How to make contour plots transparent - in matlab R2015a?

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ilupi
ilupi on 22 Oct 2015
Answered: Will Grant on 31 Aug 2021
Hi, how can I make several contourf plots which are transparent? I tried the facealpha but the contour class has no facealpha property. Is there another way to make the contourf plots transparent?

Answers (4)

Patricia Handmann
Patricia Handmann on 1 Nov 2017
Is there some news for Matlab 2017a how to do this in a nice way?
  1 Comment
Thoralf Stein
Thoralf Stein on 4 Apr 2019
[~, hContour] = contourf(peaks(20), 10);
drawnow; % this is important, to ensure that FacePrims is ready in the next line!
hFills = hContour.FacePrims; % array of TriangleStrip objects
[hFills.ColorType] = deal('truecoloralpha'); % default = 'truecolor'
for idx = 1 : numel(hFills)
hFills(idx).ColorData(4) = 150; % default=255
end

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Walter Roberson
Walter Roberson on 22 Oct 2015
There is no known way to do this in R2014b, R2015a, or R2015b, other than to draw the contour plot yourself using other primitives such as patch()
In sufficiently old versions contourf created patch objects whose AlphaData or FaceAlpha could be adjusted. In the versions for a bit before R2014b, contour() and contourf() produced contourgroup objects which could not have their Alpha adjusted. The work-around in that era was to use contour3() which still generated patch objects, and to hack the ZData that was created to remove some NaN that prevented the faces from being filled. (I have a script somewhere that does this.)
  3 Comments
Walter Roberson
Walter Roberson on 22 Feb 2017
Unfortunately, at least up to R2016b, there is no Alpha property implemented for contour objects.

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Boris Belousov
Boris Belousov on 25 Feb 2016
As a possible workaround, you may manually define a contour of the area which you want to make transparent. Here is an example where a part of a contour plot is shaded using another contour plot.
% Axes
X = 1:8;
Y = 1:9;
% Contour plot with four levels and three contour lines
Z = [1 1 1 1 1 0 0 0;
1 1 1 1 1 0 0 0;
1 1 1 1 1 0 0 0;
1 1 1 1 1 1 0 0;
1 1 1 1 1 1 0 0;
2 2 1 1 1 1 1 0;
2 2 2 1 1 1 1 1;
3 2 2 2 2 2 1 1;
3 3 3 3 2 2 1 1];
% Top layer that shades part of the plot
Z_f = [0 0 0 0 0 0 0 1;
0 0 0 0 0 0 0 1;
0 0 0 0 0 0 1 1;
0 0 0 0 0 0 1 1;
0 0 0 0 0 0 1 1;
0 0 0 0 0 1 1 1;
0 0 0 0 1 1 1 1;
0 0 0 0 1 1 1 1;
0 0 1 1 1 1 1 1];
% Get contour for the top-layer patch and close the figure
C = contourf(Z_f,'LevelList',0.5); close all
% Close the contour by adding two extra points
x = [C(1,2:end), X(end), X(end)];
y = [C(2,2:end), Y(1), Y(end)];
% Contour plot
contourf(Z,'LevelList',[0 0.5 1.5 2.5]);
% Transparent patch
hold on
fill(x,y,'m','FaceAlpha',0.5);
hold off

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