Surface normals not properly transformed by markersize scaling.

[kbarros]

To draw an ellipsoid, one can start with a sphere mesh and apply an anisotropic scaling via markersize:

using GLMakie
pts = Point3f[[0,0,0], [1,0,0]]
markersize = Vec3f[[0.5, 0.2, 0.5], [0.5, 0.2, 0.5]]
rotations = [qrotation(Vec3f(1,0,0), 0), qrotation(Vec3f(1,1,0), π/4)]
meshscatter(pts; markersize, rotations, color=:yellow)

The shading is wrong because the surface normals have not been properly transformed. @ffreyer pointed out on Slack that only one line of the GLMakie vertex shader should be changed:

Makie.jl/GLMakie/assets/shader/particles.vert

Line 103 in 14b65a0

vec3 N = normals;

This line should read vec3 N = normals / s;. That is, it should include an element-wise division by the scaling factors. With this fix, I get the correct visual result:

Presumably a similar change is needed for WGLMakie and CairoMakie, on these lines:

Makie.jl/WGLMakie/assets/particles.vert

Line 37 in 14b65a0

frag_normal = N;

Makie.jl/CairoMakie/src/primitives.jl

Line 940 in 14b65a0

ns = map(n -> normalize(normalmatrix * n), meshnormals)

Below is a derivation explaining why its appropriate to divide by the scale factors.