You don't do this in HSV space. You do it in RGB space. With uint8 pixel values, you can have as many as 16.7 million unique colors, or as many colors as you have pixels, if the number of pixels is less than 16.7 million. So you make up an RGB histogram. Here, run this demo:
% Creates a 3D scatter plot of an RGB color gamut of a color image.
clc; % Clear the command window.
close all; % Close all figures (except those of imtool.)
imtool close all; % Close all imtool figures.
clear; % Erase all existing variables.
workspace; % Make sure the workspace panel is showing.
format longg;
format compact;
fontSize = 20;
% Read in a standard MATLAB color demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
baseFileName = 'peppers.png';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
if ~exist(fullFileName, 'file')
% Didn't find it there. Check the search path for it.
fullFileName = baseFileName; % No path this time.
if ~exist(fullFileName, 'file')
% Still didn't find it. Alert user.
errorMessage = sprintf('Error: %s does not exist.', fullFileName);
uiwait(warndlg(errorMessage));
return;
end
end
rgbImage = imread(fullFileName);
% Get the dimensions of the image. numberOfColorBands should be = 3.
[rows columns numberOfColorBands] = size(rgbImage);
% Display the original color image.
imshow(rgbImage);
title('Original Color Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
% Construct the 3D color gamut.
gamut3D = zeros(256,256,256);
for column = 1: columns
for row = 1 : rows
rIndex = redChannel(row, column) + 1;
gIndex = greenChannel(row, column) + 1;
bIndex = blueChannel(row, column) + 1;
gamut3D(rIndex, gIndex, bIndex) = gamut3D(rIndex, gIndex, bIndex) + 1;
end
end
% Get a list of non-zero colors so we can put it into scatter3()
% so that we can visualize the colors that are present.
r = zeros(256, 1);
g = zeros(256, 1);
b = zeros(256, 1);
nonZeroPixel = 1;
for red = 1 : 256
for green = 1: 256
for blue = 1: 256
if (gamut3D(red, green, blue) > 1)
% Record the RGB position of the color.
r(nonZeroPixel) = red;
g(nonZeroPixel) = green;
b(nonZeroPixel) = blue;
nonZeroPixel = nonZeroPixel + 1;
end
end
end
end
figure;
scatter3(r, g, b, 3);
xlabel('R', 'FontSize', fontSize);
ylabel('G', 'FontSize', fontSize);
zlabel('B', 'FontSize', fontSize);
