How can I set the transparency of LINE objects in MATLAB 7.14 (R2012a)?

I would like to set the transparency of LINE objects in my figures. I use the ALPHA command to set the transparency of objects in my figure. However, LINE objects do not respect ALPHA values even though I am using the OPENGL renderer.

Answer 1

[MathWorks Support Team]

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

[Walter Roberson]

Karl Oskar Pires Bjørgen comments,

This is not the simplest approach, check fabian's answer below

[Walter Roberson]

The question is specifically about R2012a, at which time the functionality described by Fabian did not exist.

Answer 2

[Fabian]

you could try setting a four-element vector for the color property of the line. works for me in 2014 b.

lh.Color=[0,0,0,0.5]

with lh handle to the line, producing a black line with transparency 0.5

[Walter Roberson]

Interesting. This does not appear to be documented.

(I do not have R2014b or later so I cannot test.)

[TheStranger]

Srry for the offtop, but any ideas why it was not put in the help? Looks like an easter egg, lol. I think someone should write a book like "MATLAB a bunch of undocumented stuff", because it is not the first time I encounter some problem, which can be solved, but the solution isn't in the help menu :)

[Walter Roberson]

You might be interested in: https://undocumentedmatlab.com/

[DGM]

@Troy Skousen's comment-as-answer moved here:

I'm using R2021b. It looks like it will not take a 4 element color any more. Any other thoughts?

[DGM]

I can't test anything beyond R2019b, but undocumented features are subject to change without notice; after all, if there's no documentation, the changes aren't documented either.

[Star Strider]

The 4-element colour vector seems to work in R2022a —

x = 0:0.01:2*pi;
y = sin(x);

figure
plot(x, y, '-', 'Color',[0.2 0.5 0.9 0.8], 'LineWidth',2.5)             % Alpha = 0.8
hold on
plot(x, y+0.25, '-', 'Color',[0.2 0.5 0.9 0.2], 'LineWidth',2.5)        % Alpha = 0.2
hold off

.

[Walter Roberson]

The 4-element colour vector continues to work in R2025a.

However, anything that causes the line object to be copied or regenerated with lose the alpha information, including:

• copyobj
• saving and loading the plot or portions of the plot
• sending the plot between parallel workers and the client
• resizing the axes
• print() or exportobj() (both of which cause the figure to be copied to a temporary figure, then internal properties are adjusted to fit the page size and resolution properties in effect, then the modified version is rendered.)