Semi-transparent markers in Matlab Figures

Question

I want to plot a scatter plot with filled markers and make them semi-transparent so when two or more markers overlap, the overlapping area will be more opaque.

I naively thought

sg = scatter(rand(1000,1),rand(1000,1), 'filled');
alpha(0.5)

would work, but it doesn't. Also

set(get(sg, 'Children'), 'FaceAlpha', 0.2)

doesn't work. Any ideas?

Answer 1

Here's some sample matlab code that makes transparent scatterplot points with patch objects:

x=randn(5000,1)*20;
y= randn(5000,1)*20;
t= 0:pi/10:2*pi;
figure();
for i=1:size(x)
    pb=patch((sin(t)+ x(i)),(cos(t)+y(i)),'b','edgecolor','none');
    alpha(pb,.1);
end

Answer 2

AFAIK, you cannot change the alpha values of the plot markers in scatter. One solution would be to patch to draw markers yourself. Alpha values can be set for patch() objects and you will get the desired effect when markers overlap. However, this can get quite cumbersome and will need to be customized to your needs.

See this related question, where the function defined in the question does exactly that. You can use that as a starting point and work from there.