Prerequisite knowledge

Prior experience programming with ActionScript 3 is required. Familiarity developing projects in the Flash Builder workspace is recommended.

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Required products

Flash Builder (Download trial)

Starling is an ActionScript 3 2D framework developed on top of the Stage3D APIs, available for the desktop in Adobe Flash Player 11 and Adobe AIR 3. Starling was designed for game development, but you can use it for many other applications. Starling makes it possible to write fast GPU accelerated applications without having to touch the low-level Stage3D APIs.
Most Flash developers want the ability to leverage GPU acceleration (through Stage3D) without having to write such higher-level frameworks and digging into the low-level Stage3D APIs. Starling is completely based on the Flash Player APIs and abstracts the complexity of Stage3D (Molehill). The result is intuitive programming for everyone.
Starling is designed for ActionScript 3 developers, especially those involved in 2D game development. To use it, you must have a basic understanding of ActionScript 3. Since Starling is lightweight, flexible and easy to use, you can also use it for other project needs, such as UI programming. The framework is designed to be as intuitive as possible, so any Java or .Net developer can begin using it right away.

Overview of Starling

Starling is intuitive and easy to learn. Flash and Flex developers will understand it immediately because it follows most of the ActionScript dogmas and abstracts the complexity of the low-level Stage3D APIs. Rather than coding against concepts like vertices buffer, perspective matrices, shader programs, and assembly bytecode, Starling uses familiar concepts such as a DOM display list, an event model, and familiar APIs like MovieClip, Sprite, TextField, and others.
Starling is a lightweight bird in many ways. The number of classes is limited (around 80 KB of code). There are no external dependencies besides Flash Player 11 or AIR 3 (and mobile support is slated for a future release). These factors keep your applications small and your workflow simple.
Starling is free and alive. It is licensed under the Simplified BSD license, so you can use it freely even in commercial applications. We are working on it every day and we count on an active community to improve it even more.
Behind the scenes, Starling uses the Stage3D APIs—which are low-level GPU APIs running on top of OpenGL and DirectX on desktop and OpenGL ES2 on mobile devices. It's important to note that Starling is the ActionScript 3 port of Sparrow, the equivalent library for iOS relying on OpenGL ES2 APIs (see Figure 1):
Stage3D (Molehill) is layered on top of Starling.

Figure 1. Stage3D (Molehill) is layered on top of Starling.

Starling recreates many APIs that are familiar to Flash developers. The graph below illustrates the graphic element APIs exposed by Starling (see Figure 2):
Starling supports DisplayObject inheritance.

Figure 2. Starling supports DisplayObject inheritance.


It's odd to consider that 2D content can be created on top of 3D GPU APIs. When it comes to Stage3D APIs, many people think that those APIs are strictly limited to 3D content. The name causes confusion: If it is called Stage3D, how can you use it to create 2D elements? Figure 3 illustrates the question about the ability to draw a MovieClip with the drawTriangles API.
Is it possible to use the drawTriangles API to create a 2D movie clip?

Figure 3. Is it possible to use the drawTriangles API to create a 2D movie clip?


GPU are efficient and can draw triangles quickly. Using the drawTriangles API, you can draw two triangles, and then sample a texture and apply it to the triangles using UV mapping. This creates a textured quad, which represents a sprite. By updating the textures on the triangles on each frame, the end result is a MovieClip.
The good news is that it is not necessary to work through those details to use Starling. You simply provide the frames, supply them to a Starling MovieClip and that's all it takes (see Figure 4).
Using the drawTriangles API and a textured quad, you can create a 2D graphic.

Figure 4. Using the drawTriangles API and a textured quad, you can create a 2D graphic.


