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To have any physical presence in the scene

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a game object must use one of the

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collider components to define it's physical shape

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and to interact with the relevant physics engine.

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Unity has 2 physics engines.

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One for 3D and one for 2D.

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These two physics engines do not interact with each other.

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For more information on physics and physics 2D

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please see the information linked below.

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When using the 2D physics engine

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a collider 2D component must be used.

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There are 4 collider 2D components to choose from.

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The circle collider 2D,

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the box collider 2D,

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the polygon collider 2D

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and the edge collider 2D.

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All of these collider components share many properties

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as they all derive from the same generic

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base collider 2D.

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Each collider 2D component however is

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optimised for a particular shape.

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The circle collider 2D works with

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round circular objects.

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The box collider 2D works well with rectangular objects.

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The polygon collider 2D work

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best with complex shapes.

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And the edge collider 2D works well for surfaces.

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The edge collider and polygon colliders are very similar.

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They are created by several points, or vertices,

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with edges connecting them.

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The main difference between them is that the polygon collider

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must be a closed shape covering an area.

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And the edge collider must be open,

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defining an edge of one or more segments.

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One good use of the edge collider

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is to make a single solid surface

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rather than a series of 2D colliders

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even if several sprites are used to create

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the visual aspect of the surface.

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When comparing all 2D colliders

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there are 2 properties exposed in the inspector

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that they all share.

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Is Trigger and Material.

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The other unique properties are for defining

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the shape of that particular 2D collider.

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The Is Trigger property will set the collider

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to behave as a trigger collider.

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With normal 2D physics collisions

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the 2D physics engine will prevent

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2D colliders from passing through each other.

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2D trigger colliders will not participate

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in physical collisions and will send 2D

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trigger messages rather than 2D collision messages.

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These trigger messages can be used to

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initiate new actions in the scene.

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For more information on Is Trigger

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and the messages sent by colliders

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please see the information linked below.

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The material property is a reference to the

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physics material 2D being used by the collider 2D

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and this can be none.

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Physics materials define how colliders

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will react when they collider

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and define both the bounciness and

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the friction of the collider's surface.

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To make a bouncy ball or slippery ice covered platform

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use a different physics material 2D

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with each collider 2D.

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For more information on physics material 2D

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please see the information linked below.

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To use a collider 2D it must be attached

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to a game object.

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One way of attaching a collider 2D component

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is to use the Add Component button

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and select Physics 2D

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and select a Collider 2D Component.

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For more information on game objects,

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components and how to use them

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please see the information linked below.

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When a collider 2D component is attached

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Unity will try to size the collider 2D to the sprite.

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If the size and shape of the collider 2D

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is undesirable then the shape can be edited by either

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changing the values in the inspector

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or changing the collider 2D itself in the scene view.

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To edit a collider 2D in the scene view

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hold down the shift key.

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This will display handles on the 2D collider's

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gizmo in the scene view.

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These handles are drag-able.

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In the case of the edge collider and the polygon collider

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dragging an existing handle will move

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the collider's vertex, and change the collider's shape

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rather than resizing the collider.

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By moving the cursor over an empty edge

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on either the edge collider or the polygon collider

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a new vertex can be created by clicking.

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This is indicated by a new handle appearing

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and the edge highlighting in green.

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In the case of the edge collider and the polygon collider

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holding the control key will allow a

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vertex to the be deleted when clicked.

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This is indicated by the handle

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and the edge highlighting in red.

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A game object does not need a rigidbodied

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2D to use a collider 2D.

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To participate in 2D collision and trigger events

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however at least 1 of the game objects

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involved must have a rigidbody 2D attached.

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A common pattern for best performance

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would be to attach collider 2D components

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but not rigidbody components

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to all of the static, or non-moving game objects

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that need to interact with 2D physics.

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And attach a collider 2D and a rigidbody 2D

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to the dynamic, or moving, game object

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that need to interact in the scene.

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For more information on physics 2D

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collider 2D, collisions and triggers

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please see the information linked below.