tformarray handle as function parameter

Question

esben plenge 2011-3-9

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https://ww2.mathworks.cn/matlabcentral/answers/2833-tformarray-handle-as-function-parameter

回答： arushi 2024-8-21

Hi, I am trying to find the largest eigenvalues of some linear transforms, and thought that I could use eigs(afun,...) for the purpose, with 'afun' being a handle to my linear transform function.

The code is inserted below, including a similar operation using imtransform, which works fine.

N = 10;
x = eye(N);
% Compose transform:
T1 = [1 0 -(N+1)/2; 0 1 -(N+1)/2; 0 0 1];
T2 = [0 1 0; -1 0 0; 0 0 1];
T3 = [1 0 (N+1)/2; 0 1 (N+1)/2; 0 0 1];
tf = T3*T2*T1
T = maketform('affine',tf');
R = makeresampler('nearest','fill');
im_1 = imtransform(x,T,R); % test 
fwd_1 = @(x) imtransform(x,T,R);
im_2 = tformarray(x,T,R,[1 2],[1 2], [N N] ,[],0); % test 
fwd_2 = @(x) tformarray(x,T, R,[1 2],[1 2],[N N] ,[],0);
[v1,d1] = eigs(fwd_1,10);
[v2,d2] = eigs(fwd_2,10);

The last code-line generates the error (Matlab v 7.6.0.324, Windows XP, 32 bit):

----------------------- ??? Subscripted assignment dimension mismatch.

Error in ==> eigs at 249 workd(:,cols(2)) = Amtimes(workd(:,cols(1)));

Error in ==> eigs_test at 27 [v2,d2] = eigs(fwd_2,10); -------------------------

Is it a bug in eigs() or are my input parameters to tformarray() or eigs() wrong?

Thanks,

Esben

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Answer 1

arushi 2024-8-21

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https://ww2.mathworks.cn/matlabcentral/answers/2833-tformarray-handle-as-function-parameter#answer_1502169

Hi Esben,

The error you're encountering is likely due to a mismatch in dimensions when using the eigs function with a function handle. The eigs function expects the function handle to represent a linear operator that can be applied to a vector, returning another vector of the same size. In your case, the function handle fwd_1 or fwd_2 should apply the transform to a vector, not a matrix.

Function Handle Expectations: The eigs function expects afun to be a function handle that accepts a vector and returns a vector of the same size. Your current setup applies the transformation to a matrix, which isn't compatible with eigs.
Matrix to Vector Conversion: Since eigs works with vectors, you need to reshape the input matrix into a vector, apply the transformation, and then reshape the result back into a vector.