First of all, can you get a better image, like with no clutter outside the rectangle and a contrasting background, not one that gets about as dark as the rectangle itself?
I'd start out by trying an edge detector like imgradient() or a texture filter like stdfilt(). Then use hough() or houghlines().
Since the camera axes were setup somewhat arbitrarily, why is it so important to get the angle of the rectangular block with respect to the edges of the image?
