How to find the location of transformed points after imwarp? (transforming isosceles trapezoid to rectangle)

I am trying to use a projective transformation to turn an isosceles trapezoid into a rectangle, and after the transformation, I'm left with excess regions and I'm stuck trying to crop out the rectangle I want. imwarp doesn't allow me to see where those vertices by which I defined my transformation went using fitgeotrans , so I don't know what vertices to put for imcrop . Below is an example.

I know the pixel size of the final picture I want, and the excess region lies outside of that pixel area so I just want to crop it out. Since the trapezoid is isosceles, I can crop it in the x-direction just by math, but the y-direction is stretched, and in my application could be stretched in many ways due to different trapezoid shapes. I'm not sure how to get those y-coordinates. Any ideas?

Answer 1

[Wen]

transform = cp2tform(movingPoints, fixedPoints, 'projective');

displayImage = imtransform(roi, transform, 'xData', [1 l], 'yData', [1 w], 'xyScale', [1 1]);

I used the not recommended functions as in here, it seems imwarp doesn't allow bounds to be specified: http://www.mathworks.com/matlabcentral/answers/69442-stretch-quadrilateral-roi-to-pre-definded-rectangle-size