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Matrix multiplication within a function not working

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Bharath Nagarajan
Bharath Nagarajan on 19 Sep 2021
Commented: Bharath Nagarajan on 19 Sep 2021
The following function creates a column vector and an image histogram:
function lutlum = lutlummer(image)
for i = 1:1:265
lutlum(i, 1) = i-1;
end
%multiplying the LUT with the luminescence histogram of the image
%Finding luminance image 'L' (Formula gained from class slides)
L = uint8(sum(bsxfun(@times,double(image),reshape([0.299 0.587 0.114],[1 1 3])),3));
%Finding luminance histogram'HL'
HL = histcounts(L,[0:256])';
lutlum = lutlum*HL;
end
The function is called with an image such that HL is a 1x2 matrix, so multiplying a 265x1 matrix ('lutlum') with a 1x2 matrix should be no problem. however, i still get this message:
Matrix dimensions must agree.
lutlum = lutlum.*HL;
Please do let me know where i am going wrong
thanking yall in advance!! :D
  4 Comments
Bharath Nagarajan
Bharath Nagarajan on 19 Sep 2021
L is supposed to be the 'Luminescence image' and it appears that it is a greyscale image. Howvere, I am trying to multiply lutlum (256x1 matrix) with HL (a 1x2 matrix). this should still work though right? since the matrix dimentions are compatable?

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Answers (2)

Sulaymon Eshkabilov
Sulaymon Eshkabilov on 19 Sep 2021
There is an err, here is how it is to be:
lutlum = HL.'*lutlum; % creates 2 - by - 256
  1 Comment
Bharath Nagarajan
Bharath Nagarajan on 19 Sep 2021
Unfortunately it still throws the same error
Error using *
Incorrect dimensions for matrix multiplication. Check that the number of columns in the first matrix matches the number of rows in the second matrix. To perform elementwise multiplication, use '.*'.
lutlum = lutlum.'*HL;
and even when the order of matrices are switched:
Error using *
Incorrect dimensions for matrix multiplication. Check that the number of columns in the first matrix matches the number of rows in the second matrix. To perform elementwise multiplication, use '.*'.
lutlum = HL.'*lutlum;

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Sulaymon Eshkabilov
Sulaymon Eshkabilov on 19 Sep 2021
There are a couple of errs in the code. Here is the corrected version:
function lutlum = lutlummer(image)
for i = 1:1:256 % Has to be 256 NOT 265. 256 similar to 265 :)
lutlum(i, 1) = i-1;
end
%multiplying the LUT with the luminescence histogram of the image
%Finding luminance image 'L' (Formula gained from class slides)
L = uint8(sum(bsxfun(@times,double(image),reshape([0.299 0.587 0.114],[1 1 3])),3));
%Finding luminance histogram'HL'
HL = histcounts(L,[0:256])'; % Note the size of HL is 256 - by - 1 NOT [1 - by - 2] as you have stated
lutlum = lutlum*HL'; % Transpose is necessary
end

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