Mayavi中quiver3d（）的Colormap

Question

I am trying to plot a vector field and I am using Mayavi2s quiver3d() function. I would like the arrows to be coloured according to their angle on the unit-sphere:

I can do this this way:

def color(theta, phi):
    while(phi < 0):
        phi += 2.0*np.pi
    h = phi / (2.0 * np.pi)
    s = 1
    v = 1.0 - 0.9999999*(theta / np.pi)
    print("h = {}, s = {}, v ={}".format(h,s,v))
    return hsv_to_rgb(h,s,v)

def plot_nice():
    x = np.array([1.,2.,3.])
    y = np.array([0.,0.,0.])
    z = y

    phi   = np.linspace(0,np.pi/2.0,3)
    theta = phi

    u = np.sin(theta) * np.cos(phi)
    v = np.sin(theta) * np.sin(phi)
    w = np.cos(theta)

    obj = quiver3d(x, y, z, u, v, w,
            line_width=3, colormap='hsv', 
            scale_factor=0.8, mode='arrow',resolution=25)

    for i in range(x.shape[0]):
        r,g,b = color(theta[i], phi[i])
        print("R: {}, G: {}, B: {}".format(r,g,b))
        obj = quiver3d(x[i], y[i], z[i], u[i], v[i], w[i],
                line_width=3, color=(r,g,b), colormap='hsv', 
                scale_factor=0.8, mode='arrow',resolution=25)
    return obj

figure(bgcolor=(1,1,1))
plot_nice()

But this is very slow if I have hundreds of arrows. I can do it in a faster way:

def plot_fast():
    x = np.array([1.,2.,3.])
    y = np.array([0.,0.,0.])
    z = y

    phi   = np.linspace(0,np.pi/2.0,3)
    theta = phi

    u = np.sin(theta) * np.cos(phi)
    v = np.sin(theta) * np.sin(phi)
    w = np.cos(theta)

    obj = quiver3d(x, y, z, u, v, w,
            line_width=3, colormap='hsv', 
            scale_factor=0.8, mode='arrow',resolution=25)
plot_fast()

But the I loose the colormap:

How do I create something like the upper plot without having to create a new plot for each arrow?

Answer 1

It is possible by manipulating the plot object after the fact.

To the quiver3d call, add the arguments scalars=np.mod(phi, 2*np.pi) (for instance) and scale_mode='none' . After the call, add the line

obj.glyph.color_mode = 'color_by_scalar'

From memory, it might be possible to control separately the color and size of the arrows but this is a bit more elaborate.