How to make a color scale like this?

Hello. How can I make a color scale with several different colors? I would like a scale exactly like this, with 100 colors. Thanks!

Answer 1

[David Goodmanson]

Hi pf,

I copied in the image in your posting (a jpeg) and called it image1. That results in a 3d array that is 180x882x3 uint8, with values from 0 to 255. Then

figure(1)
imagesc(image1)
% on the resulting image, use zoom and datatips to find array coordinates at each end
%  of the image colorbar, and in the middle of the colorbar heightwise.
% good choices are (25,110) and (868,110)

% determine the colormap.  Divide by 255 since colormap values run from 0 to 1.
rgbX = double(squeeze(image1(110,25:868,:)))/255;   % 844x3
% interpolate down to 100 colors, put colorbar on the original image
n = 100;
rgbXn = interp1(1:844,rgbX,linspace(1,844,n));      % 100x3
colormap(rgbXn)
colorbar

Answer 2

[Image Analyst]

You can determine it from your image. Essentially you can use the improfile() function:

colorMap = improfile(rgbImage, xi, yi, 100)
colorMap = squeeze(colorMap) / 255;

Here's a full demo I created for you (also attached with reference image):

% Demo to find colormap from an image of a colorbar.
% By Image Analyst, December 8, 2020.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clearvars;
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 16;
fprintf('Beginning to run %s.m ...\n', mfilename);

%-----------------------------------------------------------------------------------------------------------------------------------
% Read in image of colorbar.
folder = [];
baseFileName = 'colorbar.jpeg';
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
	% The file doesn't exist -- didn't find it there in that folder.
	% Check the entire search path (other folders) for the file by stripping off the folder.
	fullFileNameOnSearchPath = baseFileName; % No path this time.
	if ~exist(fullFileNameOnSearchPath, 'file')
		% Still didn't find it.  Alert user.
		errorMessage = sprintf('Error: %s does not exist in the search path folders.', fullFileName);
		uiwait(warndlg(errorMessage));
		return;
	end
end
rgbImage = imread(fullFileName);
[rows, columns, numberOfColorChannels] = size(rgbImage)
% Display the test image full size.
subplot(3, 1, 1);
imshow(rgbImage, []);
axis('on', 'image');
caption = sprintf('Reference Image : "%s"', baseFileName);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.

% Set up figure properties:
% Enlarge figure to full screen.
hFig1 = gcf;
hFig1.Units = 'Normalized';
hFig1.WindowState = 'maximized';
% Get rid of tool bar and pulldown menus that are along top of figure.
% set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
hFig1.Name = 'Demo by Image Analyst';

% Get the profile of line y=112 and x=25 and x=869
x1 = 25;
x2 = 869;
xi = [x1, x2]; % Take 100 samples
yi = [112, 112];

%--------------------------------------------------------------------------------------
% HERE IS THE MAIN PART OF THE ROUTINE:
% Create the colormap from the RGB profile through the relevant part of the image.
colorMap = improfile(rgbImage, xi, yi, 100)
% Colormap needs to be between 0 and 1, so divide by 255.
colorMap = squeeze(colorMap) / 255;
%--------------------------------------------------------------------------------------

% Plot r, g, and b components.
subplot(3, 1, 2);
plot(colorMap(:, 1), 'r-', 'LineWidth', 2); % Plot red component.
hold on;
plot(colorMap(:, 2), 'g-', 'LineWidth', 2); % Plot green component.
plot(colorMap(:, 3), 'b-', 'LineWidth', 2); % Plot blue component.
grid on;
title('Color Map Profiles', 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;

% Create a indexed (gray scale) image and apply our new colormap with the imshow() function.
indexedImage = uint8(repmat(1:100, [10, 1])); % Image is 100 pixels wide and 10 pixels tall.
subplot(3, 1, 3);
imshow(indexedImage, 'ColorMap', colorMap);
caxis([0, 99]);
impixelinfo; % Let user mouse around and see the indexed value (not the mapped RGB value).
colorbar; % Put up a colorbar on the right hand side.
axis('on', 'image');
title('Indexed Image with Color Map Applied', 'FontSize', fontSize, 'Interpreter', 'None');

fprintf('Done running %s.m ...\n', mfilename);