ParametricPlot3D for only part of a sphere

Question

I want to plot the region of a sphere that satisfies the equation

$$ \tan\theta\geq-\frac{rz}{rx\cos\phi + ry\sin\phi} $$

where $rx$, $ry$, $rz$ are known constants. The spherical coordinates $\theta$ and $\phi$ are linked together, making the desired region a smooth cutout of the total sphere.

The basic parametric plot code for a sphere is:

ParametricPlot3D[{Cos[ϕ]*Sin[θ], Sin[ϕ]*Sin[θ], Cos[θ]}, {ϕ, 0, 2 π}, {θ, 0, π}]

I essentially want to apply the above inequality to the limits of this plot, but cannot figure out how to successfully do so. I could just create a table of points in this region and use ListPlot3D or something, but I would prefer a smooth surface.

Answer 1

Try also using Boole. As it was commented by Jason B it requires a certain values of the PlotPoints and PlotRange:

ParametricPlot3D[{Cos[ϕ]*Sin[θ], 
   Sin[ϕ]*Sin[θ], Cos[θ]}*
  Boole[Tan[θ] + 1/(Cos[ϕ] + Sin[ϕ]) > 0], {ϕ, 
  0, 2 π}, {θ, 0, π}, PlotPoints -> 100, 
 PlotRange -> Full]

where I put x=y=z=1. It gives this:

Have fun!

Answer 2

The way I understood the question, the Cartesian parameters are fixed. I'm just going to call them $x_0$, $y_0$ and $z_0$. But since in spherical coordinates we also have $x=r\cos\phi \sin\theta$, $y=r\sin\phi\sin\theta$, $z=r\cos\theta$, we can rewrite the inequality as

$$0 \le x_0 x + y_0 y + z_0 z $$

(the radial coordinate $r$ cancels). This leads me to the following implementation using MeshFunctions:

With[{x0 = -1/2, y0 = 0, z0 = 1/2},
 SphericalPlot3D[1, θ, ϕ, Mesh -> {{0}}, PlotPoints -> 70,
   MeshShading -> {None, Orange},
  MeshStyle -> None,
  MeshFunctions -> {Function[{x, y, z, θ, ϕ, r}, x x0 + y y0 + z z0]}]]

I used SphericalPlot3D because it's fast. The mesh only has the contour 0 because that's the cutoff for the inequality. The formulation of the problem now uses only Cartesian coordinates, so I need just the corresponding first three arguments of the mesh function.

However, in multiplying the original inequality by trig functions to bring it into the simpler form above, some sign changes are lost. Thanks to theDude for pointing that out. The original inequality can easily be plotted in the same way, though:

With[{x0 = -1/2, y0 = 0, z0 = 1/2}, 
 SphericalPlot3D[1, θ, ϕ, Mesh -> {{0}}, PlotPoints -> 70,
   MeshShading -> {None, Orange}, MeshStyle -> None, 
  MeshFunctions -> {Function[{x, y, z, θ, ϕ, r}, 
     Tan[θ] + z0 /(x0 Cos[ϕ] + y0 Sin[ϕ])]}]]

The plot agrees with the result of thedude's answer, but this method is faster.

Answer 3

Another approach:

x0 = -1; y0 = 0; z0 = 1;
reg1 = ImplicitRegion[x^2 + y^2 + z^2 == 1, {{x, -1, 1}, {y, -1, 1}, {z, -1, 1}}];
reg2 = ImplicitRegion[
          Reduce[TransformedField["Spherical" -> "Cartesian", 
             Tan[θ] >= -z0/(x0 Cos[ϕ] + y0 Sin[ϕ]), {r, θ, ϕ} -> {x, y, z}] && 
             x ∈ Reals && y ∈ Reals && z ∈ Reals &&
            -1 < x < 1 && -1 < y < 1 && -1 < z < 1, {x, y, z}
          ] // Quiet,
          {x, y, z}
       ];

DiscretizeRegion[RegionIntersection[reg1, reg2], MaxCellMeasure -> 10^-4]

---

EDIT

Apparently what OP wants is

With[
 {x0 = -1/2, y0 = 0, z0 = 1/2},
 ParametricPlot3D[
  {Cos[ϕ]*Sin[θ], Sin[ϕ]*Sin[θ], 
   Cos[θ]}, {ϕ, 0, 2 π}, {θ, 0, π}, 
  RegionFunction -> 
   Function[{ϕ, θ}, 
    Tan[θ] + z0/(x0 Cos[ϕ] + y0 Sin[ϕ]) >= 0], 
  PlotRange -> All
  ]
 ]

However, I cannot tell why the output doesn't agree with my and Jens's answer. Does anyone have an explanation?

---

EDIT 2

Jens's plot yields:

if MeshFunctions -> {Function[{x, y, z, θ, ϕ, r}, Tan[θ] + z0 /(x0 Cos[ϕ] + y0 Sin[ϕ])]} is replaced with MeshFunctions -> {Function[{θ, ϕ, r}, Tan[θ] + z0 /(x0 Cos[ϕ] + y0 Sin[ϕ])]}.

Answer 4

ParametricPlot3D[
  If[Tan[θ] + -Cos[ϕ]/(
     Cos[θ] Sin[ϕ] Cos[ϕ] + 
      Sin[θ] Sin[ϕ] Sin[ϕ]) > 
    0, {Cos[ϕ] Sin[θ], Sin[ϕ] Sin[θ], 
    Cos[θ]}, Null],
  {ϕ, 0, 2 π}, {θ, 0, π},
  PlotPoints -> 200] // Quiet

or

ParametricPlot3D[{Cos[ϕ] Sin[θ], Sin[ϕ] Sin[θ], Cos[θ]}, {ϕ, 0, 2 π}, {θ, 0, π},
  RegionFunction -> Function[{θ, ϕ}, 
    Tan[θ] + -Cos[ϕ]/(Cos[θ] Sin[ϕ] Cos[ϕ] + Sin[θ] Sin[ϕ] Sin[ϕ]) > 0],
  PlotPoints -> 100] // Quiet