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avdl - Hello, World!

This page will show a "Hello, World!" program made in avdl. It will explain all the basic details to show all skills needed to make a very basic game.

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Aim

The "Hello, World!" program will display a triangle, of pale violet colour, that keeps rotating. The aim is to show a very simple program using avdl.

It will explain some basic concepts like how to:


Create a world

Create a file named hello-world.dd, and open it with a text editor.

The first thing needed, is create a new world that will host our triangle. To do that, we need to create a new class, that is a subclass of dd_world (avdl's world class).

The syntax to create a class is as follows:

(class -classname- -subclass- -declarations-)

Where:

Using that information, let's create a world called "HelloWorld":

(class HelloWorld dd_world
    (group

    (def dd_meshColour triangle)
    (def float rotation)

    (function void create (group))
    (function void update (group))
    (function void draw (group))
    (function void clean (group))

    )
)

Let's break this in parts and explain what it does.

(class HelloWorld dd_world
    (group

This creates a new world called HelloWorld, which is a subclass of dd_world. The group command is used to pass multiple expressions when one is expected. In this case, multiple variables and function declarations are passed, so we group them together.

    (def dd_meshColour triangle)
    (def float rotation)

The command def is used to declare a variable. In this case it is used to declare that these variables belong in the HelloWorld world. It's signature is this:

(def -vartype- -varname-)

Where:

The dd_meshColour is an internal class that creates a mesh with a shape and vertex colours. So the lines above create a mesh with the name triangle, and a floating number that tracks constant rotation, with the name rotation.

    (function void create (group))
    (function void update (group))
    (function void draw (group))
    (function void clean (group))

These are functions that avdl calls when specific criteria is met. The command function has the following signature:

(function -type- -name- -arguments-)

Where:

So the above lines declare a few functions that return nothing (void) and have no arguments (empty group command).


Implement the world's initialisation

In this part, the world needs to have a defined behaviour. What does the triangle mesh look like? What colour is it? What kind of rotation does it do and how fast?

To do this, the world's functions need to be implemented, starting with the create one. This function will run once when the world is marked to be created, and looks like this:

(class_function HelloWorld void create (group)
    (group

    (dd_clearColour 0.13 0.13 0.13 1)

    (= this.rotation 0)

    (this.triangle.set_primitive DD_PRIMITIVE_TRIANGLE)
    (this.triangle.set_colour 0.8 0.6 1)

    )
)

The command class_function is used to implement a function belonging to a class. It's signature is this:

(class_function -classname- -type- -name- -arguments- -statements-)

Where:

With this information, let's look at the first two lines:

(class_function HelloWorld void create (group)
    (group

These implement the function create belonging to class HelloWorld, which returns void and has no arguments. The second group command is for declaring the statements inside of it.

    (dd_clearColour 0.13 0.13 0.13 1)

The command dd_clearColour sets the colour that the window is using to clear itself. If nothing is drawn, the whole screen will have that colour. It takes 4 float numbers as arguments, that define the values red, green, blue and alpha.

    (= this.rotation 0)

This sets the initial rotation to 0.

    (this.triangle.set_primitive DD_PRIMITIVE_TRIANGLE)
    (this.triangle.set_colour 0.8 0.6 1)

These two lines define what the triangle mesh looks like.

The command set_primitive gives it a shape that is pre-defined by avdl. Possible values at the moment are DD_PRIMITIVE_TRIANGLE, DD_PRIMITIVE_RECTANGLE and DD_PRIMITIVE_BOX.

The command set_colour gives a colour to the mesh's shape. It takes three float numbers as input, that represent red, green and blue values.

Important: The mesh's shape has to have been defined before it is given a colour.


Implement the world's update functionality

It gets easier from here, now the world update function needs to be implemented. This is run once per frame, and takes care of updating the world's state. It looks like this:

(class_function HelloWorld void update (group)
    (group

    (= this.rotation (+ this.rotation 1))

    (if (>= this.rotation 360)
        (= this.rotation (- this.rotation 360))
    )

    )
)

Let's break it down again:

(class_function HelloWorld void update (group)
    (group

Like before, this defines the implementation of function update, belonging to class HelloWorld, which returns void and has no arguments.

    (= this.rotation (+ this.rotation 1))

This part, increases the rotation value by 1 every frame. This like can be interpreted as this.rotation = this.rotation + 1.

    (if (>= this.rotation 360)
        (= this.rotation (- this.rotation 360))
    )

This part is only there to wrap around the value of rotation, so when it goes above 360, it will go back to 0 and continue increasing infinitely. Given our rotation will be a full circle, this change won't be visible on the screen.


Implement the world's draw functionality

Next is the world's draw function. What does the world look like? Where is the "camera" looking from? The code looks like this:

(class_function HelloWorld void draw (group)
    (group

    (dd_translatef 0 0 -5)

    (dd_rotatef this.rotation 0 0 1)

    (this.triangle.draw)

    )
)

Let's break it down:

(class_function HelloWorld void draw (group)
    (group

As before, this implements the function draw of class HelloWorld, which returns void and has no arguments.

    (dd_translatef 0 0 -5)

The default "camera" is in the center of the world, looking at the -Z axis. So to position something directly in front of the camera, we need to move what we want to draw in the same direction, in this case, 5 points in the -Z axis.

    (dd_rotatef this.rotation 0 0 1)

This is where the rotation comes in. This line will rotate any object that we draw after it this.rotation degrees around the Z axis.

    (this.triangle.draw)

The last line, simply draws the triangle mesh in the current position (influenced by the translation and rotation above). This will draw the triangle in the middle of the screen.


Implement the world's clean functionality

This part is temporary, and is meant to be removed in a future version of avdl. Until then, the clean function has to be implemented as an empty function like this:

(class_function HelloWorld void clean (group) (group))

The avdl compiler then knows how to add the right functionality to free all resources that this world was using.


Set the project's settings

Before this project is complete, we need to define the dd_gameInit function. This is a function that runs when the game is first started, before a world is shown on the screen. In fact, this defines which world should be shown on the screen by default:

(function void dd_gameInit (group)
    (group

    (dd_setGameTitle "Hello World - avdl")

    (dd_world_set_default HelloWorld)

    )
)

Let's break it up:

(function void dd_gameInit (group)
    (group

This defines the function dd_gameInit that returns void and takes no arguments.

    (dd_setGameTitle "Hello World - avdl")

The command dd_setGameTitle will determine the text that appears on top of a window. On some operating systems, this is always visible, but on others it might not. So it's good to keep in mind that not all users will see this.

    (dd_world_set_default HelloWorld)

This will define the default world, the one that will appear on the screen as soon as everything is loaded as expected. We simply pass it the name of the class created above HelloWorld, and avdl will take care of the rest.


Final finish

That's it. The last part left is to compile the above file into an executable. This can be done with the following command:

avdl hello-world.dd

Once this is done, it will create an executable called game in the same directory. By running that, a window should pop-up with a rotating triangle with a pale violet colour.

You can find this file in the avdl repository, under samples/hello-world/hello-world.dd here: Github


What's next?

The avdl language is still under active development. As it evolved, more tutorials will be added here.

Until then, move back to the main page:

avdl


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