Cylinder unwrapping with imtransform

Hello everybody, I am trying to make the unwrapping of a photo of a cylindrical object (only one side), I applied this code I wrote:

if true
  % code
I = imread('2258.jpg'); 
axis on
box on
imshow(I)
ndims_in = 2;
ndims_out = 2;
forward_mapping = []
f = @(x, unused) arcsin(1/x);
inverse_mapping = f;
tdata = [];
tform = maketform('custom', ndims_in, ndims_out, ...
  forward_mapping, inverse_mapping, tdata);
udata = [-1 1];
vdata = [-1 1];
xdata = [-1.57 1.57];
ydata = [-1 1];
I2 = imtransform(I, tform, 'UData', udata, 'VData', vdata, ...
  'XData', xdata, 'YData', ydata);
subplot(1,2,1)
imshow(I)
subplot(1,2,2)
imshow(I2)

end

but I received lots of errors: ??? Error using ==> mldivide Matrix dimensions must agree.

Error in ==> @(x,unused)arcsin(1/x)

Error in ==> maketform>inv_composite at 592 U = feval(t.tdata(i).inverse_fcn, U, t.tdata(i));

Error in ==> tform at 56 X = feval( t.(f.fwd_fcn), U, t );

Error in ==> tforminv at 68 varargout = tform('inv', nargout, varargin{:});

Error in ==> tformarray at 241 M = tforminv(G,T);

Error in ==> imtransform at 275 B = tformarray(args.A, args.tform, args.resampler, tdims_a, tdims_b, ...

Where is the error for you? How could I solve it? Thank you a lot, Phalaen

[Image Analyst]

How about attaching '2258.jpg' so people can try your code?

Answer 1

[David Young]

I haven't tested this, but here's a quick thought: try replacing

f = @(x, unused) arcsin(1/x);

with

f = @(x, unused) asin(1./x);

as you probably want the element-by-element division rather than the matrix inverse here.

[Edit: arcsin replaced with asin to take note of Youssef Khmou's helpful comment]

[Youssef Khmou]

is it arcsin or asin ( or asind) ?

Answer 2

[Elisa]

Thank you David and Youssef, with your suggestion (f = @(x, unused) asin(1./x);) matlab gave no error messages, the problem now is that, after the computation, it returns a completely black image and not the stretched image, what could the problem be? Thank you!

[David Young]

Hard to know - maybe if you attach the image as suggested by Image Analyst it might be possible to figure it out.

Answer 3

[Elisa]

I found another problem, that I corrected: It is not f = @(x, unused) asin(1./x); but f = @(x, unused) asin(x); (I made an error in the geometrical computation), but now I received another declaration by Matlab: ??? Error using ==> imtransform>check_udata at 606 'UData' elements cannot be equal unless A has only one column.

Error in ==> imtransform>parse_inputs at 386 args.udata = check_udata(varargin{k+1}, args.A);

Error in ==> imtransform at 264 args = parse_inputs(varargin{:});