Fix clipping of rgb_from_vector()

* Clip saturation and luminance separately to make sure the result is clipped
  at the maximum possible saturation or luminance
* Ensure that the argument of np.arccos(...) is within the allowed range
  to avoid NaN

src/empyre/vis/colors.py

@@ -65,8 +65,6 @@ class Colormap3D(colors.Colormap, metaclass=abc.ABCMeta):
         """
         self._log.debug('Calling rgb_from_vector')
         x, y, z = vector
-        R = np.sqrt(x ** 2 + y ** 2 + z ** 2)
-        R_max = vmax if vmax is not None else R.max() + 1E-30
         # FIRST color dimension: HUE (1D ring/angular direction)
         phi = np.asarray(np.arctan2(y, x))
         phi[phi < 0] += 2 * np.pi
@@ -74,11 +72,14 @@ class Colormap3D(colors.Colormap, metaclass=abc.ABCMeta):
         rgba = np.asarray(self(hue))
         r, g, b = rgba[..., 0], rgba[..., 1], rgba[..., 2]
         # SECOND color dimension: SATURATION (2D, in-plane)
-        rho = np.sqrt(x ** 2 + y ** 2)
-        sat = rho / R_max
+        rho = np.sqrt(np.abs(x)**2 + np.abs(y)**2)
+        rho_max = vmax if vmax is not None else rho.max() + 1E-30
+        sat = np.minimum(rho, rho_max) / rho_max
         r, g, b = interpolate_color(sat, (0.5, 0.5, 0.5), np.stack((r, g, b), axis=-1))
         # THIRD color dimension: LUMINANCE (3D, color sphere)
-        theta = np.arccos(z / R_max)
+        z_max = vmax if vmax is not None else np.abs(z).max() + 1E-30
+        z_clipped = np.minimum(np.abs(z), z_max) / z_max
+        theta = np.arccos(z_clipped)
         lum = 1 - theta / np.pi  # goes from 0 (black) over 0.5 (grey) to 1 (white)!
         lum_target = np.where(lum < 0.5, 0, 1)  # Separate upper(white)/lower(black) hemispheres!
         lum_target = np.stack([lum_target] * 3, axis=-1)  # [0, 0, 0] -> black / [1, 1, 1] -> white!