add example

docs/documentation/cameras.md

@@ -30,6 +30,59 @@ Note that this camera is not used by `Axis`. It is used, by default, for 2D `LSc
 
 {{doc Camera3D}}
 
+## Example - Visualizing the cameras view box
+
+```julia
+using GeometryBasics, LinearAlgebra
+
+function frustum_snapshot(cam)
+    r = Rect3f(Point3f(-1, -1, -1), Vec3f(2, 2, 2))
+    rect_ps = coordinates(r) .|> Point3f
+    insert!(rect_ps, 13, Point3f(1, -1, 1)) # fix bad line
+
+    inv_pv = inv(cam.projectionview[])
+    return map(rect_ps) do p
+        p = inv_pv * to_ndim(Point4f, p, 1)
+        return p[Vec(1,2,3)] / p[4]
+    end
+end
+
+
+ex = Point3f(1,0,0)
+ey = Point3f(0,1,0)
+ez = Point3f(0,0,1)
+
+fig = Figure()
+scene = LScene(fig[1, 1])
+cc = Makie.Camera3D(scene.scene, projectiontype = Makie.Perspective, far = 3.0)
+
+linesegments!(scene, Rect3f(Point3f(-1), Vec3f(2)), color = :black)
+linesegments!(scene,
+    [-ex, ex, -ey, ey, -ez, ez],
+    color = [:red, :red, :green, :green, :blue, :blue]
+)
+center!(scene.scene)
+
+cam = scene.scene.camera
+eyeposition = cc.eyeposition
+lookat = cc.lookat
+frustum = map(pv -> frustum_snapshot(cam), cam.projectionview)
+
+scene = LScene(fig[1, 2])
+_cc = Makie.Camera3D(scene.scene, projectiontype = Makie.Orthographic)
+lines!(scene, frustum, color = :blue, linestyle = :dot)
+scatter!(scene, eyeposition, color = :black)
+scatter!(scene, lookat, color = :black)
+
+linesegments!(scene,
+    [-ex, ex, -ey, ey, -ez, ez],
+    color = [:red, :red, :green, :green, :blue, :blue]
+)
+linesegments!(scene, Rect3f(Point3f(-1), Vec3f(2)), color = :black)
+
+fig
+```
+
 ## General Remarks
 
 To force a plot to be visualized in 3D, you can set the limits to have a nonzero \(z\)-axis interval, or ensure that a 3D camera type is used.