Android Tutorial Introduction to the libGDX library

Informatix · Sep 12, 2013

Introduction to the libGDX library

What is libGDX ?

libGDX is a game engine. As we saw in the first tutorial, a game engine provides a framework to create games and covers all aspects (rendering, animation, input, music, networking, physics, ...) of various kinds of games.

libGDX is considered as one of the best and fastest engine for the Android world. It is free, rich-featured, reliable, and proved its efficiency in a lot of well-known games (Ingress, Zombie Smasher, Apparatus, Monsterama Park, Clash of the Olympians, Bumbledore, etc.)

It’s a layered framework: it goes from low-level classes for OpenGL experts to high-level classes, easy to use by beginners. It includes a scene graph (Scene2D classes), a physics engine (Box2D classes), a particle system, a map renderer, a sprite batcher, an extensive set of mathematics classes… more than 200 classes in total.

For technical reasons, the version for Basic4Android cannot be multi-platform, and by choice, the current release doesn't include the 3D classes (except the ones for the perspective camera and for the decals) and the Daydream class.

libGDX was created in 2010 by Mario Zechner (badlogicgames.com) and is maintained by M. Zechner, Nathan Sweet (esotericsoftware.com) and a community of developers.

Minimum requirements

OpenGL ES 2.0
Android Froyo (API 8)

Hardware acceleration

libGDX does not require that you enable hardware acceleration on your device because it is based on OpenGL, which interacts directly with the GPU.

Debugging

You cannot use the debugger of B4A with libGDX because most of the code of the library runs in a different thread. You have to use the Log() function to debug your game.

The library provides a debug renderer for Box2D (lgBox2DDebugRenderer), a debug renderer for Scene2D (lgScn2DDebugRenderer) and a profiler for OpenGL (lgGLProfiler).

A word about the classes

The main class is LibGDX. All other classes are prefixed by lg.
All Box2D classes are prefixed by lgBox2D. All Scene2D classes are prefixed by lgScn2D. All Map classes are prefixed by lgMap. All Math classes are prefixed by lgMath.

The LibGDX class gives access to five interfaces: Audio (lgAudio), Files (lgFiles), Graphics (lgGraphics), Input (lgInput), and Net (lgNet). You will use the Input interface, for example, to get the input from the accelerometer.

With some classes (e.g. the five interfaces), you cannot create a new instance with Dim. You have to use an instance returned by the library. For example, you cannot write:

B4X:

Dim Graphics As lgGraphics
Graphics = lGdx.Graphics

but you can write:

B4X:

Dim Graphics As lgGraphics = lGdx.Graphics

Some classes cannot be instantiated at all because they are generic classes (e.g. com.badlogic.gdx.scenes.scene2d.utils.Drawable or com.badlogic.gdx.maps.tiled.TiledMapTile). In this case, either you store their instance as an Object or you use a subclass, e.g.:

B4X:

Dim objTile As Object = CurrentLayer.GetCell(X, Y).Tile
Dim staticTile As lgMapStaticTiledMapTile = CurrentLayer.GetCell(X, Y).Tile

OpenGL ES

I explained what OpenGL is in the previous tutorial and I won't discuss it further here because the main advantage to use a game engine like libGDX is to benefit from the abstraction layer above OpenGL. However, if you need (or want) to call directly OpenGL, here's how to get access to the classes and functions:

B4X:

Dim lGdx_GL20 As lgGL20 = lGdx.Graphics.GL20

or better (includes also the constants):

B4X:

Dim GL As lgGL

Note: libGDX uses the 2D coordinate system and the color encoding of OpenGL ES, so the Y-axis is pointing upwards and each color value ranges from 0 to 1.

The libGDX life-cycle

An important thing to keep in mind about libGDX is that it runs in its own thread. Your Basic4Android application runs most of the time in a different thread called the UI thread, or main thread. That means you cannot access the other views of your activity and change their properties from the libGDX thread. Fortunately, there's a function in the LibGDX class that passes the runnable (the piece of code to execute) from a thread to the other: CallSubUI. So if you want to change the activity title, set a label text or show a MsgBox from the libGDX thread, don't forget to use this function to avoid a crash!

