I thought from your prior messages on this tile that you needed the corners to know how to warp the image. Now it looks like you're saying that you warped it somehow without the corners. But unfortunately you warped it incorrectly so that it's still not square.
But if it were the binary image, you could simply use find() in the first and last row of the binary image to find the corners.
upperLeftCornerX = find(binaryImage(1,:), 1, 'first');
upperRightCornerX = find(binaryImage(1,:), 1, 'last');
lowerLeftCornerX = find(binaryImage(end,:), 1, 'first');
lowerRightCornerX = find(binaryImage(end,:), 1, 'last');
but again, you must already have the corners to do that incorrect warping so why not just do it correctly, using the code I gave you in the other posting? There is no need to correct the skewness only part way and then try to warp it again to get it all the way fixed.