# Introduction

As of Gazebo6, it is possible to control which geometries collide. A bitmask may be applied to an SDF collision element, where the bitwise-and of two collision's bitmasks determines if they may collide.

A bitmask is a number, usually specified in hexadecimal, that is associated with some object. In our case the objects are collision geometries. When two bitmasks are compared, through a bitwise-and operation, the outcome is used to control what logic is applied. The logic in our case is whether a collision is possible or not.

Let's consider an example that consists of three boxes, boxA, boxB, and boxC. The boxes have bitmasks 0x01, 0x02, and 0x03 for boxA, boxB, and boxC respectively. In binary these bitmasks would be:

• boxA: 01
• boxB: 10
• boxC: 11

Let's assume that all three boxes are in a simulated world, where boxA is directly below boxB and boxC is directly above boxB. This creates a stack of boxes with boxA on bottom, and boxC on top. We will also assume that all the boxes are capable of colliding with the ground.

Once simulation starts, boxA and boxB will be in collision. However, the bitwise-and of their bitmasks will produce a value of zero. This indicates to the simulation engine that collisions should be ignored, and the result is boxB passes through boxA.

At this point boxA and boxB are both co-located and resting on the ground. Meanwhile, boxC is falling from its start position. BoxC will eventually hit boxA and boxB. The bitwise-and of boxC's bitmask and the bitmask belonging to both boxA and boxB will result in a value greater than zero. This non-zero result, also considered ''true'' by programming languages, indicates to the simulation engine that collisions should be generated. The result is that boxC will come to rest on top of boxA and boxB.

This example is available as a run-able Gazebo demo.

1. Run the demo world, and start in a paused state

``````  gazebo -u worlds/shapes_bitmask.world
``````
2. Press the play button to see the boxes drop.

# How to set a collide bitmask

A collide bitmask may be set using SDF, an XML file format for describing simulation properties and entities. The bitmask XML element is a child of the surface XML element. The bitmask value may be specified using either base-10 or base-16. The following is a simple example, note that some necessary SDF elements have been removed for clarity.

``````<model name="box">
<collision name="collision">
<geometry> ... </geometry>

<surface>
<contact>