Quality factor (video compression)

The quality factor (qf) of encoded video streams (a.k.a. frame quality) is the amount of bits for each pixel in a given video.

Calculation

The quality factor can be calculated by the formula:

$Q\_f\; =\; frac\{1000\; cdot\; b\}\{v\; cdot\; h\; cdot\; f\}$

where $Q\_f$ is the quality factor, $b$ is the bit rate (expressed in kbit/s), $v$ is the vertical resolution of the frame, $h$ is the horizontal resolution of the frame, and $f$ is the frame rate (expressed in frames per second).

As an example, consider a 90 minutes video with a resolution of 640x360 pixels and a frame rate of 23.976 frames per second (frame/s) to be encoded. The video is supposed to fit onto a 700 MiB CD. If you reserve 100 MiB for the audio stream, this will leave you with 600 MiB for the video stream, which equals a video bandwidth of

$b\; =\; frac\; \{600\; cdot\; 1024\; cdot\; 1024\; cdot\; 8\}\; \{90\; cdot\; 60\}\; =\; 932,067.(5)\; bit/s\; =\; 932.067(5)\; kbit/s$

At a frame rate of 23.976 frame/s the quality factor equals

$Q\_f\; =\; frac\; \{1000\; cdot\; 932.067(5)\}\; \{\; 640\; cdot\; 360\; cdot\; 23.976\; \}\; =\; 0.1687$ bits per pixel.

Accordingly you can use the formula to calculate the bit rate needed to produce an encode of a given $Q\_f$.