Android Tutorial Introduction to the libGDX library

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 :
application_lifecycle_diagram.png


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.

.....
 
Last edited:

shashkiranr

Active Member
Licensed User
Hi Fred,

In a live wallpaper application, I am loading a bitmap font using the asset manager but the font is not getting recognized when i check in the log. the font is in the assets folder.

B4X:
assetmanager.Initialize("AM")
    assetmanager.Load("font/lqbm.fnt",assetmanager.TYPE_BitmapFont)
    Log("------")
    Log("Loaded assets:")
    Dim L As List = assetmanager.LoadedAssetNames
    L.Sort(True)
    For i = 0 To L.Size - 1
        Log("- " & L.Get(i) & " (" & assetmanager.GetAssetType(L.Get(i)) & ")")
    Next
    Log("------")

Kindly let me know the mistake i am making here

Regards,
SK
 

Informatix

Expert
Licensed User
Hi Fred,

In a live wallpaper application, I am loading a bitmap font using the asset manager but the font is not getting recognized when i check in the log. the font is in the assets folder.

B4X:
assetmanager.Initialize("AM")
    assetmanager.Load("font/lqbm.fnt",assetmanager.TYPE_BitmapFont)
    Log("------")
    Log("Loaded assets:")
    Dim L As List = assetmanager.LoadedAssetNames
    L.Sort(True)
    For i = 0 To L.Size - 1
        Log("- " & L.Get(i) & " (" & assetmanager.GetAssetType(L.Get(i)) & ")")
    Next
    Log("------")
Bitmap fonts are made of two files: an image (png format) and a text file (your .fnt file). The image is referenced in the text file by the line:
page id=0 file="font.png"
You should check that the image is not missing and that the path and name of the image in the text file lead to the right file.
 

shashkiranr

Active Member
Licensed User
Hi Fred,

I created the bitmap font using libgdx nightly build as you mentioned and copied both the font file and image to the assets folder as given in the asset manger tutorial but it is not loading the font.

Edit : I am calling this in LG_Create in the live wallpaper template
 
Last edited:

Informatix

Expert
Licensed User
Hi Fred,

I created the bitmap font using libgdx nightly build as you mentioned and copied both the font file and image to the assets folder as given in the asset manger tutorial but it is not loading the font.

Edit : I am calling this in LG_Create in the live wallpaper template
Did you check the path to the image in the text file?
 

shashkiranr

Active Member
Licensed User
Hi Fred,

I checked the path as you mentioned in the bitmap font. its given as
B4X:
page id=0 file="lqbm.png"
. I have one question though i am calling this in the wallpaper service. Does it hold good because i know it works in Activity Module.

Edit : the asset manager is picking my font in activity module but not in the wallpaper service module.


Edit 2 : I got it working :) removed the assetsmanager and used the lw.files.internal to access the assets folder.

Regards,
SK
 
Last edited:

shashkiranr

Active Member
Licensed User
Hi Fred,

In my wallpaper application, I have initially loaded the texture with an image "1.png". When the user selects a different image, im using
B4X:
Texture.InitializeWithFile(LW.Files.internal("SelectedImage"))
to load the texture with this new image but im getting a force close.
I even tried to dispose the texture and initialize it again with the new image and got the same result.

How do you clear the old image and assign the texture with the new one.

Kindly let me know your inputs.

Regards,
SK
 

Informatix

Expert
Licensed User
Hi Fred,

I checked the path as you mentioned in the bitmap font. its given as
B4X:
page id=0 file="lqbm.png"
. I have one question though i am calling this in the wallpaper service. Does it hold good because i know it works in Activity Module.

Edit : the asset manager is picking my font in activity module but not in the wallpaper service module.


Edit 2 : I got it working :) removed the assetsmanager and used the lw.files.internal to access the assets folder.

Regards,
SK
I never tried the Asset Manager in a Wallpaper service so thanks for the report.
It's not really an issue as the Asset Manager is slow and should only be used when you need to load a lot of resources in the background; it is not suited for a live wallpaper app where no loading time should occur.
 

Informatix

Expert
Licensed User
Hi Fred,

In my wallpaper application, I have initially loaded the texture with an image "1.png". When the user selects a different image, im using
B4X:
Texture.InitializeWithFile(LW.Files.internal("SelectedImage"))
to load the texture with this new image but im getting a force close.
I even tried to dispose the texture and initialize it again with the new image and got the same result.

How do you clear the old image and assign the texture with the new one.

Kindly let me know your inputs.

Regards,
SK
Do you have this problem only in the LiveWallpaper service?
 

shashkiranr

Active Member
Licensed User
Yes . I am currently working on the live wallpaper. I have not tried texture in activity module. Is there a workaround for it?
 

Informatix

Expert
Licensed User
Yes . I am currently working on the live wallpaper. I have not tried texture in activity module. Is there a workaround for it?
In my LiveWallpaper_Smileys example, I replaced the code that stops/restarts the effect by:
B4X:
If LW.Input.JustTouched Then
      texSmiley.dispose
      texSmiley.Initialize("droid.png")
End If
... and that works fine. You have to find the problem in your code because it's not related to libGDX.
 

Informatix

Expert
Licensed User
Hi Fred,

In my wallpaper application, I have initially loaded the texture with an image "1.png". When the user selects a different image, im using
B4X:
Texture.InitializeWithFile(LW.Files.internal("SelectedImage"))
to load the texture with this new image but im getting a force close.
I even tried to dispose the texture and initialize it again with the new image and got the same result.

How do you clear the old image and assign the texture with the new one.

Kindly let me know your inputs.

Regards,
SK
I tried to load a bitmap font with the asset manager in a live wallpaper service and had no problem (with both methods: finishloading and update). Maybe your font file is corrupted or has a specific problem (could you send it to me?).
 

shashkiranr

Active Member
Licensed User
Hi Fred,

i also used the font in the assets example but it dint work, I have attached the font which i have created using Hiero for your reference.
 

Attachments

  • fonts.zip
    25.9 KB · Views: 208

Carcas

Member
Licensed User
HI,
Forgive the simple question.
I'm used to the colors B4A ... (colors.argb (255,123,23,79))
How can I convert to libgdx (. Color.argb (?,?,?,?)) ?

There is not much documentation on the web :(
 

Informatix

Expert
Licensed User
HI,
Forgive the simple question.
I'm used to the colors B4A ... (colors.argb (255,123,23,79))
How can I convert to libgdx (. Color.argb (?,?,?,?)) ?

There is not much documentation on the web :(
From the tutorial:
"each color value ranges from 0 to 1"

From the help text of the setColorRGBA function (used many times in the examples):
"R Red, from 0 to 1
G Green, from 0 to 1
B Blue, from 0 to 1
A Alpha, from 0 to 1"

And here's the first link returned by Google for "color libGDX": http://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/graphics/Color.html

Not documented indeed....
 

Carcas

Member
Licensed User
I've already read this but maybe I have not explained well

For example, I have this color ARGB (255,84,84,84) --GRAY

I would like the exact same color with float numbers.
I saw toFloatBits but I do not understand how to use it

I dont Know Java
 

Informatix

Expert
Licensed User
I've already read this but maybe I have not explained well

For example, I have this color ARGB (255,84,84,84) --GRAY

I would like the exact same color with float numbers.
I saw toFloatBits but I do not understand how to use it

I dont Know Java
You don't need to know Java, just very basic maths. If the max value for a color is 1 for libGDX, then you have simply to divide your color ranging from 0 to 255 by 255. So 0 -> 0, 128 -> 0.5, 255 -> 1, etc.
 
Last edited:
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