Game maker tutorial : How to make arms with joints Before you read further Targeted audience Beginners to intermediate game maker developers

Prerequisites Understanding of base GML language. Loop ,conditional and with constructs.

Covered materials Mathematical notions : Sine and Cosine. Messaging between objects through instance variables. How to set up a complex system.

Defining goal Goal definition We want to simulate a robot arm like the one you can see in pharmaceutics or car factories.

Analysis of such arm As we can see in the following picture, the arm is composed of multiple parts. The depicted one has 3 segments and 3 joints. The next thing we will have to do is defining a data structure to describe the arm. In Game Maker, we cant use structured objects, unless we use empty GameMaker Objects (GMO). Drawing 1: An arm state and mirror array to living GMO.

Instead we will use plain old arrays to hold all the arm

Author : [email protected]

Data Structure definition We will hold the data in 3 arrays, one will contains the length of each segment, the other the relative angle of the joint to the horizontal and the last one the effective angle of each segment to the horizontal. The gray lines represent the next segment at angle “0” in relative position. The horizontal black lines are … the horizontals. The black sectors denotes the relatives angles. The gray sectors denotes the absolute angles. You can see that the gray sectors are simply the sum of the previous black and gray sectors. That means that you can calculate each segment angle with this formula :

S angle (i)=S angle (i−1)+S join (i)

Illustration 1: Arm and angles

Updating data structure We will need to compute the position of each segment and they angle to the normal. For the last, we will use the above formula. For the position, we will simply say that the beginning of one segment is positioned at the end of the previous segment. GameMaker don't offer this information, so we will calculate it. We have the following information about the previous segment: •

Current angle

•

Current coordinates

•

Current length

Here is how we use them :

x=x '+cos(angle)×lenght y= y ' −sin(angle)×lenght

It's that easy ! We did do “-sin()” to go counter-clockwise like GM does.

Author : [email protected]

Game Maker implementation

In this implementation we want to be able to add segment and controls without having to add new code (Always think like that, it's a huge time saver for the future since it ease re-usability for you and others) Arm controller will be an object that will keep the arm in a consistent state and won't be interactive. Controls permit to move the arm. Note that you do not need to make one object per segment, it's only needed when you want different length.

Author : [email protected]

Setting-up the arm object

This is enough ! The presence of the variable “arm_serial” will permit the arm_controller to properly pick the object while initializing !

Author : [email protected]

Setting-up up/down controllers

In the create event :

target_index will be properly set by the arm_controller initialization! In the Left Button event :

Author : [email protected]

Setting-up the arm_controller object

In the step event :

Line 4 to 7 is the application of the first formula to set-up angles. Line 11 to 16 is the application of the second formula to set-up segments positions.

Author : [email protected]

In the create event :

We initialize the 3 arrays and the up/down controllers. We check instance variable existence to pick up the right objects and we already left room to have multiple arms, arm_controllers and so on ;) Exercise 1: Set-up the whole stuff to have two independent arms.(Only some objects to duplicate then change only some literal values) Exercise 2a: Use inheritance to avoid duplicating whole piece of code. Exercise 2b: Use the ability to have per instance “create event” in Game Maker to avoid using inheritance and code duplication ! Author : [email protected]