How to convert a vector to rgb colormap?

Question

Ali Nouri 2019-12-30

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链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/498482-how-to-convert-a-vector-to-rgb-colormap

评论： Image Analyst 2020-1-2

Hello guys

can anybody tell me how to convert a ector to rgb colormap??

I have have vector, which its value goes from -1 to 1. I want the define a colormap, where the value of the 0.2 to 1 is red and -0.2 to -1 is blue and the rest is green.

thanks in advance.

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Answer 1

Image Analyst 2019-12-30

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/498482-how-to-convert-a-vector-to-rgb-colormap#answer_408180

在 MATLAB Online 中打开

Do this:

% Colormap "where the value of the 0.2 to 1 is red and
% -0.2 to -1 is blue, and the rest is green.
numColors = 256;
vec = linspace(-1, 1, numColors);
cmap = zeros(numColors, 3);
% Make everything green to start with
cmap(:, 2) = 1;
% Make red from where vec goes from 0.2 to 1.
redIndexes = vec >= 0.2 & vec <= 1;
cmap(redIndexes, 1) = 1;
cmap(redIndexes, 2) = 0;
cmap(redIndexes, 3) = 0;
% Make blue from where vec goes from -1 to -0.2.
blueIndexes = vec >= -1 & vec <= -0.2;
cmap(blueIndexes, 1) = 0;
cmap(blueIndexes, 2) = 0;
cmap(blueIndexes, 3) = 1;
cmap % Echo to command window so we can see it.

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Ali Nouri 2020-1-1

在 MATLAB Online 中打开

numColors = 31;
vec = [-0.00653341206474312
       0.00324578781459654
      0.000552586889809205
        0.0449766491863441
       0.00356180692563616
        0.0746463180472126
        0.0078011233071506
      -0.00078555536582595
      -0.00374634887393066
       0.00109938285489642
         0.369351654077307
      -0.00454936207691688
      2.20408232101434e-05
      0.000254609487960349
      -0.00135086171535819
       -0.0695803908468877
        0.0116165165761393
      -0.00197408951942686
     -2.47282982787341e-05
         0.375027165631663
        -0.047418741927062
        -0.840829144608918
      -0.00254093170763818
        0.0010078132834115
     -0.000870537370561834
        -0.019269131980187
      0.000548413597787709
      -0.00772253261710933
       0.00372968305677465
        0.0221712405360291
       0.00246775692879843];
cmap = zeros(numColors, 3);
% Make everything white to start with
cmap(:, :) = 1;
% Make red from where vec goes from 0.2 to 1.
redIndexes = vec >= 0.02 & vec <= 1;
cmap(redIndexes, 1) = 1;
cmap(redIndexes, 2) = 0;
cmap(redIndexes, 3) = 0;
% Make green from where vec goes from -1 to -0.2.
greenIndexes = vec >= -1 & vec <= -0.02;
cmap(greenIndexes, 1) = 0;
cmap(greenIndexes, 2) = 1;
cmap(greenIndexes, 3) = 0;
cmap; % Echo to command window so we can see it.
%B=sortrows((cmap));
%colormap(B);
colormap(cmap)
colorbar;
caxis([-1 1])
set(get(colorbar,'YLabel'),'String','risk')

As you can see that I use colormap(cmap), but something seems wrong, can you see what is wrong with the way i try to make plot.

Ali Nouri 2020-1-1

在 MATLAB Online 中打开

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
numColors = 31;
%value of 31 country
vec = [-0.00653341206474312
       0.00324578781459654
      0.000552586889809205
        0.0449766491863441
       0.00356180692563616
        0.0746463180472126
        0.0078011233071506
      -0.00078555536582595
      -0.00374634887393066
       0.00109938285489642
         0.369351654077307
      -0.00454936207691688
      2.20408232101434e-05
      0.000254609487960349
      -0.00135086171535819
       -0.0695803908468877
        0.0116165165761393
      -0.00197408951942686
     -2.47282982787341e-05
         0.375027165631663
        -0.047418741927062
        -0.840829144608918
      -0.00254093170763818
        0.0010078132834115
     -0.000870537370561834
        -0.019269131980187
      0.000548413597787709
      -0.00772253261710933
       0.00372968305677465
        0.0221712405360291
       0.00246775692879843];
cmap = zeros(numColors, 3);
% Make everything white to start with
cmap(:,:) = 1;
% Make red from where vec goes from 0.2 to 1.
redIndexes = vec >= 0.2 & vec <= 1;
cmap(redIndexes, 1) = 1;
cmap(redIndexes, 2) = 0;
cmap(redIndexes, 3) = 0;
% Make green from where vec goes from -1 to -0.2.
greenIndexes = vec >= -1 & vec <= -0.2;
cmap(greenIndexes, 1) = 0;
cmap(greenIndexes, 2) = 1;
cmap(greenIndexes, 3) = 0;
cmap; % Echo to command window so we can see it.
B=sortrows((cmap),1);
colormap(B);
%colormap(cmap)
colorbar;
caxis([-1 1])
set(get(colorbar,'YLabel'),'String','risk')
% shaperead for country
land = shaperead('cntry02.shp', 'UseGeoCoords', true);
%map over europe
w = worldmap('europe');
setm(gca,'ffacecolor','b')   %using this line the seas are colored in blue
%facecolor with new colormap
faceColors = makesymbolspec('Polygon',...
             {'INDEX', [1 31], 'FaceColor', ...
              colormap});
% geshow with new facecolor
geoshow(land,'DisplayType', 'polygon',....
        'SymbolSpec',faceColors)
   

Walter Roberson 2020-1-1

Your colorbar looks like that because you did not use linspace() like we said. You used your unsorted vec, and you used one colormap entry for each entry in vec.

Is your task to create a colormap and apply it to an image (or imagem or surface plot or patch) ? If so then you need to create a sorted vector of equal-spaced values -- the linspace that we have been repeatedly using in the code -- and in that case you would get a sensible colormap.

Or is your task to take a vector of values and convert each one into an RGB color directly, such as you might do for creating an RGB image yourself without a colormap, or such as you might do for creating a scatter plot in which each point is individually colored? That is what your code in https://www.mathworks.com/matlabcentral/answers/498482-how-to-convert-a-vector-to-rgb-colormap#comment_782315 is doing, converting each value in the vector into an RGB color directly, with the resulting array of color entries being your cmap array. That is a valid thing to do, very useful sometimes, but it will not give you an array you can use for a colorbar.

It is valid to combine the two approaches, to create a direct array of RGB values, and to also create a linear colormap using linspace() that you use to colorbar(), just for user comprehension, even though the colormap is not influencing what displays on the screen in that case.