Lab vs RGB vs HSV color spaces for color detection?

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David 2013-8-5

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https://ww2.mathworks.cn/matlabcentral/answers/84068-lab-vs-rgb-vs-hsv-color-spaces-for-color-detection

评论： Walter Roberson 2018-7-27

I'm having trouble understanding why there exist multiple color spaces. That is, informatically, all three spaces have the same color information content, so why would one perform better or worse for selecting out, say, red?

I saw from the example demo by ImageAnalyst that lab space is very good for detecting a particular color selected by the user, but I just don't get why the performance should be different for any color space? does it have to do with some computational issues?

thank you for your time.

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Answer 1

Image Analyst 2013-8-5

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You have to look at the shape of the gamut in the different color spaces to understand which color space will be best. Unfortunately MATLAB does not have a gamut visualizer like imageJ does ( http://rsb.info.nih.gov/ij/plugins/color-inspector.html) so you'll have to use that to understand your gamut. I've been telling the Mathworks for years that it's my number one most requested upgrade they can do. For example imagine that you have a gamut that is like a cigar shape extending out into the orange region of LAB/HSV space. You're not going to be able to carve out those pixels with a box, which is what you get if you set thresholds in LAB color space, but if you set thresholds on H, you can carve out a sector going radially, for example you can get all hues going from 40 to 50 degrees, which is the orange region.

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Image Analyst 2013-8-5

在 MATLAB Online 中打开

Code for a shortcut to the imageJ gamut inspector:

% Windows installation instructions:
% First, download and install ImageJ from here:
% http://rsb.info.nih.gov/ij/download.html
% Then download the 3D Color Inspector plug-in for ImageJ here:
% http://rsb.info.nih.gov/ij/plugins/color-inspector.html
% and save it here:
% C:/Program Files/ImageJ/plugins/Color
%
% Now we need to enable the plug-in to work with MATLAB.
% First open your classpath file:
% C:\Program Files\MATLAB\R2013a\toolbox\local\classpath.txt
% for editing.  Then add the following lines as new lines to your classpath file:
% C:/Program Files/ImageJ/ij.jar
% C:\Program Files (x86)\ImageJ\plugins\Color\Color_Inspector_3D.jar
clc;  % Clear the MATLAB command window.
jarFileName = 'C:\Program Files (x86)\ImageJ\plugins\Color\Color_Inspector_3D.jar';
if ~exist(jarFileName, 'file')
  warningMessage = sprintf('Error: Color Inspector jar file does not exist:\n%s', jarFileName);
  warndlg(warningMessage);
  return;
end
% User must browse for the file they want to open.
% Set up a starting/initial folder that they will start browsing from.
startingFolder = pwd; % Change to something specific if you want.
if ~exist(startingFolder, 'dir')
  startingFolder = pwd;
end
% Bring up the "Open File" dialog box.
[baseFileName, folder] = uigetfile({'*.jpg;*.tif;*.png;*.gif;*.bmp','Image Files (jpg,tif,png,bmp,gif)';...
'*.*','All Files (*.*)' },'Specify image file',  startingFolder);
if baseFileName ~= 0
  % If they didn't click Cancel, build the full filename:
  fullFileName = fullfile(folder, baseFileName)
  % Now pass that into the 3D Color Inspector plugin, and you're done.
  color_inspector(fullFileName)
end

Image Analyst 2013-8-5

在 MATLAB Online 中打开

My MATLAB code for a color gamut visualizer, though it's primitive compared to the ImageJ plug-in.

% 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);
msgbox('Click the rotation icon to change your point of view.');