custom colormap with missing colors

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André
André 2018-7-4
评论： André 2018-7-10
If I create the following image, with 3 possible values: 1, 2 and 10.
im = zeros(128,128)
im(1:32,:) = 1
im(33:64,:) = 2
im(65:end,:) = 10
ax = imshow(im,[])
%I apply the following colormap:
cmap = [0.9020 0.0980 0.2941
0.2353 0.7059 0.2941
0.8000 0.8000 0.8000]
colormap(ax.Parent,cmap)
I obtain an image with only 2 colors, when the colormap has 3 colors. Why isn't the colormap being respected? I want these specific colors for each value of the image. And I want my colormap to include only these colors, no more and no less. And no, my image does not contain values from 3 to 9. It is just 1, 2 and 10 and it is supposed to be like this.
Any help appreciated
André

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Stephen23
Stephen23 2018-7-4
编辑：Stephen23 2018-7-4
You could:
1. specify the limits, and provide the axes handle when calling colormap:
imshow(im,[1,3]);
colormap(gca,cmap)
2. treat it as an indexed image (note that out-of-range indices are truncated to the end colors):
>> imshow(im,cmap)
3. convert it to a TrueColor (RGB) image:
>> [~,idx] = ismember(im,[1,2,10]);
>> rgb = ind2rgb(idx,cmap);
>> imshow(rgb)
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Stephen23
Stephen23 2018-7-4
编辑：Stephen23 2018-7-4
@André: Thank you. You might like to read these:
https://www.mathworks.com/help/matlab/creating_plots/how-patch-data-relates-to-a-colormap.html
https://www.mathworks.com/help/matlab/ref/matlab.graphics.primitive.patch-properties.html#buduav0-1-CDataMapping
You can easily specify the patch 'CDataMapping' to be 'direct', in which case the patch CData corresponds one-to-one with the axes colormap, as does the colorbar. Like this:
>> cmap = [0,0,1;0,1,0;1,0,0;0,1,1;1,1,0;1,0,1]; % six ugly colors.
>> X = [0,1,0,1;1,2,1,2;1,2,1,2;0,1,0,1]; % four squares
>> Y = [0,0,1,1;0,0,1,1;1,1,2,2;1,1,2,2]; % four squares
>> patch(X,Y,[1;2;3;6],'FaceColor','flat','CDataMapping','direct')
>> colormap(cmap)
>> colorbar()
>> caxis([0.5,6.5])
Note the third input to patch: you can see how each patch C value corresponds to the colormap row/color. Note how all colormap colors are shown exactly in the colorbar, in the correct order:
A colorbar is just a narrow axes on one side of the figure, so you could easily create this yourself, which gives you complete control over its location, size, tick labels, etc.
André
André 2018-7-10
Exactly what I needed. Thanks a lot.

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