OpenCV warpperspective

Question

For some reason whenever I use OpenCV's warpPerspective() function, the final warped image does not contain everything in the original image. The left part of the image seems to get cut off. I think the reason why this is happening is because the warped image is created at the leftmost position of the canvas for the warpPerspective(). Is there some way to fix this? Thanks

Answer 1

The problem occurs because the homography maps part of the image to negative x,y values which are outside the image area so cannot be plotted. what we wish to do is to offset the warped output by some number of pixels to 'shunt' the entire warped image into positive coordinates(and hence inside the image area).

Homographies can be combined using matrix multiplication (which is why they are so powerful). If A and B are homographies, then AB represents the homography which applies B first, and then A.

Because of this all we need to do to offset the output is create the homography matrix for a translation by some offset, and then pre-multiply that by our original homography matrix

A 2D homography matrix looks like this :

[R11,R12,T1] [R21,R22,T2] [ P , P , 1] 

where R represents a rotation matrix, T represents a translation, and P represents a perspective warp. And so a purely translational homography looks like this:

[ 1 , 0 , x_offset] [ 0 , 1 , y_offset] [ 0 , 0 ,    1    ] 

So just premultiply your homography by a matrix similar to the above, and your output image will be offset.

(Make sure you use matrix multiplication, not element wise multiplication!)