Why does imwarp returns single integer uint8 on Homography Projective Transform?

I am trying to map a captured image quadrangle to deskewed view as if viewed from top. My image contains an A4 sheet of paper with some text on it. I have calculated the Homography Matrix H using the corner points and corresponding deskewed points ( 0,0 0,h w,0 & w,h ).

I am expecting an image as output as per imwarp documentation but I am getting a single integer. What am i doing wrong?

Here is the code for doing transformation:

Trial>> H

H =

    1.4827    0.1074 -508.2923
    0.0122    0.9982 -387.1204
    0.0000    0.0000    1.0000

Trial>> tf = projective2d(H);
Trial>> tf

tf = 

    projective2d with properties:

                   T: [3×3 double]
      Dimensionality: 2

Trial>> imwarp(Image, tf)

ans =

    uint8

     213

Trial>>

[Av Pa]

2 observations

1. x,y co-ordinates of the identified corners were exchanged. ie x was y and y contained value of x.

2. After switching the values for calculating x and y and taking transpose of the Homography matrix, I get transformed image. But the image is flipped for some reason.

[Matt J]

I suggest that you attach the images and the input/output points so that we can attempt to reproduce what you see.

Answer 1

[Pratik Jain]

the format that the imwarp uses is the transpose of the standard Homography matrix.

just use

tf = projective2d(H')

notice the H' (H transpose).

Hope that helps