Uses of layers in Unity

You can use layers to optimize your project and workflow. Common uses of layers include: * Layer-based renderingThe process of drawing graphics to the screen (or to a render texture). By default, the main camera in Unity renders its view to the screen. More info
See in Glossary. * Layer-based collisionA collision occurs when the physics engine detects that the colliders of two GameObjects make contact or overlap, when at least one has a Rigidbody component and is in motion. More info
See in Glossary.

Use the Camera culling mask with layers.

You can render only the objects in a particular layer, or selection of layers, if you use the CameraA component which creates an image of a particular viewpoint in your scene. The output is either drawn to the screen or captured as a texture. More info
See in Glossary’s culling maskAllows you to include or omit objects to be rendered by a Camera, by Layer.
See in Glossary. To change the culling mask, select the camera you want to use, and select the Culling Mask dropdown in the InspectorA Unity window that displays information about the currently selected GameObject, asset or project settings, allowing you to inspect and edit the values. More info
See in Glossary window. If you clear the checkbox of a layer, it doesn’t render in the sceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
See in Glossary.

Note: UI(User Interface) Allows a user to interact with your application. More info
See in Glossary elements and screen space canvas children are exceptions to this and render regardless.

Ray cast with layers

You can use layers to specify which GameObjects that a ray cast can intersect with. To make a ray cast ignore a GameObjectThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary, you can assign it to the Ignore Raycast layer, or pass a LayerMask to the ray cast API call.

If you don’t pass a LayerMask to the ray cast API call, Unity uses Physics.DefaultRaycastLayers which matches every layer except Ignore Raycast.

The Physics.Raycast function uses a bitmask, and each bit determines if a layer is ignored by rays or not. If all bits in the layerMask are on, the ray collides against all collidersAn invisible shape that is used to handle physical collisions for an object. A collider doesn’t need to be exactly the same shape as the object’s mesh - a rough approximation is often more efficient and indistinguishable in gameplay. More info
See in Glossary. If the layerMask = 0, there are no collisions.

For example, if you want to cast a ray on layer 8, see the code sample below:

int layerMask = 1 << 8;

// Does the ray intersect any objects which are in layer 8?
if (Physics.Raycast(transform.position, Vector3.forward, Mathf.Infinity, layerMask))
{
    Debug.Log("The ray hit the player");

You can then do the inverse, so that the ray collides with all layers except layer 8.

void Update ()
{
    // Bit shift the index of the layer (8) to get a bit mask
    int layerMask = 1 << 8;

    // This casts rays only against colliders in layer 8.
    // But to collide against everything except layer 8, use the ~ operator because it inverts a bitmask.
    layerMask = ~layerMask;

    RaycastHit hit;
    // Does the ray intersect any objects excluding layer 8.
    if (Physics.Raycast(transform.position, transform.TransformDirection (Vector3.forward), out hit, Mathf.Infinity, layerMask))
    {
        Debug.DrawRay(transform.position, transform.TransformDirection (Vector3.forward) * hit.distance, Color.yellow);
        Debug.Log("Did Hit");
    }
    else
    {
        Debug.DrawRay(transform.position, transform.TransformDirection (Vector3.forward) *1000, Color.white);
        Debug.Log("Did not Hit");
    }
}

Note: If you don’t pass a layerMask to the Raycast function, it still ignores colliders that use the IgnoreRaycast layer.