how can i transform the image from irregular rectangle to rectangle and finding the exact corners of the resultant image

hi..... I am new to matlab. my task is transformation of image from irregular rectangle form the rectangle and finding the corners of the resultant image. i am using imtransform command for transformation of image, i getting the rectangle. and also getting the corners of the resultant image, but the problem is the corners of the resultant rectangle is not exact values. i am using the resultant rectangle corners for finding the point in that rectangle. so please help me.. up to now my code and image is attached below ...

a=imread('c\image1.bmp'); % original image i.e irregular rectangle.
b=im2bw(a,0.2); 
%%%finding corners of the orginal image%%
[y,x] = find(b);
[~,loc] = min(y+x);
C = [x(loc),y(loc)];
OTL=[x(loc),y(loc)]   %%top left coordinates
[~,loc] = min(y-x);
C(2,:) = [x(loc),y(loc)];
OTR=[x(loc),y(loc)]    %%top right coordinates
[~,loc] = max(y+x);
C(3,:) = [x(loc),y(loc)];
OBR=[x(loc),y(loc)]     %%bottom right coordinates
[~,loc] = max(y-x);
C(4,:) = [x(loc),y(loc)];
OBL=[x(loc),y(loc)]    %%bottom left coordinates 

%%find the location of new corners%%
L = mean(C([1 4],1));
R = mean(C([2 3],1));
U = mean(C([1 2],2));
D = mean(C([3 4],2));
C2 = [L U; R U; R D; L D];
%%image transformation %%
T = cp2tform(C ,C2,'projective');
IT= imtransform(b,T,'XData',[1 size(a,2)],...
   'YData',[1 size(a,1)]);  % getting the same resolution of  original image

find each of new corners

[y,x] = find(IT);
[~,loc] = min(y+x);
FC= [x(loc),y(loc)];
FTL=[x(loc),y(loc)]
[~,loc] = min(y-x);
FC(2,:) = [x(loc),y(loc)];
FTR=[x(loc),y(loc)]
[~,loc] = max(y+x);
FC(3,:) = [x(loc),y(loc)];
FBR=[x(loc),y(loc)]
[~,loc] = max(y-x);
FC(4,:) = [x(loc),y(loc)];
FBL=[x(loc),y(loc)]

and my results are

ORINAL IMAGE CORNERS 
OTL= 339 122
OTR= 1139 264
OBR= 1002 895
OBL= 221 718
 RESULATANT IAMGE CORNERS
FTL= 280 193
FTR= 1074 195
FBR= 1070 806
FBL= 280 806

i am getting the above results.. in that results i am using the FTL and FBR values for point detection in .net code in that code i am using that values the exact values are not found. please help me for matlab code for transformation of image and getting the resultant corners. thanking you advance.