Transparent GLMakie background

transparent_glmakie_background.jl

C_1 = C_a * α_a + C_b1 * (1 - α_a)
C_2 = C_a * α_a + C_b2 * (1 - α_a)

C_1 - C_2 = (C_b1 - C_b2) * (1 - α_a)
1 - (C_1 - C_2) / (C_b1 - C_b2) = α_a

(C_1 - (C_b1 * (1 - α_a))) / α_a = C_a


##

function alpha_colorbuffer(scene)
    bg = scene.backgroundcolor[]
    scene.backgroundcolor[] = RGBAf(0, 0, 0, 1)
    b1 = copy(Makie.colorbuffer(scene))
    scene.backgroundcolor[] = RGBAf(1, 1, 1, 1)
    b2 = Makie.colorbuffer(scene)
    scene.backgroundcolor[] = bg
    return map(infer_alphacolor, b1, b2)
end

function infer_alphacolor(rgb1, rgb2)
    rgb1 == rgb2 && return RGBAf(rgb1.r, rgb1.g, rgb1.b, 1)
    c1 = Float64.((rgb1.r, rgb1.g, rgb1.b))
    c2 = Float64.((rgb2.r, rgb2.g, rgb2.b))
    alpha = @. 1 - (c1 - c2) * -1 # ( / (0 - 1))
    meanalpha = clamp(sum(alpha) / 3, 0, 1)
    meanalpha == 0 && return RGBAf(0, 0, 0, 0)
    c = @. clamp((c1 / meanalpha), 0, 1)
    return RGBAf(c..., meanalpha)
end

f, _ = scatter(randn(100, 3),
    color = 1:100,
    colormap = (:viridis, 0.5),
    axis = (type = Axis3,),
    markersize = 20
)

save("test_opaque.png", f.scene)
save("test_alpha.png", alpha_colorbuffer(f.scene))