To better understand how Starling reduces the complexity, examine the code you would have to write to display a simple textured quad using the low-level Stage3D APIs:
// create the vertices var vertices:Vector.<Number> = Vector.<Number>([ -0.5,-0.5,0, 0, 0, // x, y, z, u, v -0.5, 0.5, 0, 0, 1, 0.5, 0.5, 0, 1, 1, 0.5, -0.5, 0, 1, 0]); // create the buffer to upload the vertices var vertexbuffer:VertexBuffer3D = context3D.createVertexBuffer(4, 5); // upload the vertices vertexbuffer.uploadFromVector(vertices, 0, 4); // create the buffer to upload the indices var indexbuffer:IndexBuffer3D = context3D.createIndexBuffer(6); // upload the indices indexbuffer.uploadFromVector (Vector.<uint>([0, 1, 2, 2, 3, 0]), 0, 6); // create the bitmap texture var bitmap:Bitmap = new TextureBitmap(); // create the texture bitmap to upload the bitmap var texture:Texture = context3D.createTexture(bitmap.bitmapData.width, bitmap.bitmapData.height, Context3DTextureFormat.BGRA, false); // upload the bitmap texture.uploadFromBitmapData(bitmap.bitmapData); // create the mini assembler var vertexShaderAssembler : AGALMiniAssembler = new AGALMiniAssembler(); // assemble the vertex shader vertexShaderAssembler.assemble( Context3DProgramType.VERTEX, "m44 op, va0, vc0\n" + // pos to clipspace "mov v0, va1" // copy uv ); // assemble the fragment shader fragmentShaderAssembler.assemble( Context3DProgramType.FRAGMENT, "tex ft1, v0, fs0 <2d,linear, nomip>;\n" + "mov oc, ft1" ); // create the shader program var program:Program3D = context3D.createProgram(); // upload the vertex and fragment shaders program.upload( vertexShaderAssembler.agalcode, fragmentShaderAssembler.agalcode); // clear the buffer context3D.clear ( 1, 1, 1, 1 ); // set the vertex buffer context3D.setVertexBufferAt(0, vertexbuffer, 0, Context3DVertexBufferFormat.FLOAT_3); context3D.setVertexBufferAt(1, vertexbuffer, 3, Context3DVertexBufferFormat.FLOAT_2); // set the texture context3D.setTextureAt( 0, texture ); // set the shaders program context3D.setProgram( program ); // create a 3D matrix var m:Matrix3D = new Matrix3D(); // apply rotation to the matrix to rotate vertices along the Z axis m.appendRotation(getTimer()/50, Vector3D.Z_AXIS); // set the program constants (matrix here) context3D.setProgramConstantsFromMatrix(Co ntext3DProgramType.VERTEX, 0, m, true); // draw the triangles context3D.drawTriangles( indexBuffer); // present the pixels to the screen context3D.present();
The code sample above creates a square 2D instance (see Figure 5).
The result of using the drawTriangles API and a textured quad to create a 2D object.

Figure 5. The result of using the drawTriangles API and a textured quad to create a 2D object.


The code sample shown above is admittedly pretty complex. That is the cost of having access to low-level APIs. On the plus side, you can control many aspects, but it takes a considerable amount of code to set everything up.
With Starling, you can write the following code instead:
// create a Texture object out of an embedded bitmap var texture:Texture = Texture.fromBitmap ( new embeddedBitmap() ); // create an Image object our of the Texture var image:Image = new Image(texture); // set the properties quad.pivotX = 50; quad.pivotY = 50; quad.x = 300; quad.y = 150; quad.rotation = Math.PI/4; // display it addChild(quad);
As an ActionScript 3 developer, familiar with using the Flash APIs, you can begin working with these APIs exposed immediately, while all the complexity of the Stage3D APIs is handled behind the scenes.
If you experiment using the redraw regions feature, Starling renders everything on Stage3D, not the classic display list, as expected. The screenshot below illustrates this behavior. The quad rotates on each frame, and the redraw regions only show the FPS counter, not the Stage3D content (see Figure 6).
Example of content rendered using Stage3D.

Figure 6. Example of content rendered using Stage3D.


Remember that with the Stage 3D architecture, the content is completely rendered and composited by the GPU. As a result, the redraw regions feature used for the display list running on the CPU cannot be used.

Layering restrictions

When you work with Starling (and Stage 3D), remember that there's one limitation when developing content. As mentioned earlier, Stage3D is literally a new rendering architecture inside Flash Player. A GPU surface is placed under the display list, meaning that any content running inside the display list will be placed above Stage3D content. At the time of this writing, content running in the display list cannot be placed under the Stage3D layer (see Figure 7).
The stacking order of content rendered with Stage3D.

Figure 7. The stacking order of content rendered with Stage3D.


Also notice that the Stage3D object cannot be transparent. If this was possible, you could play video using the Stage Video technology (introduced in Flash Player 10.2) and overlay the video with content rendered through Stage3D. Hopefully this feature will be enabled in a future release of Flash Player.

