quality factor

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.


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.

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