Since libGDX runs in a different thread, you have to inform its library of the events of your activity : Create, Pause and Resume. First, create an instance of libGDX in Globals :

B4X:

Dim lGdx As libGDX

In Activity_Create (and nowhere else), add the Initialize function :

B4X:

lGdx.Initialize(False, "LG") 'fills the activity with the libGDX surface, uses OpenGL 1 for compatibility and prefixes the events with LG

In Activity_Resume, add the following line :

B4X:

If lGdx.IsInitialized Then lGdx.Resume

In Activity_Pause, add the following line :

B4X:

If lGdx.IsInitialized Then lGdx.Pause

You could initialize libGDX differently. For example, it could be limited to a view with InitializeView. You could also define a configuration (lgConfiguration class) and pass it to libGDX. Example:

B4X:

Dim Config As lgConfiguration

'Disables the accelerometer and the compass
Config.useAccelerometer = False
Config.useCompass = False

'Uses a WakeLock (the device will stay on)
Config.useWakelock = True

'Creates the libGDX surface
lGdx.Initialize2(Config, "LG")

Once done, your library is ready to raise the events of its life-cycle : Create, Resize, Render, Pause, Resume, Dispose. These events are the place to put all the code of your game. Don't put anything in the usual activity events. They are reserved for your other views and are raised by the UI thread.

Create :

Create is the first raised event. It is raised soon after the initialization of the library and the creation of the OpenGL surface.
In this event, initialize your renderer and your input processors, load your resources (we'll see that in detail later) and initialize your game data.

Resize :

Resize is raised when the size of the libGDX surface changes. Under Android, that should only happen when you start the application and when it is restarted after a rotation or resumed.
It is raised at least once, after Create, and, when the application is resumed, just before Resume.
In this event, initialize the camera viewport. It's probably the only use you will find for it.
This event returns the new width and height in pixels.

Render :

Render is raised as soon as possible after Create and Resize.
It's where things are drawn. It's also where you have to put the logic of your game, but I would not recommend putting hundreds of lines of code here. Instead, create new subs and new modules and call them from this event.
The first lines in Render should be to clear the screen. Example:

B4X:

lGdx_GL.glClearColor(0, 0, 1, 1) 'Blue background
lGdx_GL.glClear(lGdx_GL.GL10_COLOR_BUFFER_BIT)

Pause :

Pause is raised when the activity is sent in the background, rotated or exited.
It's the right place to save your game data.
Note that the OpenGL context is destroyed when the app goes in the background, so all your unmanaged textures and pixmaps have to be reloaded or recreated in the Resume event when the app returns to the foreground.

Resume :

Contrary to the Resume event of your activity, this one is not raised after Create, only when the application returns from a pause.
As the OpenGL context is destroyed when the app goes in the background, all your unmanaged* textures and pixmaps have to be reloaded or recreated when this event is raised. More info here. *Not loaded by an asset manager.

Dispose :

Dispose is called when the activity is exited, after Pause, or when the device is rotated.
In this event, release all the used resources by calling the Dispose function of objects (if they have one).

The life-cycle :

Multiple screens

A game is made of many screens. You could create an activity for each one, but that would not be very convenient because you'd have to reinitialize the library in each activity and reload some resources. Moreover, that would not ease any graphical transition between screens. In fact, most games are made with a very small number of activities and make use of a screen manager instead. The screen manager stores the reference of the different screens and allows switching between them. Each screen has its own life-cycle.
To create a screen manager with two screens, for example, declare them in Globals:

B4X:

Dim lGdx_ScrMgr As lgScreenManager
Dim lGdx_Screen(2) As lgScreen

Then add these lines in the Create event handler:

B4X:

'Creates two screens
lGdx_ScrMgr.Initialize(lGdx)
lGdx_Screen(0) = lGdx_ScrMgr.AddScreen("LGS1")
lGdx_Screen(1) = lGdx_ScrMgr.AddScreen("LGS2")

Show the first screen:

B4X:

lGdx_ScrMgr.CurrentScreen = lGdx_Screen(0)

When you want to change the current screen, just change the value of the CurrentScreen property. That will raise the Hide event of the previous screen and the Show event of the new one.

The screens have the same life-cycle as the library, and thus the same events except that Create is named Show and Dispose is named Hide.

Input processor and gesture detector

To get the input events raised by your players, you have to declare input processors. libGDX has an input processor for keyboard and touch events (lgInputProcessor) and a specialized input processor for gestures (lgGestureDetector).
Start by declaring them in Globals:

B4X:

Dim lGdx_IP As lgInputProcessor
Dim lGdx_GD As lgGestureDetector

Initialize them in the Create event (or the Show event of a screen if you want different processors for different screens):

B4X:

lGdx_IP.Initialize("IP")
lGdx_GD.Initialize("GD")

And add the event handlers that you need:

B4X:

