Light direction in surfl

Hi, I don't understand why it is not centered.

x = -1:.1:1;
y = -1:.1:1;
[X,Y] = meshgrid(x,y);
z = X.^2 + Y.^2;
surfl(x,y,z, [0 0 1])

Answer 1

[Walter Roberson]

The bright area is mostly because of the specular lighting that is being introduced.

doc specular says

R = specular(Nx,Ny,Nz,S,V) returns the reflectance of a surface with normal vector components [Nx,Ny,Nz]. S and V specify the direction to the light source and to the viewer, respectively. You can specify these directions as three vectors[x,y,z] or two vectors [Theta Phi (in spherical coordinates).

The specular highlight is strongest when the normal vector is in the direction of (S+V)/2 where S is the source direction, and V is the view direction.

The S that is used is the one you pass in, [0 0 1], but the V that is being used is being calculated by taking view() of the axes. In a new axes that has not had anything drawn in it yet, the default view is [-37.5, 30] . And if you do not have hold on in effect, then the newplot() that is automatically done resets the axes to this view.

You have two main choices here:

1. clear the axes, view() as appropriate, and do hold on before doing the surfl() so that the specular values are calculated according to your desired view; OR
2. when you call surfl(), pass in the optional 5th parameter, k with a low value in the 3rd component ks so that the specular lighting is not given much weight.