Patch transparency in 3D

If I plot two overlaid patches in 2D with the same color, the overlapping section is twice as dark:

figure(1), clf, hold on, box on
patch([0 0 1 1],[0 1 1 0],'b','facealpha',0.5)
patch([0 0 1 1]+0.5,[0 1 1 0]+0.5,'b','facealpha',0.5)
xlim([-1 2])
ylim([-1 2])

In contrast, if I plot two overlaid patches in 3D, the overlapping section is no darker. The following code creates a cone with a base using the patch command:

Res = 50;

% Define the angles that will create the cylinder in polar coordinates
theta = linspace(0,2*pi,Res);

% Define the radius of the cylinder
r = 1;

% Create the end of the cylinders
h = 3;
x_end = r.*cos(theta);
y_end = r.*sin(theta);
z_end = 0.*x_end-h/2;

figure(1), clf; hold on, grid on
% axis vis3d
FA = 0.3;

% Plot the cone
P = patch(x_end,y_end,z_end,'b'); set(P,'edgecolor','k','facealpha',FA) % The cone base
for f = 1:Res-1
   P = patch([0 x_end([f+1 f])],[0 y_end([f+1 f])],[h/2 -h/2 -h/2],'b');
   set(P,'edgecolor',0.3.*ones(1,3),'facealpha',FA)
end
xlabel('X')
ylabel('Y')
zlabel('Z')

w = 1;
xlim([-w w])
ylim([-w w])
zlim([-2 2])
% axis off
view([-37.5 30])

What am I missing? Thanks!

[KSSV]

I find figure to be normal.....which region you are talking about?

Answer 1

[Michael Bode]

From this perspective, when we look at the region circled in red, we're looking through two surfaces. I would expect to see the base of the cone in a darker shade than ...

Ohhh. We're always looking through two surfaces. My mistake!