`poly!` on orthographic projection results in weird polygons

[tiemvanderdeure]

I imagine this is a known problem but cannot find an issue:

using GeoMakie, CairoMakie

fig = Figure()
ga = GeoAxis(
    fig[1, 1],
    dest="+proj=ortho"
)

poly!(ga, GeoMakie.land())
fig

looks like:

Does this have something to do with polygons that are partly visible?

This is on GeoMakie 0.7.16

[tiemvanderdeure]

I figured GlobeAxis is probably a better fit here - but here I run into a different issue where I think both sides of the globe are visible at the same time

fig = Figure()
ga = GlobeAxis(
    fig[1, 1],
)
poly!(ga, GeoMakie.land(), strokewidth = 1, alpha = 0.5, color = :grey)
fig

[asinghvi17]

For GlobeAxis do something like

meshimage!(ax, -180..180, -90..90, fill(colorant"white", 1, 1); zlevel = -300_000)

that gives it an inner "core" that is invisible.

Cairo does that for orthographic unfortunately, GLMakie is probably better there. Not sure how we can fix. It's something to do with how Cairo interprets NaNs - I had a couple of PRs but they never got merged.

[tiemvanderdeure]

yeah I tried the meshimage trick but it doesn't seem to work.

fig = Figure()
ga = GlobeAxis(
    fig[1, 1],
)
meshimage!(ga, -180..180, -90..90, fill(colorant"lightblue", 1, 1); zlevel = -300_000)
poly!(ga, GeoMakie.land(), strokewidth = 1, alpha = 0.5, color = :grey)
fig

The core/background is plotted but the other side of the globe is still visible

[asinghvi17]

Oh is this cairo? yeah that won't work 😅 you need a GL backend for proper depth buffering

[tiemvanderdeure]

yeah I tried Cairo so far but GLMakie runs into slightly different issues:

GlobeAxis with a white core leads to some weird clipping:

fig = Figure()
ga = GlobeAxis(
    fig[1, 1],
)
Makie.update_cam!(ga; longlat = (20, 20))
meshimage!(ga, -180..180, -90..90, fill(colorant"white", 1, 1); zlevel = -300_000)
poly!(ga, GeoMakie.land(), alpha = 0.5, color = :grey)
fig

GeoAxis with orthographic projection almost does the trick but there is still some pieces of the continents missing at the edges (some bits of Siberia and South America missing)

fig = Figure()
ga = GeoAxis(
    fig[1, 1],
    dest="+proj=ortho +lat_0 = 10 + lon_0 = 20",
    ygridvisible = false,
    xgridvisible = false,
    xticklabelsvisible = false,
    yticklabelsvisible = false
)

poly!(ga, GeoMakie.land(), color = :lightgrey)
fig

[asinghvi17]

Oof so for globeaxis this needs mesh refinement. I have been meaning to get on that for a while but never figured it out fully. Will set a claude on it to see.

For geoaxis though I guess we'd want spherical polygon clipping so that you don't run into coordinate singularities? I have no idea how cartopy etc do this though.