiB / 1642 MiB

There are 3 sec­tions, the dif­fer­ence is the “motion” val­ue. It is a kind of mul­ti­pli­ca­tor depend­ing on the amount of motion in the video. For the videos I made myself (fam­i­ly videos, and even some videos of vol­ley ball games), the first sec­tion seems to be just fine. So I reduced the orig­i­nal MP4 file to about 50% (not vis­i­ble here is the audio size, nor­mal­ly I copy the orig­i­nal audio unmod­i­fied).

For the curi­ous ones, the for­mu­la is

width_in_pixels * height_in_pixels * fps * motion_value * 0.07

for the bps val­ue. The CBR val­ue is

bps * playtime_in_seconds / 8

and the VBR val­ue is

34 * CBR_value.

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