MATLAB: Contourf and colormap limits

caxiscontourfMATLAB

Hi there!

Let's say I want to plot this:

contourf(peaks,16)      
caxis([-2 2])
colorbar

Now I get:

But I want to show black contour lines only in the caxis limits AND also the color levels in the same limits, i.e. 16 color levels in [-2 2], not just 6 like in the picture.

Any ideas? Thanks

Best Answer

The problem is your data goes from [-6.5 8]. The 16 contour lines have been drawn for that. Then when you call caxis, you get rid of any color distinction above 2 or below -2 (>2 all have same color, <-2 all have same color). This does not affect where the actual contor lines go, though. Just the range of values used to assign colors.

Instead, specify (in increasing order) the values you want the contour lines at.

Compare the output of these 3 plots

p = peaks;
figure
contourf(p,16) 
colorbar
figure
contourf(p,16) 
caxis([-2 2])
colorbar
figure
contourf(p,linspace(-2,2,16)) 
colorbar

I think the last one does what you want.

Related Solutions

MATLAB: Do colorbar limits not match the values specified by the CAXIS function in MATLAB 7.0 (R14) when using the CONTOUR or CONTOURF function

This bug has been fixed in Release 14 Service Pack 3 (R14SP3). For previous product releases, read below for any possible workarounds:

A bug in MATLAB 7.0 (R14) causes color axis limits on a graph created using the CONTOUR or CONTOURF functions to be set incorrectly.

To work around this issue, use the CONTOUR/CONTOURF function with the 'v6' switch. For example,

contourf('v6',peaks)      % contour('v6',peaks)
caxis([-2 2])
colorbar

MATLAB: How to designate color in contour plot with specific values

Here's a demo.

Produce the contour plot.
Set the binaryThreshold variable that defines the border of white/black
Redefine the colormap

% Create contour plot
Z = peaks;
[~,c] = contourf(Z);
cb = colorbar(); 
% Set binary threshold to separate the two colors. 
% See c.LevelList for list of contour levels, although you can
% set this value to any value within the range of levels.  
binaryThreshold = 2.0; 
% Determine where the binary threshold is within the current colormap
% All changes to contour plot should be made prior to this.
ax = gca(); 
crng = caxis(ax);  % range of color values (same as min|max of c.LevelList) 
clrmap = ax.Colormap; 
nColor = size(clrmap,1);
binThreshNorm = (binaryThreshold - crng(1)) / (crng(2) - crng(1));
binThreshRow = round(nColor * binThreshNorm); % round() results in approximation
% Change colormap to binary
% White section first to label values less than threshold.
newColormap = [ones(binThreshRow,3); zeros(nColor-binThreshRow, 3)]; 
ax.Colormap = newColormap; 
% Add gray contour line labels
c.LineColor = [.5 .5 .5]; 
c.LineWidth = 2; 
% Add frame to colorbar 
cb.LineWidth = .7;

Note that since all colormaps have a discrete number of colors, the border between black & white is very closely approximated and will most likely not cause a problem. For example, with a colormap of 256 colors, if the threshold is set to 33% of the color range, the transition would happen between color #84 and #85 which would be rounded to 84. If this poses a problem, you can either slightly reduce or increaes the threshold or you can change round() to ceil() or floor().

Best Answer

Related Solutions

MATLAB: Do colorbar limits not match the values specified by the CAXIS function in MATLAB 7.0 (R14) when using the CONTOUR or CONTOURF function

MATLAB: How to designate color in contour plot with specific values

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