Setting up a project

Visit the official Github page to download Starling. You may also find it helpful to visit the Starling website
Starling is licensed under the simplified BSD license, so you can use Starling in any type of commercial or non-commercial project. You can contact the Starling framework team if you need more information.
After you download Starling, you can reference the Starling library just like any other AS3 library. In order to use the new version of Flash Player 11 beta, you must target SWF version 13 by passing in an extra compiler argument to the Flex compiler: -swf-version=13. Follow these steps if you are using the Adobe Flex SDK:
  1. Download the new playerglobal.swc for Flash Player 11.
  2. Download the Flex 4.5 SDK ( from the Flex 4.5 SDK table.
  3. Install the build in your development environment.
  4. In Flash Builder, create a new ActionScript project by choosing File > New > ActionScript project.
  5. Open the Property inspector (right-click and chose the Properties option). In the list on the left, select ActionScript Compiler.
  6. Use the Configure Flex SDK's option in the upper right corner to point the project to Flex build 20967. Click OK.
  7. Configure your project to target SWF version 13.
  8. Open the Property inspector and select ActionScript Compiler from the list on the left.
  9. Add to the "Additional compiler arguments" input: -swf-version=13. This ensures the outputted SWF targets SWF version 13. If you compile on the command-line and not in Flash Builder, you must add the same compiler argument.
  10. Verify that you have installed the new Flash Player 11 build in your browser.

Setting up a scene

Now that you've prepared your development environment, you are ready to dig into the code and see what you can do with the framework. Working with Starling is very easy; simply create a Starling object and add it to your main class. In this article, when referring to objects like MovieClip, Sprite, and others, I am referring to the Starling APIs—not the native objects from Flash Player.
First, the Starling constructor expects multiple arguments. Here is the signature:
public function Starling(rootClass:Class, stage:flash.display.Stage, viewPort:Rectangle=null, stage3D:Stage3D=null, renderMode:String="auto")
Actually, only the first three are really used commonly. The rootClass argument expects a reference to a class extending starling.display.Sprite and as a second argument, our stage, then a Stage3D object:
package { import flash.display.Sprite; import flash.display.StageAlign; import flash.display.StageScaleMode; import starling.core.Starling; [SWF(width="1280", height="752", frameRate="60", backgroundColor="#002143")] public class Startup extends Sprite { private var mStarling:Starling; public function Startup() { // stats class for fps addChild ( new Stats() ); stage.align = StageAlign.TOP_LEFT; stage.scaleMode = StageScaleMode.NO_SCALE; // create our Starling instance mStarling = new Starling(Game, stage); // set anti-aliasing (higher is better quality but slower performance) mStarling.antiAliasing = 1; // start it! mStarling.start(); } } }
Below, the Game class creates a simple quad when added to the Stage:
package { import starling.display.Quad; import starling.display.Sprite; import; public class Game extends Sprite { private var q:Quad; public function Game() { addEventListener(Event.ADDED_TO_STAGE, onAdded); } private function onAdded ( e:Event ):void { q = new Quad(200, 200); q.setVertexColor(0, 0x000000); q.setVertexColor(1, 0xAA0000); q.setVertexColor(2, 0x00FF00); q.setVertexColor(3, 0x0000FF); addChild ( q ); } } }
The code above adds a listener to the Event.ADDED_TO_STAGE event and initializes the application in the event handler. That way, you can safely access the Stage.
Note: Pay attention to this subtle detail: The Game class shown above extends the Sprite class from the starling.display package, not the flash.display.package. Always check your import statements and make sure you are not using the native API instead of the Starling APIs.
Just as expected in Flash, objects in Starling have a default position of 0,0. So add a few lines to center the quad on the Stage:
q.x = stage.stageWidth - q.width >> 1; q.y = stage.stageHeight - q.height >> 1;
Test the project so far to see the result (see Figure 8).
The quad is centered on the Stage.

Figure 8. The quad is centered on the Stage.


Note that the anti-aliasing value allows you to set the type of anti-aliasing desired. Generally speaking, a value of 1 is totally acceptable but you can go further. The framework supports anti-aliasing values from 0 to 16, but the list below reflects the most common values:
  • 0: No anti-aliasing.
  • 2: Minimal anti-aliasing.
  • 4: High quality anti-aliasing.
  • 16: Very high quality anti-aliasing.
You'll rarely need to use a setting above 2, especially for 2D content. However, you'll need to decide on a case by case basis, depending on your projects. In Figure 9, compare the two screenshots to see the slight difference between two anti-aliasing values (1 and 4).
Compare the visual difference between applying an anti-aliasing value of 1 (left) and 4 (right).

