Visualization problem of the back side of a surface mesh


I have a problem with a 3D animation. This is a torus which reverses itself. The problem is that when it is reversed, it is ugly because the normals are not oriented as they should. How to solve this problem?


using Meshes
using MeshViz
using GLMakie
using Makie
using Printf

# parameterization of the Clifford torus
function Clifford(u, v)
  [cos(u), sin(u), cos(v), sin(v)] / sqrt(2)

# a 4D rotation
function rotate4D(x, t)
  cost = cos(t)
  sint = sin(t)
    -x[1]*sint + x[4]*cost,
     x[2]*cost - x[3]*sint,
     x[2]*sint + x[3]*cost,
    -x[1]*cost - x[4]*sint

# modified stereographic projection
function stereog(p)
  r = acos(p[4]) / pi / sqrt(1-p[4]*p[4])
  (r*p[1], r*p[2], r*p[3])

# topology
nu = 300
nv = 300
topo = GridTopology((nv, nu), (true, true))

# subdivisions
u_ = LinRange(0, 2*pi, nu+1)[1:nu]
v_ = LinRange(0, 2*pi, nv+1)[1:nv]

# draw the Clifford torus
function drawCT(nplots, alpha1, alpha2)
  t_ = LinRange(0, 2*pi, nplots+1)[1:nplots]
  for i in 1:nplots
    fig, ax, _ = viz( # invisible bounding box
      Meshes.Box(Meshes.Point(-1, -1, -1), Meshes.Point(1, 1, 1)); 
      alpha = 0
    ax.show_axis = false
    points = [
      stereog(rotate4D(Clifford(u, v), t_[i])) 
      for u in u_ for v in v_
    mesh = SimpleMesh(Meshes.Point.(points), topo)
    viz!(mesh; color = :maroon1)
    scale!(ax.scene, 1.65, 1.65, 1.65)
    Makie.rotate!(fig.scene, Meshes.Vec3f(1, 0, 0), alpha1)
    Makie.rotate!(ax.scene, Meshes.Vec3f(0, 0, 1), alpha2)
    png = @sprintf "zzpic%03d.png" i, fig)
1 Like

@ffreyer I think you fixed this in your current lighting PR? At least I think I remember some mentioning of backlighting changes and specular highlights etc.

You can fix that with backlight = 1f0 regardless of the pr.

We could make that the default, but I’m not sure if we should. For a static surface it makes sense that only one side gets light from a single light source and with backlight = 1f0 both sides would.

I did not know this was an option! It is not currently listed under the “3D shading attributes” when you ?mesh. Searching the online documentation, it appears to only turn up for band.

Still, great to know it’s easy to get surfaces lit on both sides.

Thank you. Where should I set this option? In viz?