Apply 4 color 2d colormap to data

Question

Elin Jacobs 2022-12-9

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https://ww2.mathworks.cn/matlabcentral/answers/1874937-apply-4-color-2d-colormap-to-data

回答： Jaswanth 2024-2-9

Hi,

I have two 1800x3600 matrices with values between 0-1, and have generated a four-color 2d colormap with the below code, where the x axis would correspond to the values in one matrix and y axis would correspond to the values in the other. I want to use the scheme to colorcode each element with the same index in the two matrices, simlar to the attached figure. Any pointers to accomplish this would be appreciated!

c{1} = [1 0 0]; % specify 4 colors
c{2} = [0 0 0];
c{3} = [1 0 1];
c{4} = [0 0 1];
C = reshape(vertcat(c{:}), 2, 2, []);
n = 700; % number of pixels
img = zeros(n, n, 3);
for i=1:3
    img(:,:,i) = interp2([0 1], [0 1], C(:,:,i), linspace(0,1,n), linspace(0,1,n).');
end
imshow(img);

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Answer 1

Jaswanth 2024-2-9

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Hi Elin Jacobs,

To color code each element with the same index in the two matrices using the colormap you have generated, you can map the values from each matrix to the corresponding x and y coordinates in the color map image.

Assuming you have matrix data available, following steps post defining the color and interpolating the colormap as per the code you have provided could be helpful:

Initialize the RGB Image: An empty RGB image ‘rgbImage’ is initialized with the same dimensions as ‘matrix1’ (and ‘matrix2’), with three layers to represent the red, green, and blue color channels.

rgbImage = zeros(size(matrix1, 1), size(matrix1, 2), 3);

Map Data to Colormap and Create RGB Image: Each element in ‘matrix1’ and ‘matrix2’ corresponds to a coordinate on the colormap image. The ‘round’ function calculates the nearest integer pixel coordinate on the colormap image based on the value from ‘matrix1’ (x-axis) and ‘matrix2’ (y-axis). The corresponding color from the colormap image is then assigned to the pixel at position (i, j) in 'rgbImage.'

for i = 1:size(matrix1, 1)
    for j = 1:size(matrix1, 2)
        x_coord = round(matrix1(i, j) * (n - 1)) + 1;
        y_coord = round(matrix2(i, j) * (n - 1)) + 1;
        rgbImage(i, j, :) = colormapImage(y_coord, x_coord, :);
    end
end
imshow(rgbImage);