How can I create a custom 3-color gradient scale using imagesc?

Hello, I am very new to Matlab and would appreciate any insight, and detailed explanations if possible.

I am using imagesc to plot a heatmap of my data that ranges from 0 to 100, in increments of 5 (21 different possible values 0,5,10,15,20,25,30,35,40,45,50,55,60,65,70,75,80,85,90,95,100). I would like to create a 3-color gradient scale (such as is possible with conditional formatting in excel). I would like the gradients to go from blue (0to50), yellow(55to80), and red(85to100). Is this possible?

Thank you in advance.

Answer 1

[Image Analyst]

Try this:

% Initialize a color map array of 101 colors.
colorMap = zeros(101, 3);
% Make blue ramp
colorMap(1:51, 3) = linspace(0, 1, 51-1+1);
% Make yellow ramp
colorMap(52:86, 1) = linspace(0, 1, 86-52+1);
colorMap(52:86, 2) = linspace(0, 1, 86-52+1);
% Make red ramp
colorMap(87:101, 1) = linspace(0, 1, 101-87+1)
% Create a sample image of a ramp.
heatMap = repmat(0:100, [200, 1]);
% Display it.
imagesc(heatMap);
% Apply the colormap and show the colorbar
colormap(colorMap);
colorbar;

[lenny466]

Hi Image Analyst, I think you showed me exactly what I asked (in retrospect), but it's not what I'm going for. Let me try to clarify. In excel I can choose to use conditional formatting of the color based on the data values. I can vary the scale based on set a set min, midpoint, and max. I'm looking for a way to translate this into Matlab, basically. After extensively looking into this, I think imagesc is the way to go for plotting the data; it's just the color scaling that's messing me up. Please see the attached snippet that shows my excel analogy.

[Image Analyst]

I don't see how what I provided doesn't do that. Each array element (pixel) has a value, and the color it appears as is the color you made in colorMap. If you want something like that, try this:

% Create a sample image of a ramp.
heatMap = repmat(0:100, [200, 1]);
% Display it.
imagesc(heatMap);

% Initialize a color map array of 101 colors.
colorMap = jet(20);
% Apply the colormap and show the colorbar
colormap(colorMap);
colorbar;
axis image;

The convention in MATLAB is to have higher values at the top of the colorbar, not the bottom like you have it there in Excel, but maybe you can flip it. Just use flipud(colorMap) to flip the colormap, then write the tick marks manually.

[lenny466]

Hi Image Analyst, Thank you for responding; I really appreciate your help. I was able to figure out how to add the jet(20) color map before you responded, but I am facing two issues with that.

1) I would like to have the yellow center shifted to the "80" point (vs "65") and 2) How can I make the NaNs in my data show up as white/background color? I have searched all over for a good answer to this and have tried many different things, but I can't find a good solution. Currently, the NaNs are set as the lowest value blue color.

Any help would be appreciated, and again, I'm super thankful!