Question

I’m doing video stabilization using optical flow. To make calcOpticalFlowPyrLK work faster I’m downscaling the original image 2x and running the function on that.

How can I modify the homograph matrix (retrieved via findHomography) to be able to warpPerspective the original, larger, image.

Answer 1

Let B be the transformation you have computed, you can multiply B by another homography, A, to get AB = C, where C is a homography that does both transformations, this is equivalent to apply first B and then A. To find A you can use getPerspectiveTransform.

Edit: by AB I meant matrix multiplication, not element-wise multiplication.

Edit 2: to get A you pass the four corners of the two images in the same order to getPerspectiveTransform such that the corners of the downsampled image are the source points and the corners of the original image are the destination points.