Can you help me to correct this error?

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Tomas
Tomas 2012-11-23
Hi, can you help me to correct this error
[p C]=size(lab)
lchan=lab(:,:,1);
L=60;
Lr = L .* ones(p, C);
d
??? Error using ==> times
Matrix dimensions must agree.
Error in ==> lab at 60
Lr = L .* ones(p, C);
thanks

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回答（3 个）

Matt Fig
Matt Fig 2012-11-23
编辑：Matt Fig 2012-11-23
What are p and C?
L = 60;
Lr = L .* ones(3, 5); % No error...(the .* is not necessary)
You have a variable named lab and a function named lab?? This is a very bad idea and may (somehow) be at the root of your problem....
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Matt Fig
Matt Fig 2012-11-23
There is more to the story then. See, this does not error on my machine:
p = 400;
C = 1800;
L = 60.0221;
Lr = L .* ones(p, C);
So I am afraid I am out of ideas here. There is something else going on in the code somewhere, but without seeing the code I cannot say. Someone else may be able to guess the problem.
Walter Roberson
Walter Roberson 2012-11-24
Tomas, what are the values of p and C ? And is the error still occurring?

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Walter Roberson
Walter Roberson 2012-11-24
Your RGB image is 3 dimensional, not 2 dimensional. Row, Column, Color-plane. Your lab equivalent of the RB is 3 dimensional as well. When you take the size() of a 3 dimensional array, you need to have either only 1 output or else 3 or more outputs. If you only have two outputs as in
[p C]=size(lab)
then p will be assigned the number of rows (as expected) but C will be assigned the product of all the remaining dimensions, and so will be columns times number of color planes.
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Matt Fig
Matt Fig 2012-11-24
编辑：Matt Fig 2012-11-24
That doesn't explain the error, however...
>> clear all
>> rgb=imread('office_2.jpg'); % Image in the IPT
>> cform = makecform('srgb2lab'); % On the FEX
>> lab = applycform(rgb,cform);
>> [p C]=size(lab);
>> % lchan=lab(:,:,1);
>> L=60.0221;
>> Lr = L .* ones(p, C); % No error....
>> whos
Name Size Bytes Class Attributes
C 1x1 8 double
L 1x1 8 double
Lr 600x2709 13003200 double
cform 1x1 13376 struct
lab 600x903x3 1625400 uint8
p 1x1 8 double
rgb 600x903x3 1625400 uint8

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Image Analyst
Image Analyst 2012-11-24
编辑：Image Analyst 2012-11-24
Like Walter said, your problem is (partially) that your C is the number of columns TIMES the number of color channels (which is 3). You should be doing it this way, as you'll notice that I ALWAYS do it in code I post, even when I suspect the image is gray level:
[rows columns numberOfColorChannels] = size(lab);
If you do it this way:
[rows columns] = size(lab);
as you did, then columns will not be the number of columns in the image but will be columns * numberOfColorChannels. Please see this blog entry by Steve Eddins for further discussion: http://blogs.mathworks.com/steve/2011/03/29/even-more-information-about-the-size-function/
But like Matt said, that wouldn't cause an error if L was just some number like 60. So, I suspect that you actually did something like
L = lab(:, :, 1); % Extract the L color channel.
and then L would have the size rows*columns, but your ones() function would return an array rows*columns*numberOfColorChannels. Hence the sizes don't match up. So I don't know why you said L was 60 when it was not, but that's your problem. I speculate that you'd like to construct a constant luminance image. In that case, you set
Lr = 60 * ones(size(L));
and then you invert the transform to get back to RGB.

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