[FFmpeg-user] lenscorrection and vignette filters

[Michael Koch]

> Please consider sending a patch that updates the documentation.

I recommend to make the following changes in chapter 38.73.1:

The filter accepts the following options:

cx

    Relative x-coordinate of the center of the distortion. This value
    has a range [0,1] and is expressed as fractions of the image width.
    The default value is 0.5.

cy

    Relative y-coordinate of the center of the distortion. This value
    has a range [0,1] and is expressed as fractions of the image height.
    The default value is 0.5.

k1

    Coefficient of the quadratic correction term. 0 means no correction.
    The default value is 0. A positive value shifts the border towards
    the center, and the new border of the output image is filled with an
    unpredictable color. A negative value shifts the border away from
    the center, so that some content is lost in the output image.

k2

    Coefficient of the double quadratic correction term. 0 means no
    correction. The default value is 0.

This is the formula that describes the distortion of the lens:

r_img = r_src * (1 + k1 * (r_src / r_0)^2 + k2 * (r_src / r_0)^4)

where r_img is the distance from the center of distortion in the 
distorted image, r_src is the distance from the center of distortion in 
object space and r_0 is halve of the image diagonal in object space. The 
lenscorrection filter does apply the inverse of the above formula.

Michael