No effect of the FaceLighting property on a single patch?

Hi, I was trying the FaceLighting property on a single patch but I could not see any difference between "with" and "without" that FaceLighting property:

X = [-24 44 44 -24];
Y = [15 15 15 15];
Z = [0 0 22 22];
patch(X,Y,Z,'red','FaceLighting', 'Gouraud');
view(30,30)

Do you know why? What mistake did I make?

[Sim]

I get the same result:

figure()
patch(X,Y,Z,'red','FaceLighting', 'Gouraud');
view(30,30)

figure()
patch(X,Y,Z,'red');
view(30,30)

Answer 1

[Turlough Hughes]

There are two reasons.

First is that you need to add a light object to the axes. To do that, use the light function.

The second reason, is that a unifaceted patch with Gouraud or Flat lighting is actually the same lighting effect. I suggest reading about how Gouraud lighting is calculated - in short, the shades over each face depend on intensities calculated at each vertex which depend the direction of the nearest surface or surfaces. For a unifaceted patch, the vertices' intesities are based on the direction of a single surface, i.e. they are all equal. However, when a vertex joins multiple faces, the vertex intensity is calculated from an average direction of the adjoining faces.

The following shows how lighting changes for a multifaceted patch, while a unifaceted patch (the same yellow one shown in the cube) results in the same distributions for Gouraud and Flat lighting:

clear, close all
vert = [0 0 0; 1 0 0;1 1 0; 0 1 0; 0 0 1; 1 0 1; 1 1 1; 0 1 1];
fac = [1 2 6 5; 2 3 7 6; 3 4 8 7; 4 1 5 8; 1 2 3 4; 5 6 7 8];
figure()
subplot(2,2,1), title('Flat Lighting')
hp(1) = patch('Vertices',vert,'Faces',fac,...
    'FaceVertexCData',hsv(6),'FaceColor','flat',...
    'FaceLighting','flat');                     % note, flat is the default
light('Position',[2 2 2]), view(30,30)
subplot(2,2,2), title('Gouraud Lighting')
hp(2) = patch('Vertices',vert,'Faces',fac,...
    'FaceVertexCData',hsv(6),'FaceColor','flat',...
    'FaceLighting','gouraud');
light('Position',[2 2 2]), view(30,30)
subplot(2,2,3), title('Flat Lighting')
hp(3) = patch([1 1 1 1], [0 1 1 0], [0 0 1 1],[1 1 0],...
    'FaceLighting','flat');
light('Position',[2 2 2]), view(30,30)
subplot(2,2,4), title('Gouraud Lighting')
hp(4) = patch([1 1 1 1], [0 1 1 0], [0 0 1 1],[1 1 0],...
    'FaceLighting','Gouraud');
light('Position',[2 2 2]), view(30,30)
[hp(:).DiffuseStrength] = deal(0.8);

[Sim]

Many many thanks! A great explantion @Turlough Hughes !!!

But then I was thinking... If "a unifaceted patch with Gouraud or Flat lighting is actually the same lighting effect", how can I still get the nice effect shown in the yellow (or pink/purple) surface of the "subplot(2,2,2), title('Gouraud Lighting')" when I have a single unifaceted patch (as in my example)? Would it be possible somehow (maybe with other methods/functions) ?

Btw, thanks a lot for your amazing reply! Astonishing!

All the Best

[Turlough Hughes]

Actually, yes, patch allows you to manually modify the vertex norms. From the previous examples you could write the following:

hp(4).VertexNormals = hp(2).VertexNormals([2 3 7 6],:)

Or just the figure on it's own would be the following:

clear, close all
figure()
hp = patch([1 1 1 1], [0 1 1 0], [0 0 1 1],[1 1 0],...
    'FaceLighting','Gouraud');
view(30,30)

light('Position',[2 2 2]), view(30,30)
hp.VertexNormals = [-1 1 -1; -1 -1 -1; -1 -1 -1; -1 1 -1];

and from your original question:

figure()
X = [-24 44 44 -24];
Y = [15 15 15 15];
Z = [0 0 22 22];
hp2 = patch(X,Y,Z,'red','FaceLighting', 'Gouraud');
light('Position',[1 -1 1])
view(30,30)
% Actually here the graphics don't always update after changing
% the norms (seems like a bug tbh), so drawnow is a workaround.
drawnow
hp2.VertexNormals([1 4],:) = repmat([1 -1 1], 2, 1);

[Sim]

AMAZING! Exactly what I needed!

If possible, I would press the button "Accept this Answer" a second time !!

All the best :)