help required using colormap function in matlab

Hello Everyone,

I am using surf function to plot a 2D matrix Z. The matrix has 61 rows and 98 columns. The data varies from 26.5 to 33.5 , and in few places I have some zeros. Till now i was using the default colourmap for plotting my data using surf command. But now I want more control over the colour pattern. I was reading through the documentation of colormap. It says that

"Specify an array of color data that is equal in size to the z- data and is used for indexed colors"

I create a matrix C of same size as Z but I still did not understand what values to put so that I can use the surf function as

figure(),surf(Z,C)

Answer 1

[Daniel Shub]

It is not clear what you are trying to do. In the simplest case you might be trying to do

Z = 7*rand(61,98)+26.5;
C = ndgrid(1:size(Z,1), 1:size(Z,2));

but I suspect you are trying to change the colormap

set(gcf, 'ColorMap', [1:64; 64:-1:1; [32:-1:1, 64:-1:33]]'/64)
colorbar

[Ricky]

Daniel I am new to MATLAB it would be helpful if you can explain what you are doing in the above code

      % set(gcf, 'ColorMap', [1:64; 64:-1:1; [32:-1:1, 64:-1:33]]'/64)
        colorbar   % i know this for displaying the colorbar in the plot

[Daniel Shub]

The [1:64; 64:-1:1; [32:-1:1, 64:-1:33]]'/64 makes a 64x3 matrix with values between 0 and 1 for use as a color map. Each row corresponds to an RGB value. The set(gcf, 'ColorMap', ... tells MATLAB to use that colormap for the current figure (gcf). The color map maps the z value from (x,y,z) space to a color. MATLAB automatically scales the map to match the range of the data. So in the case of surf(Z), 26.5 corresponds to green and 33.5 corresponds to pink.

[Ricky]

Thanks Daniel for your reply can you please also tell the reason for choosing a 64*3 matrix. The default colormap also uses a 64*3 matrix. I created a color matrix C of size 61*98 what values should I fill in it. Thanks for helping

[Daniel Shub]

A 61x98 matrix is not a color map. A color map must be Nx3 where the three values in each row represents a RGB values. I chose N to be 64 since 64 different colors tends to be enough to be percieved as visually continuous. If you have discrete data with only 8 values, then you could set N to be 8.

In surf(Z) MATLAB plots a 3D surface with the height of the surface defined by Z. It then uses the color map to assign color such that smaller Z have values from the "bottom" of the color map and larger Z have values from the "top" of the map. What those colors are depends on the color map.

In surf(Z,C) Z is still the height, but MATLAB choses the color from the color map based on C.

Answer 2

[Sean de Wolski]

You can modify it interactively by using the colormapeditor

surf(peaks);
colormapeditor

(Also available through: Edit Menu -> Colormap)