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

Question

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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>>

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Av Pa 2018-7-15

编辑：Av Pa 2018-7-15

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.