Build a matrix where each column of the image is a single color

Question

Sarah Sadeq 2016-9-29

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回答： Geoff Hayes 2016-9-29

have a homework assignment that I just need help understanding. We are to make a 4x6x3 matrix, where each column of the image is a single color, in rainbow order (Red, Orange, Yellow, Green, Blue, Purple), but we must use for loops. I wrote this code, however, I'm not getting a rainbow? Can someone help me out?

mat=zeros(4,3,3);
row1=[255,0,0];
row2=[0,255,0];
row3=[0,0,255];
row4=[0,255,255];
row5=[255,0,0];
row6=[255,0,255];
mycolormap = reshape([row1;row2;row3], 1, 3, 3); 
for i=1:4
  mat(i,:,:)=mycolormap;
end
imagesc(mat);

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Answer 1

Geoff Hayes 2016-9-29

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https://ww2.mathworks.cn/matlabcentral/answers/305122-build-a-matrix-where-each-column-of-the-image-is-a-single-color#answer_236715

在 MATLAB Online 中打开

Sarah - what is your intent with the line of code

mycolormap = reshape([row1;row2;row3], 1, 3, 3);

where you only use the first three colours that you have defined and so the final image has only three colours in it? Also, I'm not sure why you call them rows when it is the columns that you want to colour. In fact, your for loop is iterating over each row and not each column. Remember that the second dimension corresponds to the column and so what you really want to do is

for k=1:size(mat,2) % 2 for the second dimension i.e. column

Defining the rainbow matrix is a good step, though you may want to define the data type as well as an 8-bit unsigned int

rainbowImg = zeros(4,6,3,'uint8');

Next, iterate over each column of the image and colour it appropriately

 for k=1:size(rainbowImg,2)
    if k==1
        % red
        rainbowImg(:,k,1) = 255;
    elseif k==2
        % orange - what is the RGB code for this colour?
    % etc. for each column
    end
 end

Since this is homework, the above code should get you started with the first column. You would just need to do something similar for every other column. Note that there are other ways to do this too outside of using for loops.