Change opacity of lines

Question

I want to change the opacity of lines I plotted in a figure in MATLAB. I read somewhere about alpha and facealpha commands but couldn't figure how to use them. Could someone please tell me how to do this?

Answer 1

From MathWorks Support Team on How can I set the transparency of LINE objects in MATLAB 7.14 (R2012a)? (mirror):

Line series objects do not support transparency (alpha) values. The following workaround can be used to overcome this limitation:

1. Please refer to the following MATLAB Central link: http://www.mathworks.com/matlabcentral/fileexchange/36953-patchline/content/patchline.m

2. The above function can be used to introduce transparency in LINE objects as follows:

EXAMPLE 1:

n = 10;
xs = rand(n,1);
ys = rand(n,1);
zs = rand(n,1)*3;
plot3(xs,ys,zs,'r.')
xlabel('x');ylabel('y');zlabel('z');
p  = patchline(xs,ys,zs,'linestyle','--','edgecolor','g','linewidth',3,'edgealpha',0.2);

EXAMPLE 2:

t = 0:pi/64:4*pi;
p(1) = patchline(t,sin(t),'edgecolor','b','linewidth',2,'edgealpha',0.5);
p(2) = patchline(t,cos(t),'edgecolor','r','linewidth',2,'edgealpha',0.5);
l = legend('sine(t)','cosine(t)');
tmp = sort(findobj(l,'type','patch'));
for ii = 1:numel(tmp)
    set(tmp(ii),'facecolor',get(p(ii),'edgecolor'),'facealpha',get(p(ii),'edgealpha'),'edgecolor','none')
end

patchline.m (by Brett Shoelson, 20 Jun 2012 (Updated 01 Sep 2016)):

function p = patchline(xs,ys,varargin)
% Plot lines as patches (efficiently)
%
% SYNTAX:
%     patchline(xs,ys)
%     patchline(xs,ys,zs,...)
%     patchline(xs,ys,zs,'PropertyName',propertyvalue,...)
%     p = patchline(...)
%
% PROPERTIES: 
%     Accepts all parameter-values accepted by PATCH.
% 
% DESCRIPTION:
%     p = patchline(xs,ys,zs,'PropertyName',propertyvalue,...)
%         Takes a vector of x-values (xs) and a same-sized
%         vector of y-values (ys). z-values (zs) are
%         supported, but optional; if specified, zs must
%         occupy the third input position. Takes all P-V
%         pairs supported by PATCH. Returns in p the handle
%         to the resulting patch object.
%         
% NOTES:
%     Note that we are drawing 0-thickness patches here,
%     represented only by their edges. FACE PROPERTIES WILL
%     NOT NOTICEABLY AFFECT THESE OBJECTS! (Modify the
%     properties of the edges instead.)
%
%     LINUX (UNIX) USERS: One test-user found that this code
%     worked well on his Windows machine, but crashed his
%     Linux box. We traced the problem to an openGL issue;
%     the problem can be fixed by calling 'opengl software'
%     in your <http://www.mathworks.com/help/techdoc/ref/startup.html startup.m>.
%     (That command is valid at startup, but not at runtime,
%     on a unix machine.)
%
% EXAMPLES:
%%% Example 1:
%
% n = 10;
% xs = rand(n,1);
% ys = rand(n,1);
% zs = rand(n,1)*3;
% plot3(xs,ys,zs,'r.')
% xlabel('x');ylabel('y');zlabel('z');
% p  = patchline(xs,ys,zs,'linestyle','--','edgecolor','g',...
%     'linewidth',3,'edgealpha',0.2);
%
%%% Example 2: (Note "hold on" not necessary here!)
%
% t = 0:pi/64:4*pi;
% p(1) = patchline(t,sin(t),'edgecolor','b','linewidth',2,'edgealpha',0.5);
% p(2) = patchline(t,cos(t),'edgecolor','r','linewidth',2,'edgealpha',0.5);
% l = legend('sine(t)','cosine(t)');
% tmp = sort(findobj(l,'type','patch'));
% for ii = 1:numel(tmp)
%     set(tmp(ii),'facecolor',get(p(ii),'edgecolor'),'facealpha',get(p(ii),'edgealpha'),'edgecolor','none')
% end
%
%%% Example 3 (requires Image Processing Toolbox):
%%%   (NOTE that this is NOT the same as showing a transparent image on 
%%%         of the existing image. (That functionality is
%%%         available using showMaskAsOverlay or imoverlay).
%%%         Instead, patchline plots transparent lines over
%%%         the image.)
%
% img = imread('rice.png');
% imshow(img)
% img = imtophat(img,strel('disk',15));
% grains = im2bw(img,graythresh(img));
% grains = bwareaopen(grains,10);
% edges = edge(grains,'canny');
% boundaries = bwboundaries(edges,'noholes');
% cmap = jet(numel(boundaries));
% ind = randperm(numel(boundaries));
% for ii = 1:numel(boundaries)
% patchline(boundaries{ii}(:,2),boundaries{ii}(:,1),...
%     'edgealpha',0.2,'edgecolor',cmap(ind(ii),:),'linewidth',3);
% end
%
% Written by Brett Shoelson, PhD
% [email protected]
% 5/31/2012
% 
% Revisions:
% 6/26 Improved rice.png example, modified FEX image.
%
% Copyright 2012 MathWorks, Inc.
%
% See also: patch, line, plot

[zs,PVs] = parseInputs(varargin{:});
if rem(numel(PVs),2) ~= 0
    % Odd number of inputs!
    error('patchline: Parameter-Values must be entered in valid pairs')
end

% Facecolor = 'k' is (essentially) ignored here, but syntactically necessary
if isempty(zs)
    p = patch([xs(:);NaN],[ys(:);NaN],'k');
else
    p = patch([xs(:);NaN],[ys(:);NaN],[zs(:);NaN],'k');
end

% Apply PV pairs
for ii = 1:2:numel(PVs)
    set(p,PVs{ii},PVs{ii+1})
end
if nargout == 0
    clear p
end

function [zs,PVs] = parseInputs(varargin)
if isnumeric(varargin{1})
    zs = varargin{1};
    PVs = varargin(2:end);
else
    PVs = varargin;
    zs = [];
end

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