Hi,
I understand that you want to convert the existing shape into rectangle.
The polyshape syntax takes the points from matrix P and creates a shape by connecting each point in a linear fashion.
By changing the order of coordinates in matrix P, you would get a rectangle.
Below code gives the rectangle.
Hope it helps!
P = [0 0; 1 0; 1 1; 0 1; 0 0];
pgon = polyshape(P)
%Plot the polygon and compute its area.
plot(pgon)
A = area(pgon)