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Camera Ray Casting (Pixel Picking)

Code#

Cast a ray from a window pixel:

if (loop.Input.KeyboardAndMouse.NewMouseClicks.Count > 0) {
    var clickLocation = loop.Input.KeyboardAndMouse.NewMouseClicks[0].Location; // (1)!
    var ray = renderer.CastRayFromRenderSurface(clickLocation); // (2)!
}

  1. This example converts the location of a mouse click on a Window to a ray starting at the camera's near plane.

    You don't need to use a mouse click however; CastRayFromRenderSurface() simply takes an XYPair<int> indicating which pixel on the Window the resultant Ray should eminate from.

  2. The clickLocation parameter indicates which pixel on the render output surface (e.g. window) the ray should be cast from.

    The Camera and Window (or RenderOutputBuffer) that this renderer was built with will be used to create the resultant Ray.

Cast a ray from a camera's near plane:

// With a camera instance
var ray = camera.CastRayFromNearPlane(); // (1)!
var ray = camera.CastRayFromNearPlane(new XYPair<float>(0f, 0f)); // (2)!

// Without a camera instance
var ray = CameraUtils.CreateRayFromPerspectiveCameraParameters( // (3)!
    modelMatrix,
    projectionMatrix,
    rayCoord // (4)!
);
var ray = CameraUtils.CreateRayFromPerspectiveCameraParameters(
    cameraPosition,
    cameraViewDirection,
    cameraUpDirection,
    nearPlaneDistance,
    farPlaneDistance,
    verticalFov,
    aspectRatio,
    rayCoord
);

  1. This overload casts a ray from the very centre of the camera's near plane.

  2. This overload takes an XYPair<float> specifying where on the near plane the ray should originate from.

    This coordinate pair is expected as a normalized device coordinate, e.g. X and Y both in the range -1 to 1 (where (-1, -1) is the bottom left corner).

    (0, 0) indicates the ray should emanate from the very centre of the camera's view (identical to the overload on the line above).

  3. CreateRayFromPerspectiveCameraParameters works assuming a perspective projection. Use CreateRayFromOrthographicCameraParameters for an orthographic projection.

    The default for most cameras is a perspective projection.

  4. This argument specifies where on the near plane the ray should originate from; and is expected as a normalized device coordinate, e.g. X and Y both in the range -1 to 1 (where (-1, -1) is the bottom left corner).

Explanation#

The code above demonstrates how to "pixel pick" in TinyFFR; this is where you convert a mouse click on a window to a ray cast out from the camera in to the 3D world.

The resultant Ray will have the following properties:

StartPoint

This Location will indicate the point on the camera's near plane that the ray originates from.

Direction

This Direction indicates where the ray points out from the StartPoint and is guaranteed to point within the camera's frustum.

A Ray has infinite length. If you want to limit its length to the camera's far plane, you can create a BoundedRay like so: var boundedRay = ray.ToBoundedRay(camera.FarPlaneDistance - camera.NearPlaneDistance);

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