Culling (visibility helpers)

Mibo.Elmish.Culling is a helper module that keeps visibility math separate from your renderer and your spatial partitioning (the data structure that organizes objects by position).

It operates on geometric primitives:

• A view frustum (built from a camera or light View×Projection matrix; the frustum is the pyramid-shaped volume the camera sees)
• A bounding sphere / bounding box to test against it
• 2D rectangle overlap

3D: frustum culling

Build a frustum from a View×Projection matrix and test geometry against it. The frustum type is backend-specific: raylib ships its own Frustum (it has no native one), while MonoGame uses its native BoundingFrustum:

// raylib: Mibo.Elmish.Frustum over System.Numerics.Matrix4x4
let frustum = Frustum(viewProjection)

// MonoGame: Microsoft.Xna.Framework.BoundingFrustum
let frustum = BoundingFrustum(viewProjection)

if Culling.isVisible frustum entitySphere then
    // submit draw commands
    ()

Or for axis-aligned bounding boxes:

if Culling.isVisibleBox frustum nodeBounds then
    ()

_Where does the View×Projection matrix come from?_ Neither backend's camera carries ready-made matrices. On MonoGame, build them from the Camera3D fields: Matrix.CreateLookAt(cam.Position, cam.Target, cam.Up) for the view, and Matrix.CreatePerspectiveFieldOfView(cam.FovY, aspect, cam.NearPlane, cam.FarPlane) for the projection. On raylib, capture the pair inside BeginMode3D (Rlgl.GetMatrixModelview() * Rlgl.GetMatrixProjection()), or build it from Raylib.GetCameraMatrix3D(camera) and a perspective matrix.

2D: rectangle overlap

Use Camera2D.viewportBounds with Culling.isVisible2D (the rectangle type is the backend's native one: float Rectangle on raylib, int Rectangle on MonoGame):

// raylib: viewportBounds takes the camera by reference (&)
let viewBounds = Camera2D.viewportBounds &camera viewportWidth viewportHeight

// MonoGame: immutable camera, passed by value
let viewBounds = Camera2D.viewportBounds camera viewportWidth viewportHeight

if Culling.isVisible2D viewBounds spriteBounds then
    ()

What this is not

This module doesn't try to be your spatial index.

• If you have many objects: use a grid / quadtree / BVH / octree.
• Use these helpers at the edge: "is this node/object worth considering for rendering?"

See also: Camera and Rendering overview.