ffmpeg - 如何估计 h264 视频目标大小的 CRF

Question

According to the x264 encode documentation , in the CRF topic, it is informed that for every 6 additional points, the file size is reduced by approximately 50%.

From this statement, it is assumed that the CRF has an exponential influence in reducing the final size of the video. But what is the exponential ratio?

I wouldn't want to use CRF18 with max-rate adjusted to reach a final size, as maxrate damage complex video scenes more strongly, as bitrate reduction via max_rate is not uniform, but it hits the scenes that require the highest bitrate first.

Answer 1

It is not possible to estimate a final video size without assuming the encode situation of the video to be converted. If the initial file has not been reencoded at all, converting to CRF18 (imperceptible loss of quality to the eye) will already cause a large decrease in file size.

To estimate final size of different CRF values, let's proceed with the assumption below: Initial file encoded in "CRF18 and present slow".

By sliding dozens of conversions from this initial file, keeping the present slow and increasing the CRF by 0.2 points, it is possible to generate a data matrix to assess the influence of the final file size from changes in the CRF.

Having the data, just apply an exponential regression to find the factor that minimizes estimation error. The exponential factor I found in my tests was: 12.85% for each additional CRF point

So we can estimate that:

size final = (size_crf18 * (1-0.1285)^(crf-18))

And isolating the CRF variable, estimate the CRF needed to achieve an objective video size

CRF needed = 18 + LOG(size_goal/size_crf18)/LOG(1-0,1285)

Example

• Initial video with encode profile in CRF 18 preset slow, 3,500mb. Target to downsize to 2,000mb

Required CRF = 18+LOG(2000/3500)/LOG(1-0.1285) = 22.07

I recommend rounding to 1 decimal point and adding 0.2 points (lower 3% in final size) to minimize the estimation error exceeding the maximum desired size. Resulting in a required CRF22.3/slow to reduce a original file in h264/crf18/slow from 3500MB to 2000MB.

Final thoughts

It is worth noting that the exponential ratio of 12.85% between CRF and file size seems adequate ONLY when all other flags are the same. Changing other flags like present and maxrate can drastically influence the file size.

As a future vision, it would be interesting to estimate the exponential ratio of the CRF considering different combinations of present between initial and final file, since an initial file in present ultrafast offers ample opportunity for size reduction if encoded in a preset slow.