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

bmpcolormapcustom colormapimwriteMATLAB

I am trying to use the imwrite() function to create and save a .bmp file using a custom colormap, 'myMap'. The colormap contains values between 0 and 1 and is a variable of type double with size [51 3]. My image matrix is of type double and contains values 0 to 1. When I produce this image with imagesc() it comes out fine:
figure
imagesc(myImage)
axis equal
axis tight
colormap(myMap)
However, when I use imwrite to save it as a .bmp file the image is saved in all one dark grey color:
imwrite(myImage, myMap, 'grid.bmp')
I have tried converting myImage to uint8 format and scaling myMap into [256 3] size, but neither works. What am I doing wrong? Thanks.

Best Answer

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,...)