Learning OpenCV, 2nd Edition

transformation. This is because the transformation requires division by the final dimension (usually 𝑍; see

Chapter 11) and thus loses a dimension in the process.

As with affine transformations, image operations (dense transformations) are handled by different functions

than transformations on point sets (sparse transformations).

cv::warpPerspective(), Dense perspective transform

The dense perspective transform uses an OpenCV function that is analogous to the one provided for dense

affine transformations. Specifically, cv::warpPerspective() has all of the same arguments as

cv::warpAffine(), except with the small, but crucial, distinction that the map matrix must now be 3-

by-3.

void cv::warpPerspective(

cv::InputArray src, // Input Image

cv::OutputArray dst, // Result image

cv::InputArray M, // 3-by-3 transformation matrix

cv::Size dsize, // Destination image size

int flags = cv::INTER_LINEAR, // Interpolation, and inverse

int borderMode = cv::BORDER_CONSTANT, // Pixel extrapolation method

const cv::Scalar& borderValue = cv::Scalar() // Used for constant borders

);

Each element in the destination array is computed from the element of the source array at the location given

by:

𝑑𝑠𝑡 𝑥, 𝑦 = 𝑠𝑟𝑐

𝑀

!!

𝑥 + 𝑀

!"

𝑦 + 𝑀

!"

𝑀

!"

𝑥 + 𝑀

!"

𝑦 + 𝑀

!!

,

𝑀

!"

𝑥 + 𝑀

!!

𝑦 + 𝑀

!"

𝑀

!"

𝑥 + 𝑀

!"

𝑦 + 𝑀

!!

As with the affine transformation, the location indicated by the right side of this equation will not ...