Warp Affine using R

Lampros Mouselimis

2023-07-08

The getAffineTransform() and warpAffine() functions of the OpenImageR package is an RcppArmadillo re-implementation of existing Python Code and this vignette shows how these functions can be used from within R based on the author’s .ipynb file

require(OpenImageR)

path = system.file('tmp_images', 'landscape.jpg', package = "OpenImageR")

img = readImage(path)
print(dim(img))

## [1] 600 600   3

Input Image

r = ncol(img)
c = nrow(img)
offset = 50

original_points = matrix(data = c(0, 0, r, 0, 0, c),
                         nrow = 3,
                         ncol = 2,
                         byrow = TRUE)

transformed_points = matrix(data = c(offset, 0, r, offset, 0, c-offset),
                            nrow = 3,
                            ncol = 2,
                            byrow = TRUE)

M_aff = getAffineTransform(original_points = original_points,
                           transformed_points = transformed_points)

The following is the Affine transformation matrix,

print(M_aff)

##            [,1]        [,2] [,3]
## [1,] 0.91666667 -0.08333333   50
## [2,] 0.08333333  0.91666667    0

The Affine transformation matrix can be used as input in the warpAffine() function,

res_3d = warpAffine(img = img,
                    M = M_aff,
                    R = r,
                    C = c,
                    verbose = TRUE)

## time to complete : 0.0644 secs

str(res_3d)

##  num [1:600, 1:600, 1:3] 0 0 0 0 0 0 0 0 0 0 ...

The next image shows the output based on the input data and parameters,

imageShow(res_3d, clear_viewer = FALSE)

Output of Warp Affine function