MATLAB: What should I do to convert matrix X and map in imread function into the way it suppose to be???

imread

when I use imread(X, map)… function for a picture and write whos, matrix X and matrix map suppose to be a [n m] and [n 3] respectively. However, when I do it, matrix X and map will be shown as [n m 3] and [0 0] respectively! what should I do to convert it into the way it suppose to be????

Best Answer

The map is returned as empty if it is a truecolor image. If you want to convert it to an indexed image for some reason, use rgb2ind()

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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 get a dithered color image

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);

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

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