physics_joint_revolute_create

Create a revolute joint connecting two instances at one common point.

Syntax:

GML Code Example

physics_joint_revolute_create(inst1, inst2, w_anchor_x, w_anchor_y, ang_min_limt, ang_max_limit, ang_limit, max_motor_torque, motor_speed, motor, col)


Argument Description
inst1 The first instance to connect with the joint
inst2 The second instance to connect with the joint
w_anchor_x The x coordinate for the joint, within the game world
w_anchor_y The y coordinate for the joint, within the game world
ang_min_limit The lower permitted limit for the joint angle
ang_max_limit The upper permitted limit for the joint angle
ang_limit Whether the to limit the angle of the joint (true) or not (false)
max_motor_torque Sets the maximum motor torque used to achieve the desired motor speed (in Newtons per meter)
motor_speed This is the speed at which the motor should rotate
motor Whether the motor should be active (true) or not (false)
col Whether the two instances can collide (true) or not (false)


Returns: index of the joint


Description

A revolute joint forces two bodies to share a common anchor point (often called a hinge point) and the joint has a single degree of freedom - the relative rotation of the two bodies around this point. To specify a revolute you need to provide two instances and a single anchor point in the room, as you can see in the image provided:

If you look at the image, you can see that the two instances have been created to over-lap and at the point where they are touching, we have defined a revolute joint. Now, this joint can be limited in its freedom of rotation thanks to the "ang_min_limit" and "ang_max_limit" values. How does this work? Well, let's look at another image:

As you can see, angles in the physics world are not the same as the standard GameMaker: Studio angles where right is 0 degrees and then it goes anti-clockwise so that up is 90, left is 180, and down is 270. No, when dealing with the revolute joint, the 0 degrees axis runs from the joint position to the origin of the second instance defined by the function and the angles are then calculated in a clockwise direction. If you switch on angle limiting, the limits are defined as being relative to this 0 degree axis and the limit range should include zero, otherwise the joint will lurch when the room begins. Finally, you can define the joint as having a motor or not. This means that when uninfluenced by a collision the joint will move in a direction, which is defined by the motor speed with a positive number being clockwise and a negative number being anti-clockwise. The "max_motor_torque" argument is for limiting the speed of the rotation so that you don't get a perpetually accelerating motor and to limit the influence that a collision may have on the rotation. In this way you can use a joint motor to simulate joint friction by setting joint speed to zero and maximum torque to some small, but significant value. The motor will try to prevent the joint from rotating, but will yield to a significant load.

As with all the joints, if you set the "col" value to true then the two instances can interact and collide with each other but only if they have collision events, however if it is set to false, they will not collide no matter what.


Example:

GML Code Example

var mainFixture, o_id;
 mainFixture = physics_fixture_create();
 physics_fixture_set_circle_shape(mainFixture, sprite_get_width(sprite_index) / 2);
 o_id=instance_create(x+25, y, obj_Door);
 physics_fixture_bind(mainFixture, id);
 physics_fixture_bind(mainFixture, o_id);
 physics_joint_revolute_create(id, o_id, x+25, y, -90, 90, 1, 0, 0, 0, 0);
 physics_fixture_delete(mainFixture);
 

The above code creates and defines a new fixture and then creates an instance of "obj_Door", binding the created fixture to the two new objects. They are then joined by a revolute joint with no motor and the angles limited to a +/- 90 degree swing. Finally the fixture is deleted as it is no longer needed.





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