Added friction triggers documentation

Docs/physics_control.md

@@ -1,61 +1,91 @@
+<h1>Friction Triggers</h1>
+
+**Friction Triggers** are box triggers that can be added on runtime and let users define a different friction of the vehicles' wheels when being inside those type of triggers. For example, this could be useful for making slippery surfaces in certain regions of a map dynamically.
+
+In order to spawn a friction trigger using PythonAPI, users must first get the trigger.friction blueprint definition, and then set the following necessary attributes to that blueprint definition:
+
+- *friction*: The friction of the trigger box when vehicles are inside it.
+- *extent_x*: The extent of the bounding box in the X coordinate in centimeters.
+- *extent_y*: The extent of the bounding box in the Y coordinate in centimeters.
+- *extent_z*: The extent of the bounding box in the Z coordinate in centimeters.
+
+Once done that, define a transform to specify the location and rotation of the friction trigger, and simply call the `spawn_actor(trigger_blueprint, trigger_transform, attach_to=None)` method inside `carla.World`.
+
+**Example**
+```py
+# Get Blueprint definition
+bp = carla_world.get_blueprint_library().find('trigger.friction')
+
+# Fill definition's attributes
+friction = 0.0
+trigger_size = 500.0
+bp.set_attribute('friction', str(friction))
+bp.set_attribute('extent_x', str(trigger_size))
+bp.set_attribute('extent_y', str(trigger_size))
+bp.set_attribute('extent_z', str(trigger_size))
+
+# Define trigger box transform
+t = carla.Transform()
+t.location = carla.Location(44.4, 0.5, 0.08)
+t.rotation = carla.Rotation(pitch=0.0, yaw=0.0, roll=0.0)
+carla_world.spawn_actor(bp, t, attach_to=None)
+```
+
 <h1>Vehicle Physics Control</h1>
 
-Physics control properties can be tuned for vehicles and its wheels
+Physics control properties can be tuned for vehicles and its wheels. These changes are applied **only** on runtime, and values set back to default ones when the execution ends.
 
-These properties are controlled through a `VehiclePhysicsControl` object, which also contains a property to control each wheel's physics through a `WheelPhysicsControl` object.
+These properties are controlled through a `carla.VehiclePhysicsControl` object, which also provides the control of each wheel's physics through a `carla.WheelPhysicsControl` object.
 
-- *torque_curve*: Curve that indicates the torque measured in Nm for a specific RPM of the vehicle's engine
-- *max_rpm*: The maximum RPM of the vehicle's engine
-- *moi*: The moment of inertia of the vehicle's engine
+- *torque_curve*: Curve that indicates the torque measured in Nm for a specific RPM of the vehicle's engine.
+- *max_rpm*: The maximum RPM of the vehicle's engine.
+- *moi*: The moment of inertia of the vehicle's engine.
 - *damping_rate_full_throttle*: Damping rate when the throttle is maximum.
-- *damping_rate_zero_throttle_clutch_engaged*: Damping rate when the thottle is zero with clutch engaged
-- *damping_rate_zero_throttle_clutch_disengaged*: Damping rate when the thottle is zero with clutch disengaged
-
-- *use_gear_autobox*: If true, the vehicle will have automatic transmission
-- *gear_switch_time*: Switching time between gears
-- *clutch_strength*: The clutch strength of the vehicle. Measured in Kgm^2/s
-
-- *mass*: The mass of the vehicle measured in Kg
-- *drag_coefficient*: Drag coefficient of the vehicle's chassis
-- *center_of_mass*: The center of mass of the vehicle
-- *steering_curve*: Curve that indicates the maximum steering for a specific forward speed
+- *damping_rate_zero_throttle_clutch_engaged*: Damping rate when the thottle is zero with clutch engaged.
+- *damping_rate_zero_throttle_clutch_disengaged*: Damping rate when the throttle is zero with clutch disengaged.
+- *use_gear_autobox*: If true, the vehicle will have automatic transmission.
+- *gear_switch_time*: Switching time between gears.
+- *clutch_strength*: The clutch strength of the vehicle. Measured in Kgm^2/s.
+- *mass*: The mass of the vehicle measured in Kg.
+- *drag_coefficient*: Drag coefficient of the vehicle's chassis.
+- *center_of_mass*: The center of mass of the vehicle.
+- *steering_curve*: Curve that indicates the maximum steering for a specific forward speed.
 - *wheels*: List of `carla.WheelPhysicsControl` objects.
 
 
 ## `carla.WheelPhysicsControl`
+The properties that can be changed from a `carla.WheelPhysicsControl` are the following:
 
 - *tire_friction*: Scalar value that indicates the friction of the wheel.
 - *damping_rate*: The damping rate of the wheel.
 - *max_steer_angle*: The maximum angle in degrees that the wheel can steer.
 - *is_steerable*: If true, the wheel will steer.
 
-
 ##### Example
 ```py
-
 # Create front and back Wheels Physics Control
 front_wheel = carla.WheelPhysicsControl(
-    tire_friction =4.5,
-    damping_rate = 1.0,
-    max_steer_angle = 70.0,
-    is_steerable=False
+    tire_friction=4.5,
+    damping_rate=1.0,
+    max_steer_angle=70.0,
+    is_steerable=True
 )
 
 back_wheel = carla.WheelPhysicsControl(
     tire_friction=4.5,
     damping_rate=1.0,
-    is_steerable=True
+    is_steerable=False
 )
 
 wheels = [front_wheel, front_wheel, back_wheel, back_wheel]
 
 # Create Vehicle Physics Control
 torque_curve = [carla.Vector2D(x=0, y=400),
-                carla.Vector2D(x=1315, y=655)]
+                carla.Vector2D(x=1300, y=600)]
 
 steering_curve = [carla.Vector2D(x=0, y=1),
-                carla.Vector2D(x=100.0, y=1),
-                carla.Vector2D(x=300.12, y=1)]
+                  carla.Vector2D(x=100, y=1),
+                  carla.Vector2D(x=300, y=1)]
 
 physics_control = carla.VehiclePhysicsControl(
     torque_curve=torque_curve,
@@ -69,7 +99,7 @@ physics_control = carla.VehiclePhysicsControl(
     clutch_strength=10,
     mass=10000,
     drag_coefficient=0.25,
-    center_of_mass=carla.Vector3D(x=00, y=0, z=-10),
+    center_of_mass=carla.Vector3D(x=0, y=0, z=-10),
     steering_curve=steering_curve,
     wheels=wheels
 )