Sub IP_KeyDown(KeyCode As Int) As Boolean
   Return False
End Sub

Sub IP_KeyUp(KeyCode As Int) As Boolean
   Return False
End Sub

Sub IP_KeyTyped(Character As Char) As Boolean
   Return False
End Sub

Sub IP_TouchDown(ScreenX As Int, ScreenY As Int, Pointer As Int) As Boolean
   Return False
End Sub

Sub IP_TouchDragged(ScreenX As Int, ScreenY As Int, Pointer As Int) As Boolean
   Return False
End Sub

Sub IP_TouchUp(ScreenX As Int, ScreenY As Int, Pointer As Int) As Boolean
   Return False
End Sub

Sub GD_TouchDown(X As Float, Y As Float, Pointer As Int) As Boolean
   Return False
End Sub

Sub GD_Fling(VelocityX As Float, VelocityY As Float) As Boolean
   Return False
End Sub

Sub GD_LongPress(X As Float, Y As Float) As Boolean
   Return False
End Sub

Sub GD_Pan(X As Float, Y As Float, DeltaX As Float, DeltaY As Float) As Boolean
   Return False
End Sub

Sub GD_Pinch(InitialPointer1 As lgMathVector2, InitialPointer2 As lgMathVector2, Pointer1 As lgMathVector2, Pointer2 As lgMathVector2) As Boolean
   Return False
End Sub

Sub GD_Tap(X As Float, Y As Float, Count As Int) As Boolean
   Return False
End Sub

Sub GD_Zoom(InitialDistance As Float, Distance As Float) As Boolean
    Return False
End Sub

Description of events :

Fling: The user quickly dragged a finger across the screen, then lifted it. Useful to implement swipe gestures.
Pan: The user is dragging a finger across the screen. The detector reports the current touch coordinates as well as the delta between the current and previous touch positions. Useful to implement camera panning in 2D.
Pinch: Similar to zoom. The detector reports the initial and current finger positions instead of the distance. Useful to implement camera zooming and more sophisticated gestures such as rotation.
Tap: The user touched the screen and lifted the finger. The finger must not move outside a specified square area around the initial touch position for a tap to be registered. Multiple consecutive taps will be detected if the user performs taps within a specified time interval.
Zoom: The user has placed two fingers on the screen and is moving them together/apart. The detector reports both the initial and current distance between fingers in pixels. Useful to implement camera zooming.

The other events are self-explanatory.

.....

ilan · Nov 17, 2016

hi,

is it possible to set a lgScn2DLabel Alignment to top and right?
i can set only 1 option but i need TOP and RIGHT.

regards, ilan

wonder · Nov 18, 2016

B4X:

'Align RIGHT (given Left and Width):
x = (Parent.Left + Parent.Width) - Child.Width

'Example: Screen (parent) / Label (child) align RIGHT
x = 100%x - Label.Width

B4X:

'Align RIGHT (given CenterX and Width):
x = (Parent.cX + Parent.Width / 2) - Child.Width

For TOP it's the same philosophy.

wonder · Nov 18, 2016

My turn now.

I have no way to test this until tomorrow, so that's why I'm asking now:
Say I have stuff being rendered into a FrameBuffer.
What would be the fastest way to send the rendered frame across a network?

B4X:

Dim Pixmap As lgPixmap = FBO.GetDefaultFrameBufferPixmap(0, 0, lGdx.Graphics.Width, lGdx.Graphics.Height, True)
Dim PixmapIO As lgPixmapIO
PixmapIO.WritePNG(lGdx.Files.external("screenshot.png"), Pixmap)
Pixmap.dispose
'Send the PNG file to IP xxx.xxx.xxx.xxx

OR

B4X:

Dim raf As RandomAccessFile
raf.Initialize(...) --> point it somehow to an IP address
Dim bytes() As Byte

bytes = FBO.GetDefaultFrameBufferPixels(0, 0, FBO.Width, FBO.Height, False)
raf.WriteBytes(bytes, 0, bytes.Length, 0) 'Not sure about the syntax here

'This solution looks obviously faster!!!

OR

B4X:

'*Your solution here*

Note: I don't care how are the bytes written (given integrity is maintained), PNG or not, flipped or not, I just want the fastest way to "get the bytes out".

ilan · Nov 18, 2016

wonder said:

B4X:

'Align RIGHT (given Left and Width):
x = (Parent.Left + Parent.Width) - Child.Width

'Example: Screen (parent) / Label (child) align RIGHT
x = 100%x - Label.Width

B4X:

'Align RIGHT (given CenterX and Width):
x = (Parent.cX + Parent.Width / 2) - Child.Width

For TOP it's the same philosophy.

a lgscn2dlabel has an option to set alignment

B4X:

lbl.SetAlignment2(lbl.ALIGN_Center)

or

B4X:

lbl.SetAlignment2(lbl.ALIGN_Right)

the problem is that if i set it to the right it is centered (on the height) and i want it on the top right corner

if i set first right and then top it takes only the last setting.

