MATLAB: How to assign the colorbar for a subimage in a figure in MATLAB 7.8 (R2009a)

colorbarcolormapMATLABsubimage

I would like to assign a colorbar to a subimage in the figure and have it indexed by this range of the map.

Best Answer

The colorbar can be attributed to a subimage in the figure using the colormap function. To set the range of the colormap indexed by the colorbar, set its 'YLim' property to this range.

The following code illustrates this concept:

load trees 
subplot(1,2,1);
subimage(X,map);
subplot(1,2,2);
subimage(X,winter(64));
fullmap = [map;winter]; %map and winter for second image
colormap(fullmap);  %set colormap
colorbar('YLim',[size(map,1), size(map,1)+64]); %colorbar's ylimits correspond to winter

Related Solutions

MATLAB: Using imwrite() to create a .bmp file with custom colormap

The colormap for the imwrite must be in the range 0 to 1.

When you imwrite with a colormap the data must be integral. If it is type double or single then 1 is subtracted from it and the result is converted to uint8. In other words the values must already be colormap indices.

You need:

Numcolor = size(myMap, 1);
Imgmin = min(MyImage(:)) ;
Imgmax = max(MyImage(:)) ;
mappedImage = uint8( (MyImage-Imgmin)./(Imgmax-ImgMin).* (Numcolor-1) ) ;

Then imwrite(mappedImage, myMap,...)

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.

Best Answer

Related Solutions

MATLAB: Using imwrite() to create a .bmp file with custom colormap

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)

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