Add an example of how to use Healpix.jl with GeoMakie. (#246)

* Add an example of how to use Healpix.jl with GeoMakie.

* Add the example to make.jl, overview to examples.md, cardmeta

* Correct some oversights

docs/Project.toml

@@ -26,6 +26,7 @@ Geodesy = "0ef565a4-170c-5f04-8de2-149903a85f3d"
 GeometryBasics = "5c1252a2-5f33-56bf-86c9-59e7332b4326"
 GeometryOps = "3251bfac-6a57-4b6d-aa61-ac1fef2975ab"
 GraphMakie = "1ecd5474-83a3-4783-bb4f-06765db800d2"
+Healpix = "9f4e344d-96bc-545a-84a3-ae6b9e1b672b"
 ImageIO = "82e4d734-157c-48bb-816b-45c225c6df19"
 ImageTransformations = "02fcd773-0e25-5acc-982a-7f6622650795"
 Interpolations = "a98d9a8b-a2ab-59e6-89dd-64a1c18fca59"

docs/make.jl

@@ -31,6 +31,7 @@ examples = String[
     "geostationary_image.jl",
     "multiple_crs.jl",
     "rasters.jl",
+    "healpix.jl",
     "is_it_a_plane.jl",
     joinpath("cartopy", "annotation.jl"),
     joinpath("cartopy", "katrina.jl"),

docs/src/examples.md

@@ -33,6 +33,7 @@ german_lakes
 meshimage
 projections
 tissot
+healpix
 ```
 
 ## Examples from Cartopy

examples/healpix.jl

@@ -0,0 +1,45 @@
+#=
+# Healpix.jl 
+
+[Healpix.jl](https://github.com/ziotom78/Healpix.jl) is an equal-area "pixelization" of the 2-sphere.  Here, we show how to plot a Healpix.jl pixelization via GeoMakie.
+
+This is currently a manual process, but we hope to add recipes soon!
+
+=#
+
+# Construct a synthetic Healpix map
+
+using CairoMakie, GeoMakie
+
+using Healpix
+nside = 8
+m = HealpixMap{Float64, RingOrder}(nside)
+m.pixels[:] = 1:length(m.pixels)
+m
+#
+img, _, _ = Healpix.equirectangular(m)
+heatmap(img)
+# Now we can plot it on a GeoAxis with a Mollweide projection:
+meshimage(-180..180, -90..90, reverse(img; dims = 1); npoints = 200, axis = (; type = GeoAxis, dest = "+proj=moll"))
+# Finally, we can also try to obtain the image as a Mollweide projected image via Healpix, and then plot it directly.
+# For more information on what we're doing here, see the [Multiple CRS](@ref) example.
+img, _, _ = Healpix.mollweide(m)
+heatmap(img)
+# 
+fig = Figure()
+ax = GeoAxis(fig[1, 1]; dest = "+proj=moll")
+hp_bbox = Makie.apply_transform(ax.transform_func[], Makie.BBox(-180, 180, -90, 90)) 
+# The rectangle above is the bounding box of the full Earth (or sky, in this case) in the Mollweide projection.
+mini = minimum(hp_bbox)
+maxi = maximum(hp_bbox)
+meshimage!(ax, mini[1]..maxi[1], mini[2]..maxi[2], reverse(img; dims = 1); npoints = 200, source = "+proj=moll")
+lines!(ax, GeoMakie.coastlines(); color = :black, xautolimits = false, yautolimits = false)
+fig
+# Note how the meshimage looks a bit pixelated there - that is because of the mollweide projection!
+
+#=
+```@cardmeta
+Description = "Spherical pixelizations from Healpix.jl"
+Cover=fig
+```
+=#