Multiply two matrix with different dim in matlab

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I downloaded dftfilt that a function for image filtering in matlab! but it has some problem the following is code of that:

function g = dftfilt(f, H)
 F = fft2(f, size(H, 1), size(H, 2));
 g = real(ifft2(H.*F));
 g = g(1:size(f,1),1:size(f,2));

but in line 3 gives me an error that says: Error using .* Matrix dimensions must agree.

Error in dftfilt (line 3) g = real(ifft2(H.*F));

how can I fix this error? :( please help me thanks

What I have tried:

function g = dftfilt(f, H)
 F = fft2(f, size(H, 1), size(H, 2));
 g = real(ifft2(H.*F));
 g = g(1:size(f,1),1:size(f,2));

Posted 18-May-16 5:08am

amir.nazarizadeh

Updated 18-May-16 7:03am

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Sergey Alexandrovich Kryukov 18-May-16 11:23am

Multiplication of matrices of arbitrary dimension is not defined. The height of one must match the width of another one. More exactly... please see Solution 1.
—SA

Matt T Heffron 18-May-16 13:04pm

I think this might be an issue with the wrong multiplication operator in Matlab. See my Solution.

2 solutions

Solution 2

In matlab the .* operator is an element-by-element multiplication.
The matrices must be the same dimensions.
If you intended a true matrix multiplication then just use the * operator, and the matrices must have the correct relative dimensions as noted in the Wikipedia article quoted by Sergey.

Posted 18-May-16 7:03am

Matt T Heffron

Comments

Sergey Alexandrovich Kryukov 18-May-16 13:09pm

I see, thank you; 5ed.
—SA

Matt T Heffron 18-May-16 15:49pm

Thank you

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Sergey Alexandrovich Kryukov · Accepted Answer · 2016-05-18T05:31:00

Multiplication of matrices of arbitrary dimension is not defined. The height of one must match the width of another one.

Matrix multiplication is extremely simple (in contrast to division/inversion). Please see, for example:
Matrix multiplication — Wikipedia, the free encyclopedia[^].
Pay attention,

This article tells you:

…if A is an n × m matrix and B is an m × p matrix, their matrix product AB is an n × p matrix, in which the m entries across the rows of A are multiplied with the m entries down the columns of B.

Also note that this is said about the product AB, in the given order. If n≠m or m≠p, multiplication BA is not defined, only AB product can be calculated. Matrix multiplication operator (even for p≡m≡n) is not commutative.

—SA