What is the Kush Gauge™?
It’s a rule of thumb to calculate the needed bitrate for H.264 encoded video; it was written by Kush Amerasinghe, a computer scientist. In this context, the word “gauge” means “a device used to make measurements”
How does it work?
Quoting Kush’s document:
“to estimate the optimal H.264 bit rate value that would give what is considered “good quality” results for a given video, you could multiply the target pixel count by the frame rate; then multiply the result by a factor of 1, 2 or 4, depending on its motion rank; and then multiply that result by 0.07 (the constant, Ed.) to get the bit rate in bps (divide that by 1,000 to get a kbps estimate or by 1,000,000 to get a Mbps estimate).”
The Kush Gauge™ formula:
width x height x FPS x motion rank x constant = final bitrate in bps
1280 × 720 @24fps, medium motion (rank 2):
1280 × 720 × 24 × 2 × 0.07 = 3,096,576 bps = ~ 3000 kbps
What is the motion rank?
The amount of motion could be called “motion rank”; from Kush’s document:
“As a general rule, try to simplify it into three ranks: Low, Medium, High. To define these ranks in real-world terms:
• Low motion is a video that has minimal movement. For example, a person talking in front of a camera without moving much while the camera itself and the background is not moving at all.
• Medium motion would be some degree of movement, but in a more predictable and orderly manner, which means some relatively slow camera and subject movements, but not many scene changes or cuts or sudden snap camera movements or zooms where the entire picture changes into something completely different instantaneously.
• High motion would be something like the most challenging action movie trailer, where not only the movements are fast and unpredictable but the scenes also change very rapidly.”
What’s about the constant:
from Kush’s document:
“I sought to develop a base number … (that) can produce real-world bit-rate estimates. After numerous experiments, I noticed a certain pattern of what could be considered a “constant” or base value (for most commonly used video frame-size and frame-rate ranges). When rounded off, that value is 0.07 bps per pixel, per frame, per motion rank value.”
spoRv Corollary to the Kush Gauge™:
To calculate video bitrate for a codec different from H.264, the Kush Gauge costant value should be changed accordingly to the codec used.
(these constant values will be threated in the next post)
Comments, improvements, corrections are welcome!
Reference (PDF documents): H.264 FOR THE REST OF US (page 21)
Don’t forget to take a look at the updated spoRv Corollaries to the Kush Gauge – now with brand new factors added!
Or go directly to the new spoRv Gauge Calculator to find out the optimal bitrate needed for a good quality encode.