Understanding GLMakie 3D meshscatter transparency/alpha

[trahflow]

I’m trying to visualize a weighted 3D point set using meshscatter, where the weight is visualized using transparency: Low weight means high transparency.
To do this I set a color alpha value for each point.

The result looks like this:

There are a couple of things I don’t understand:

1. Why do single spheres/points have this dark/light pattern?
2. Why are points transparent to some point but not others?
   I don’t see a pattern related to depth here.
   ssao=true/false does not make a difference.

Code to reproduce the image:

using GLMakie

points = rand(Float32, 3, 20)
alpha = rand(Float32, 20)
    
basecolor = RGBf(0.0, 0.44705883, 0.698039230)
markercolor = RGBAf.(basecolor, alpha)

fig = Figure();
ax = Axis3(fig[1, 1])
ax.scene.lights = [  # define lights by hand, just to be sure
    AmbientLight(RGBf(0.2, 0.2, 0.2)),  DirectionalLight(RGBf(1, 1, 1), Vec3f(0, 0, -1))
]


meshscatter!(ax, points; color=markercolor, markersize=0.1f0, ssao=true)

Point coordinates used in my example

0.08144003, 0.76462823, 0.3078106
0.08184856, 0.88391453, 0.70793194
0.2750585, 0.21764505, 0.36415952
0.30778933, 0.5571264, 0.57384634
0.5351146, 0.94121045, 0.6286627
0.13961536, 0.97269565, 0.39499444
0.34692007, 0.49153197, 0.2805946
0.9911157, 0.22105312, 0.42372352
0.78404695, 0.2793542, 0.5839179
0.5117918, 0.8036711, 0.278471
0.7802961, 0.45787352, 0.6333635
0.9258438, 0.5479086, 0.713095
0.86555547, 0.012489796, 0.27193218
0.13506001, 0.05365455, 0.31954545
0.8792992, 0.48838484, 0.50887954
0.374256, 0.25085568, 0.28193265
0.4643891, 0.5176311, 0.13900834
0.06747496, 0.99334013, 0.67928374
0.28540635, 0.6764825, 0.38197
0.12089592, 0.3304742, 0.31492037

Order independent transparency (via transparency=true) gives a somewhat reasonable look to the above MWE, but in my actual data (with many more points) that’s not what I want, because I want to keep some perception of depth.

[sdanisch]

Have you tried transparency true for the large dataset and evaluated how much of the depth perception is kept?

[trahflow]

I actually have several larger datasets.

This is how transparency=false vs transparency=true looks like for one of them:

The problem here is also than with transparency=true my fake “shadow” (which is just a 2d meshscatter of grey circles) shines through.

[trahflow]

If I change the camera angle to get a “top view” the “two half spheres” artefact does dissapear (but the transparency issue remains):

But with just a slight tilt these artifacts do appear:

until they look like two half-spheres.
This is independent of the direction in DirectionalLight.
I also tried with WGLMakie with the same result.