Informatix · Nov 18, 2016

ilan said:
hi,

is it possible to set a lgScn2DLabel Alignment to top and right?
i can set only 1 option but i need TOP and RIGHT.

regards, ilan

Did you try to add the ALIGN constants?
And what's the result of SetAlignment(ALIGN_TOP, ALIGN_RIGHT)?

inakigarm · Nov 18, 2016

ilan said:
a lgscn2dlabel has an option to set alignment

B4X:

lbl.SetAlignment2(lbl.ALIGN_Center)

Looking at https://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/utils/Align.html and Vader B4X browser object seems that wrapped library doesn't include this constant type (ALIGN_topright)
But maybe you can type directly the int constant value for topright =18
(https://libgdx.badlogicgames.com/ni...values.html#com.badlogic.gdx.utils.Align.left)

ilan · Nov 18, 2016

Informatix said:
And what's the result of SetAlignment(ALIGN_TOP, ALIGN_RIGHT)?

i tried it but the second argument is only if you have multi lines so i did not get the result.

inakigarm said:
Looking at https://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/utils/Align.html and Vader B4X browser object seems that wrapped library doesn't include this constant type (ALIGN_topright)
But maybe you can type directly the int constant value for topright =18
(https://libgdx.badlogicgames.com/ni...values.html#com.badlogic.gdx.utils.Align.left)

thanx i will try this

ilan · Nov 18, 2016

inakigarm said:
include this constant type (ALIGN_topright)

this dosnot work (i cannot build the apk, unknown member)

inakigarm said:
But maybe you can type directly the int constant value for topright =18

PERFECT!!!! thank you very much

Informatix · Nov 18, 2016

ilan said:
i tried it but the second argument is only if you have multi lines so i did not get the result.

And did you try to add the constants? You did not answer.

Informatix · Nov 18, 2016

wonder said:
My turn now.

I have no way to test this until tomorrow, so that's why I'm asking now:
Say I have stuff being rendered into a FrameBuffer.
What would be the fastest way to send the rendered frame across a network?

B4X:

Dim Pixmap As lgPixmap = FBO.GetDefaultFrameBufferPixmap(0, 0, lGdx.Graphics.Width, lGdx.Graphics.Height, True) Dim PixmapIO As lgPixmapIO PixmapIO.WritePNG(lGdx.Files.external("screenshot.png"), Pixmap) Pixmap.dispose 'Send the PNG file to IP xxx.xxx.xxx.xxx

OR

B4X:

Dim raf As RandomAccessFile raf.Initialize(...) --> point it somehow to an IP address Dim bytes() As Byte bytes = FBO.GetDefaultFrameBufferPixels(0, 0, FBO.Width, FBO.Height, False) raf.WriteBytes(bytes, 0, bytes.Length, 0) 'Not sure about the syntax here 'This solution looks obviously faster!!!

OR

B4X:

'*Your solution here*

Note: I don't care how are the bytes written (given integrity is maintained), PNG or not, flipped or not, I just want the fastest way to "get the bytes out".

What do you want to achieve? Transmit a snapshot through the network? Between apps using libGDX?

ilan · Nov 18, 2016

Informatix said:
And did you try to add the constants? You did not answer.

sorry but i dont understand what you mean with constants

i tried:

B4X:

    menushopprices(2).SetAlignment2(menushopprices(2).ALIGN_Top)
    menushopprices(2).SetAlignment2(menushopprices(2).ALIGN_Right)

but only the last command is performed (.ALIGN_Right)

then i tried:

B4X:

    menushopprices(2).SetAlignment(menushopprices(2).ALIGN_Top, menushopprices(2).ALIGN_Right)

result is only on top (and centered!) because second argument is for the lines alignment

wonder · Nov 18, 2016

Informatix said:
What do you want to achieve? Transmit a snapshot through the network? Between apps using libGDX?

I want to build a LibGDX based screen-casting solution [Android ---> PC].
That's why I don't care how the bytes are to be written. The PC "decoder" will be fast enough to decode and render on screen (B4J Canvas or C++ OpenGL).

Note: The frameBuffer doesn't necessarily have to be rendered on the Android screen.
For example, just a black screen with text "Casting in progress..."

