MATLAB: How to get a dithered color image

colormapditheringImage Processing Toolboxrgb image

I know that

X = dither(RGB, map);

returns an indexed image approximation to the given RGB image. I have two questions:

(1) Is there a second step that converts the indexed image to an RGB image using the provided colormap?

(2) How to design a colormap that matches the original colors of the image?

Note: I want a dithered color image not an indexed image.

Any hint/help is appreciated!

Best Answer

I've never used dither() so I don't know what it does. But to answer your questions:

(1) Yes. It's called ind2rgb().

rgbImage = ind2rgb(indexedImage, customColorMap);

(2) Call rgb2ind():

[indexedImage, customColorMap] = rgb2ind(grayImage, numColors);

Related Solutions

MATLAB: How to use RGB2IND with the colormap approximation method to convert images with a pre-specified colormap in Image Processing Toolbox 3.2 (R13)

To convert an RGB image into an indexed image using a pre-specified colormap, use the DITHER function rather than the RGB2IND function, specifiying the following values for the Qm and Qe parameters to DITHER: Qm = 8 and Qe = 7.

The following example illustrates this procedure.

% Create a colormap.
gmap = [0:255;0:255;0:255]'/255;
% Create an RGB image.
rgb1 = zeros(256,256,3);                
rgb1(:,:,1) = repmat([0:255]/255,256,1);
rgb1(:,:,2) = repmat([0:255]/255,256,1);
rgb1(:,:,3) = repmat([0:255]/255,256,1);
% Use DITHER to convert the image,
% specifying the colormap and values
% for the Qe and Qm parameters.
rgb2 = dither(rgb1,gmap,8,7);          
% To test this solution, convert the indexed image back into 
% an RGB image and find the difference between the
% original image and the newly created image.
rgb3 = ind2rgb(rgb2, gmap);             
diff = sum(sum(sum(rgb1-rgb3)))

The reason for using DITHER instead of RGB2IND is that when running RGB2IND with a specified colormap and the 'nodither' option, the function uses a method that quantizes the search space. Specifically, RGB2IND uses the following code:

dither(rgb,map,Qm,Qe)

With Qm = 5 and Qe = 4. Since Qe < Qm, no dithering is performed, which is the same when you specify the ‘nodither’ option with RGB2IND. Since Qm < 8, several of the 2^8 colors in the colormap get grouped together to perform the search. This is done to get reasonable performance since a brute-force nearest neighbor search is quite expensive to do for every pixel in an image.

Hence, to get the exact same colors back, using DITHER with Qm = 8 and Qe = 7. However, this is computationally more expensive than the previous method as the syntax forces DITHER to search through the entire colormap.

MATLAB: How to calculate intensity of a particular color in an RGB image

To get a matrix with the gray level intensities of the three different color channels, use imsplit:

[r, g, b] = imsplit(rgbImage);

g will be a matrix of your green intensities. If you want the overall intensity considering the other color channels, you can use

hsvImage = rgb2hsv(rgbImage);
[h, s, v] = imsplit(hsvImage);

and look at the v channel, or

labImage = rgb2lab(rgbImage);
[L, a, b] = imsplit(labImage);

and look at the L image, or

ycbcrImage = rgb2ycbcr(rgbImage);
[Y, Cb, Cr] = imsplit(ycbcrImage);

and look at the Y image.

Best Answer

Related Solutions

MATLAB: How to use RGB2IND with the colormap approximation method to convert images with a pre-specified colormap in Image Processing Toolbox 3.2 (R13)

MATLAB: How to calculate intensity of a particular color in an RGB image

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