Figure 9. Compare the visual difference between applying an anti-aliasing value of 1 (left) and 4 (right).


Experiment with applying values above 2 to set the quality needed for your project. Of course, choosing a high value impacts the performance. Note that Stage3D is not affected by the Stage quality of the SWF file.
Here are descriptions of the other APIs available for use with the Starling object:
  • enableErrorChecking: Allows you to enable or disable error checking. Specifies whether errors encountered by the renderer are reported to the application. When enableErrorChecking is set to true, the clear() and drawTriangles() methods called internally by Starling are synchronous and can throw errors. When enableErrorChecking is set to false, the default, the clear(), and drawTriangles() methods are asynchronous and errors are not reported. Enabling error checking reduces rendering performance. Only enable error checking when debugging a project and disable it before deploying the final version.
  • isStarted:Indicates if start was called.
  • juggler: A juggler is a simple object. It simply saves a list of objects implementing IAnimatable and advances their time if told to do so (by calling its own advanceTime: method). When an animation is completed, it throws it away.
  • start: Starts the rendering and event handling.
  • stop: Stops the rendering and event handling. Use this method to pause the rendering when the game goes into the background to save resources.
  • dispose: Call this method when you want to dispose the entire content currently being rendered on the GPU memory. This API internally disposes everything (shader programs, textures, and everything else).
Once your Starling object has been created, a debug trace is outputted automatically, displaying information about the renderer. By default, when the SWF file is correctly embedded in the page or when testing in standalone Flash Player, Starling will output the following:
[Starling] Initialization complete. [Starling] Display Driver:OpenGL Vendor=NVIDIA Corporation Version=2.1 NVIDIA-7.2.9 Renderer=NVIDIA GeForce GT 330M OpenGL Engine GLSL=1.20 (Direct blitting)
Of course, the specific hardware details will vary, depending on your configuration. The message above indicates the use of GPU acceleration because it includes the details about the drivers version. For debugging purposes, you may want to force the software fallback used internally by the Flash Player to learn how your content will perform when running on software.
Add the following code to inform Starling that you want to use the software fallback (software rasterizer):
mStarling = new Starling(Game, stage, null, null, Context3DRenderMode.SOFTWARE);
When you are using software, the output message confirms that you are running in software mode:
[Starling] Initialization complete. [Starling] Display Driver:Software (Direct blitting)
Be sure to test your content in software mode too, to better understand its performance when running in this mode. Your content may fallback to software if the user's configuration is using old drivers (for consistency reasons, all drivers older than 2009 are blacklisted).
In the next section, you'll take a look at the requirements for Stage3D when embedding your SWF file in a page.

Wmode requirements

You have to remember that in order to enable Stage 3D and GPU acceleration, you have to use wmode=direct as the embed mode in the page. If you do not specify any value or choose an other value than "direct"—like "transparent", "opaque", or "window"—Stage 3D will not be available. Instead, you will get a runtime exception informing you that the creation of the Context3D object failed when requestContext3D on Stage3D is called.
Figure 10 illustrates the runtime exception dialog box.
Runtime exception, when Context3D is not available.

Figure 10. Runtime exception, when Context3D is not available.


It is important to handle this situation if your application is embedded using the wrong wmode. You need to react appropriately by displaying a message explaining the issue. Fortunately, Starling handles this automatically for you and display the message in Figure 11.
Warning message when the application is not correctly embedded.

Figure 11. Warning message when the application is not correctly embedded.


Stage quality

As a Flash developer, the concept of stage quality is not new to you. Remember that when working with Stage3D, and as a result Starling, the stage quality has no impact on the performance.

Progressive enhancements

When GPU acceleration cannot be leveraged, Stage3D will fallback to software and use internally a software fallback engine, called SwiftShader (Transgaming). To make sure that your content runs well in such a scenario, you need to detect when you are running in software and remove potential effects that would be slow in software.
In the context of 2D content, software fallback should be able to handle many objects and provide good performance, but still, to detect this, you can access the Context3D object from the Starling object by using the static property context:
// are we running hardware or software? var isHW:Boolean = Starling.context.driverInfo.toLowerCase().indexOf("software") == -1;
It is a good practice to always design your content with software fallback in mind, it will offer a progressive experience, ensuring the best experience whatever the scenario.

Where to go from here

Hopefully this article has inspired you to experiment working with 2D objects using the Stage3D APIs. To learn more about Starling, check out the following online resources: