MATLAB: Map from value to colormap

colormap

I use pcolor to create a figure out of some matrix of data.

h=pcolor(X,Y,M);

Is it possible to receive for a specific index in the matrix in which color (RGB values) pcolor used (assuming a constant colormap)?

Or should i ask it – If I have a colormap and caxis – is there any function that will give me the color of a value (mapping from value to rgb value in the colormap)?

Thanks

Best Answer

Please check the solution provided at the below link by Jason D. Yeatman which does the exact thing that you want http://web.stanford.edu/~jyeatman/AFQ/doc/utilities/vals2colormap.html#_top

function rgb = vals2colormap(vals, colormap, crange)
0002 % Take in a vector of N values and return and return a Nx3 matrix of RGB
0003 % values associated with a given colormap
0004 %




0005 % rgb = AFQ_vals2colormap(vals, [colormap = 'jet'], [crange])
0006 %
0007 % Inputs:
0008 % vals     = A vector of values to map to a colormap or a cell array of
0009 %            vectors of values
0010 % colormap = A matlab colormap. Examples: colormap = 'autumn';
0011 %            colormap = 'jet'; colormap = 'hot';
0012 % crange   = The values to map to the minimum and maximum of the colormap.
0013 %            Defualts to the full range of values in vals.
0014 %
0015 % Outputs:
0016 % rgb      = Nx3 matrix of rgb values mapping each value in vals to the
0017 %            corresponding rgb colors.  If vals is a cell array then rgb
0018 %            will be a cell array of the same length
0019 %
0020 % Example:
0021 % vals = rand(1,100);
0022 % rgb = AFQ_vals2colormap(vals, 'hot');
0023 %
0024 % Copyright Jason D. Yeatman, June 2012

Related Solutions

MATLAB: Draw pixels within threshold interval

Try this demo. It uses the thresholds to get a mask than erases outside that mask. Then it makes a colormap and applies it only within the threshold range. Adapt as needed.

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 22;
%--------------------------------------------------------------------------------------------------------





%    READ IN IMAGE
folder = pwd;
baseFileName = 'cameraman.tif';
grayImage = imread(baseFileName);
% Get the dimensions of the image.
% numberOfColorChannels should be = 1 for a gray scale image, and 3 for an RGB color image.
[rows, columns, numberOfColorChannels] = size(grayImage)
if numberOfColorChannels > 1
	% It's not really gray scale like we expected - it's color.
	% Use weighted sum of ALL channels to create a gray scale image.
	grayImage = rgb2gray(grayImage);
	% ALTERNATE METHOD: Convert it to gray scale by taking only the green channel,
	% which in a typical snapshot will be the least noisy channel.
	% grayImage = grayImage(:, :, 2); % Take green channel.
end
%--------------------------------------------------------------------------------------------------------
% Display the image with the original, linear colormap.
subplot(2, 2, 1);
cmap = jet(256);
imshow(grayImage, 'Colormap', cmap);
axis('on', 'image');
title('Original Color Map', 'FontSize', fontSize, 'Interpreter', 'None');
impixelinfo;
colorbar;
hFig = gcf;
hFig.WindowState = 'maximized'; % May not work in earlier versions of MATLAB.
drawnow;
set(gca,'ColorScale','log')
%--------------------------------------------------------------------------------------------------------
% Show the histogram:
subplot(2, 2, 2);
imhist(grayImage);
grid on;
title('Histogram', 'FontSize', fontSize, 'Interpreter', 'None');
minGL = 150;
maxGL = 190;
xline(minGL, 'Color', 'r', 'LineWidth', 2);
xline(maxGL, 'Color', 'r', 'LineWidth', 2);
%--------------------------------------------------------------------------------------------------------
% BINARIZE THE IMAGE
mask = grayImage >= minGL & grayImage <= maxGL;
%--------------------------------------------------------------------------------------------------------
% Display the mask.
subplot(2, 2, 3);
imshow(mask);
axis('on', 'image');
title('Mask', 'FontSize', fontSize, 'Interpreter', 'None');
impixelinfo;
colorbar;
drawnow;
%--------------------------------------------------------------------------------------------------------
% Apply the mask
maskedImage = grayImage; % Initialize
maskedImage(~mask) = 0; % Erase outside of mask
subplot(2, 2, 4);
imshow(maskedImage, 'Colormap', cmap);
axis('on', 'image');
caption = sprintf('Masked Image with color map applied between %d and %d', minGL, maxGL);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
impixelinfo;
colorbar;
caxis([minGL, maxGL]);
drawnow;

MATLAB: How to display the intensity of 2D image as a colormap

You need to pass in a colormap, N-by-3 array with values between 0 and 1, to to the colormap() function, NOT an image. And you should probably also pass in a handles to the axes you want to apply it to, otherwise it will apply that colormap to every axes in the figure window, which is probably not what you want. So in short, you want:

myColorMap = hsv(256);      % Create a colormap.


colormap(gca, myColorMap);  % Now apply the colormap.

You can also pass the colormap directly into imshow() and skip calling the colormap() function if you want.

See this full demo and study it:

grayImage = imread('cameraman.tif');
subplot(3, 1, 1);
imshow(grayImage);
axis('on', 'image');
title('Original Image');
subplot(3, 1, 2);
% Create a colormap.
myColorMap = hsv(256);
% Display image with that colormap.
imshow(grayImage, 'Colormap', myColorMap);
colorbar;
axis('on', 'image');
title('Overlay Image');
% Or equivalently, using the colormap() function:
subplot(3, 1, 3);
% Display image with no colormap.
imshow(grayImage);
% Create a colormap.
myColorMap = hsv(256);
% Now apply the colormap.  Pass in the axes or else it will apply the colormap
% to ALL images on the figure, which is probably not what you want.
colormap(gca, myColorMap);
colorbar;
axis('on', 'image');
title('Overlay Image');

Now, if you already have a colormap, and you want to produce an RGB image, like to write to a file on disk rather than just pseudocoloring a displayed image, you want to call ind2rgb()

rgbImage = ind2rgb(grayImage, myColorMap);

Best Answer

Related Solutions

MATLAB: Draw pixels within threshold interval

MATLAB: How to display the intensity of 2D image as a colormap

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