How do I use IMTRANSFORM function with a grid overlay for projective transformation of my image?

I would like to use the IMTRANSFORM function to perform a projective transformation of my image with a grid overlay.

Answer 1

[MathWorks Support Team]

In this example of using the IMTRANSFORM with 'projective' option and a grid overlay, we have defined 4 control points which we want to map onto some base points that form the vertices of a rectangle.

Note that the result is fuzzy because of the resampling; therefore you will not see a single white pixel at each of the four bright points in the zOut image.

% I want to map control_points to base_points

control_points = [ 3,3 ; 2,8 ; 8,3 ; 9,8 ]

base_points    = [ 3,3 ; 3,8 ; 8,3 ; 8,8 ]

% make control points white

zIn = zeros(10,10,'uint8');

for k =1:size(control_points,1)

      zIn(control_points(k,2),control_points(k,1)) = intmax('uint8');

end

iptsetpref('ImshowInitialMagnification','fit')

iptsetpref('ImshowAxesVisible','on')

% calculate transformation from control points

tform = cp2tform(control_points,base_points,'projective');

[zOut,x,y] = imtransform(zIn,tform,'bicubic');

% Calculate grids to overlay on images

[X,Y] = meshgrid(1:9,1:9);

[U,V] = tforminv(tform,X,Y);

% Create shade of gray

gray = 0.65 * [1 1 1];

% plot original image

figure, imshow(zIn)

hold on

line(U, V, 'Color',gray);

line(U',V','Color',gray);

% plot output

figure, imshow(zOut,'XData',x,'YData',y)

hold on

line(X, Y, 'Color',gray);

line(X',Y','Color',gray);