Can you help me to correct this error?

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

Answer 1

[Matt Fig]

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....

[Tomas]

cform = makecform('srgb2lab');
lab = applycform(rgb,cform);
[p C]=size(lab)

Sorry L=60.0221 calculated using this link http://www.mathworks.com/matlabcentral/fileexchange/24009-rgb2lab/content/RGB2Lab.m

[Tomas]

i changed the name of the function

[Matt Fig]

You still have not said what p and C are (and you added another unknown variable to the list: rgb)... Do this:

whos p C ones
p
C

and print the output MATLAB gives.

[Tomas]

rgb=imread('image.jpg');

>> whos p C ones
p
C
  Name      Size            Bytes  Class     Attributes

    C         1x1                 8  double              
    p         1x1                 8  double              

p =

   400

C =

        1800

[Matt Fig]

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]

Tomas, what are the values of p and C ? And is the error still occurring?

Answer 2

[Walter Roberson]

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.

[Matt Fig]

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

Answer 3

[Image Analyst]

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.