Shading goes black for meshes featuring locally inverted normals

[Kevin-Mattheus-Moerman]

• are you running newest version (version from docs) ?
• can you reproduce the bug with a fresh environment ? (]activate --temp; add Makie)
• What platform + GPU are you on? -> Ubuntu 22.04, NVIDIA T550 Laptop GPU

It looks like there is an interpolation error for color data (and/or shading?) when the normal directions are inverted. This is probably due to normal direction interpolation which will return a zero length vector when interpolating between two adjacent and inverted normal vectors.

Here I inverted the face orientations for half of a sphere. The only thing to see here is that the shading seems to flip to a different state. This looks like the desired behaviour:

However, when the normal directions are exactly negated it looks like we get funny behaviour. Here I inverted the faces halfway a quadrilateral plate mesh. As you can see we now get sudden and odd back regions:

It looks like perhaps Makie should detect that the interpolation produced a zero length vector, next it should be renormalised and be forced to point in the direction of the closest healthy vertex or face normal.

Below is some code I used to study this, which features simple sets of triangles which produces the following image:

using GLMakie
using GeometryBasics

F1 = TriangleFace{Int64}[[1,2,3],[6,4,5]]
V1 = Point3{Float64}[[0.0, 1.0, 0.0], [0.0, -1.0, 0.0], [1.0, 0.0, 0.0], [2.25, 1.0, 0.0], [2.25, -1.0, 0.0],[1.25,0,0]]
C1 = [v[1] for v ∈ V1] # x-coordinates as colors

F2 = TriangleFace{Int64}[[1,2,3],[3,4,5]]
V2 = Point3{Float64}[[0.0, 1.0, 0.0], [0.0, -1.0, 0.0], [1.0, 0.0, 0.0], [2.0, 1.0, 0.0], [2.0, -1.0, 0.0]]
C2 = [v[1] for v ∈ V2] # x-coordinates as colors

F3 = TriangleFace{Int64}[[1,2,3],[5,4,3]]
V3 = Point3{Float64}[[0.0, 1.0, 0.0], [0.0, -1.0, 0.0], [1.0, 0.0, 0.0], [2.0, 1.0, 0.0], [2.0, -1.0, 0.0]]
C3 = [v[1] for v ∈ V3] # x-coordinates as colors

F4 = TriangleFace{Int64}[[1,2,3],[4,5,6]]
V4 = Point3{Float64}[[0.0, 0.0, 0.0], [1.0, 0.0, 0.0], [1.0, 1.0, 0.0], [1.25, 0.0, 0.0], [1.25, 1.0, 0.0],[2.25, 0.0, 0.0]]
C4 = [v[1] for v ∈ V4] # x-coordinates as colors

F5 = TriangleFace{Int64}[[1,2,3],[3,4,2]]
V5 = Point3{Float64}[[0.0, 0.0, 0.0], [1.0, 0.0, 0.0], [1.0, 1.0, 0.0], [2.0, 0.0, 0.0]]
C5 = [v[1] for v ∈ V5] # x-coordinates as colors

F6 = TriangleFace{Int64}[[1,2,3],[2,4,3]]
V6 = Point3{Float64}[[0.0, 0.0, 0.0], [1.0, 0.0, 0.0], [1.0, 1.0, 0.0], [2.0, 0.0, 0.0]]
C6 = [v[1] for v ∈ V6] # x-coordinates as colors


## Visualization
fig = Figure(size=(800,800))

ax = Axis3(fig[1, 1], aspect = :data, xlabel = "X", ylabel = "Y", zlabel = "Z", title = "Separate, inverted")
hp = poly!(ax,GeometryBasics.Mesh(V1,F1), color=C1, shading = FastShading, transparency=false,colormap=Reverse(:Spectral),strokewidth=3,strokecolor=:black)

ax = Axis3(fig[1, 2], aspect = :data, xlabel = "X", ylabel = "Y", zlabel = "Z", title = "Touching, inverted")
hp = poly!(ax,GeometryBasics.Mesh(V2,F2), color=C2, shading = FastShading, transparency=false,colormap=Reverse(:Spectral),strokewidth=3,strokecolor=:black)

ax = Axis3(fig[1, 3], aspect = :data, xlabel = "X", ylabel = "Y", zlabel = "Z", title = "Touching consistent")
hp = poly!(ax,GeometryBasics.Mesh(V3,F3), color=C3, shading = FastShading, transparency=false,colormap=Reverse(:Spectral),strokewidth=3,strokecolor=:black)

ax = Axis3(fig[2, 1], aspect = :data, xlabel = "X", ylabel = "Y", zlabel = "Z", title = "Separate,inverted")
hp = poly!(ax,GeometryBasics.Mesh(V4,F4), color=C4, shading = FastShading, transparency=false,colormap=Reverse(:Spectral),strokewidth=3,strokecolor=:black)

ax = Axis3(fig[2, 2], aspect = :data, xlabel = "X", ylabel = "Y", zlabel = "Z", title = "Touching, inverted")
hp = poly!(ax,GeometryBasics.Mesh(V5,F5), color=C5, shading = FastShading, transparency=false,colormap=Reverse(:Spectral),strokewidth=3,strokecolor=:black)

ax = Axis3(fig[2, 3], aspect = :data, xlabel = "X", ylabel = "Y", zlabel = "Z", title = "Touching consistent")
hp = poly!(ax,GeometryBasics.Mesh(V6,F6), color=C6, shading = FastShading, transparency=false,colormap=Reverse(:Spectral),strokewidth=3,strokecolor=:black)

fig

[ffreyer]

We could try doing something like normal = normalize(normal + vec3(1e-6, 0, 0)) at the start of

Makie.jl/GLMakie/assets/shader/lighting.frag

Lines 40 to 59 in 46e14a8

	vec3 blinn_phong(vec3 light_color, vec3 light_dir, vec3 camdir, vec3 normal, vec3 color) {
	// diffuse coefficient (how directly does light hits the surface)
	float diff_coeff = smooth_zero_max(dot(light_dir, -normal)) +
	backlight * smooth_zero_max(dot(light_dir, normal));
	// DEBUG - visualize diff_coeff, i.e. the angle between light and normals
	// if (diff_coeff > 0.999)
	// return vec3(0, 0, 1);
	// else
	// return vec3(1 - diff_coeff,diff_coeff, 0.05);
	// specular coefficient (does reflected light bounce into camera?)
	vec3 H = normalize(light_dir + camdir);
	float spec_coeff = pow(max(dot(H, -normal), 0.0), shininess) +
	backlight * pow(max(dot(H, normal), 0.0), shininess);
	if (diff_coeff <= 0.0 || isnan(spec_coeff))
	spec_coeff = 0.0;
	return light_color * vec3(diffuse * diff_coeff * color + specular * spec_coeff);
	}

to avoid interpolating to vec3(0). This probably requires earlier normalize(normal) calls to be removed, e.g.

Makie.jl/GLMakie/assets/shader/mesh.frag

Line 115 in 46e14a8

color.rgb = illuminate(normalize(o_world_normal), color.rgb);

since this may lead to nan on some gpus. CairoMakie (primitves.jl) and WGLMakie (shaders in assets folder) should be pretty analogous.

[Kevin-Mattheus-Moerman]

Looks like I posted this before, sorry for the duplicate: #3624 (comment)