Informatix · Nov 18, 2016

ilan said:
sorry but i dont understand what you mean with constants

ALIGN_TOP+ALIGN_RIGHT

A constant is a value that cannot change. Usually constants are named with upper case. In B4A, you declare them with Dim Const.

ilan · Nov 18, 2016

Informatix said:
A constant is a value that cannot change

yes this is something i know

but i dont understand how you want me to set the constant

ALIGN_TOP+ALIGN_RIGHT when i write something like this i get unknown member

anyway as @inakigarm suggested, setting the value 18 works as expected.

Informatix · Nov 18, 2016

wonder said:
I want to build a LibGDX based screen-casting solution [Android ---> PC].
That's why I don't care how the bytes are to be written. The PC "decoder" will be fast enough to decode and render on screen (B4J Canvas or C++ OpenGL).

Transmitting the array of bytes (Pixmap.Pixels) in a zipped format is probably faster than creating and transmitting a PNG file but you have to be able to recreate the image from it on the PC side.
Note that the FastIO library from ProBundle is faster than the RandomAccessFile library.

Informatix · Nov 18, 2016

ilan said:
yes this is something i know

but i dont understand how you want me to set the constant

ALIGN_TOP+ALIGN_RIGHT when i write something like this i get unknown member

anyway as @inakigarm suggested, setting the value 18 works as expected.

According to your code above, that should be
menushopprices(2).ALIGN_Top+menushopprices(2).ALIGN_Right
That's more obvious than 18.
In fact, Alignment should work exactly like Gravity.

wonder · Nov 18, 2016

Informatix said:
Transmitting the array of bytes (Pixmap.Pixels) in a zipped format is probably faster than creating and transmitting a PNG file but you have to be able to recreate the image from it on the PC side.
Note that the FastIO library from ProBundle is faster than the RandomAccessFile library.

Glad I bought the ProBundle...

Now, why do you say Pixmap.Pixels and not FBO.GetDefaultFrameBufferPixels? They're both of type Byte().

Informatix · Nov 18, 2016

wonder said:
Glad I bought the ProBundle...

Now, why do you say Pixmap.Pixels and not FBO.GetDefaultFrameBufferPixels? They're both of type Byte().

There's not a great difference between them. GetDefaultFrameBufferPixels is a helper function that I added to LibGDX. Here's its code:

B4X:

    public static byte[] GetDefaultFrameBufferPixels (int X, int Y, int Width, int Height, boolean FlipY)
    {
        Gdx.gl.glPixelStorei(GL20.GL_PACK_ALIGNMENT, 1);
        final ByteBuffer pixels = BufferUtils.newByteBuffer(Width * Height * 4);
        Gdx.gl.glReadPixels(X, Y, Width, Height, GL20.GL_RGBA, GL20.GL_UNSIGNED_BYTE, pixels);
        final int numBytes = Width * Height * 4;
        byte[] lines = new byte[numBytes];
        if (FlipY) {
            final int numBytesPerLine = Width * 4;
            for (int i = 0; i < Height; i++) {
                pixels.position((Height - i - 1) * numBytesPerLine);
                pixels.get(lines, i * numBytesPerLine, numBytesPerLine);
            }
        } else {
            pixels.clear();
            pixels.get(lines);
        }
        return lines;
    }

wonder · Nov 18, 2016

Since I'm never going to use FlipY, I could remove it to make it lighter:

B4X:

public static byte[] GetDefaultFrameBufferPixels (int X, int Y, int Width, int Height)
{
    Gdx.gl.glPixelStorei(GL20.GL_PACK_ALIGNMENT, 1);
    final ByteBuffer pixels = BufferUtils.newByteBuffer(Width * Height * 4);
    Gdx.gl.glReadPixels(X, Y, Width, Height, GL20.GL_RGBA, GL20.GL_UNSIGNED_BYTE, pixels);
    final int numBytes = Width * Height * 4;
    byte[] lines = new byte[numBytes];
    pixels.clear();
    pixels.get(lines);
    return lines;
}

But... why return "lines" instead of "pixels"? How is "lines" affected by pixels.get(lines)?

ilan · Nov 18, 2016

Informatix said:
According to your code above, that should be
menushopprices(2).ALIGN_Top+menushopprices(2).ALIGN_Right
That's more obvious than 18.
In fact, Alignment should work exactly like Gravity.

ofcourse, you are rigt @Informatix !!

the SetAlignment2 of an label expect an int
and .ALIGN_Top = 2
and .ALIGN_Right = 16
so menushopprices(3).ALIGN_Top+menushopprices(3).ALIGN_Right = 18

did not thought about that option, thank you, i learned something new today

Android Tutorial Introduction to the libGDX library

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