VolatileBufferedToolkitImage Strategies

   
By Chet Haase, Senior Staff Engineer, Java 2D, February 15, 2005  

A common question seems to arise often from Java graphics developers about which image type or creation method to use. When exactly should you use VolatileImage? What is BufferedImage appropriate for? What about the old Toolkit images? And when is BufferStrategy more appropriate than one of these image types?

It's a pretty big topic, and the answer (like all truly great answers) is probably "It depends". But there are some general guidelines that can come in handy.

Image Types

First of all, perhaps a short dictionary of image types might help:

  • Toolkit Image: This is the oldest kind of image object in the Java API. These images are created and returned by the old 1.x APIs such as Applet.getImage() and Toolkit.createImage(). These images are created from a pointer to a data source, such as a GIF or JPG file, and return an object of type Image. They are useful for convenient loading and storage of image data for display, but getting at the actual pixel data or manipulating it is not as easy.
  • BufferedImage: This is an image type that was created in the JDK 1.2 API. They were created for easier and more powerful manipulation of the actual pixel data in an image. At first, there was no way to load an image from a data source directly into a BufferedImage; these images were used, instead, to create an arbitrary buffer for pixel data, which you could then write to, read from, or display conveniently. The main way to get actual pixel data into a BufferedImage object at first was through use of rendering operations (after getting a Graphics object for the BufferedImage), or by manually setting the pixel data through methods in BufferedImage, WritableRaster, and DataBuffer. With the advent of the ImageIO API (see below) in JDK 1.4, it became possible to create a BufferedImage object directly from a data source, just like Toolkit images (only these BufferedImage objects are writable, unlike their Toolkit image cousins).
  • VolatileImage: This image type was created in JDK 1.4 as a means of creating and managing accelerated image memory. One of the problems with hardware acceleration for images is that, on some platforms, accelerated memory can be deleted out from under you at any time. This is obviously not what you want for your typical image data. To work around that, the VolatileImage API was created to provide a notification mechanism so that you know when an image must be re-rendered due to data loss. VolatileImage objects are not loaded from image data, but are just created as empty pixel buffers (much as the initial BufferedImage objects were (see above)); to load image data into a VolatileImage, applications must load the image data through some non-Volatile means, get the Graphics object for the VolatileImage, and then copy the data into the Graphics object using drawImage().
  • Managed Images: These image objects are not specific objects or APIs in Java, but are rather a concept of how we accelerate image operations. A "managed image" is one that you create through any of the normal image creation/loading methods and which we try to accelerate for you internally, by creating an accelerated mirror copy of the image data. This type of image can benefit from hardware acceleration without falling prey to the "surface loss" issues mentioned above for VolatileImage. I'll talk more about managed images and hardware acceleration later in the article.

That's it for the basic image types. Now let's talk about how we actually create and use these image objects.

Who You Gonna Call?

Whenever I want to give myself a fright about the complexity of our APIs, I simply ponder the vast array of choices that face developers who simply want to create an image. I'm sure I'm missing some here, but let's see...

Applet:
        getImage(url)
        getImage(url, name)

BufferedImage:
        new BufferedImage(colorModel, raster, premultiplied, properties)
        new BufferedImage(width, height, type)
        new BufferedImage(width, height, type, colorModel)

BufferStrategy:
        new BufferStrategy()
        (Note: This method cannot be called directly)

Canvas:
        createBufferStrategy(numBuffers)
        createBufferStrategy(numBuffers, capabilities)

Component:
        createImage(imageProducer)
        createImage(width, height)
        createVolatileImage(width, height)
        createVolatileImage(width, height, capabilities)

GraphicsConfiguration:
        createCompatibleImage(width, height)
        createCompatibleImage(width, height, transparency)
        createCompatibleVolatileImage(width, height)
        createCompatibleVolatileImage(width, height, capabilities)

Image:
        new Image()
        (Note: This method cannot be called directly)

ImageIO:
        read(file)
        read(imageInputStream)
        read(inputStream)
        read(url)

ImageIcon:
        new ImageIcon(imageData[]).getImage()
        new ImageIcon(imageData[], description).getImage()
        new ImageIcon(image).getImage()
        new ImageIcon(image, description).getImage()
        new ImageIcon(filename).getImage()
        new ImageIcon(filename, description).getImage()
        new ImageIcon(url).getImage()
        new ImageIcon(url, description).getImage()

Toolkit:
        createImage(imageData[])
        createImage(imageData[], offset, length)
        createImage(producer)
        createImage(filename)
        createImage(url)
        getImage(filename)
        getImage(url)

Window:
        createBufferStrategy(numBuffers)
        createBufferStrategy(numBuffers, capabilities)

I am sure there are more out there, especially using approaches like the ImageIO package, which is all about reading and writing images, as you might guess from the name. But this list will do for now.

So it's a wrap. This article's pretty much finished; just use the above API calls to create your images. Left as an exercise to the reader. Q.E.D. It's obvious, isn't it?

Okay, so maybe it isn't obvious; there are a lot of methods above that all seem to need different parameters or that create different types of images.

Here's the trick: All of the above image creation methods (and any others that are not on the list) can be broken down into just a few categories. Then the plethora of ways of creating an image in one of those categories can just be seen as utility methods; different ways of getting the same result. The convenience methods may be because of logic (why do I have to get the GraphicsConfig to create an image associated with a Component? Why not use the Component directly?), or convenience (instead of using some InputStream mechanism for all image readers, we provide several ways to read the image directly including from filenames, URLs, and streams; just call the method appropriate for your situation).

So the real work in this article is to break down the categories of image types and describe which types of images and methods you may want to use in which situations. Once you get that down, the rest, as they say, is just implementation details.

Image Loading or Creation?

First of all, are you loading existing image data? Or are you creating an image buffer in memory? Image loading means that you have image data (either locally or across the network) that you want to load into your application, possibly to copy that image onto the screen or to read and operate on the data. Image creation means that you want some arbitrary image memory created for your application; perhaps you want to create a buffer for double-buffered animations, or you want a place to cache intermediate filtering results.

Image Loading

In the above method list, all of the methods that take filenames, URLs, streams, producers, and data arrays are those intended for loading existing images. In particular, all of the methods listed above for Applet, ImageIO, ImageIcon, and Toolkit are intended for image loading:

Applet:
        getImage(url)
        getImage(url, name)

ImageIO:
        read(file)
        read(imageInputStream)
        read(inputStream)
        read(url)

ImageIcon:
        new ImageIcon(imageData[]).getImage()
        new ImageIcon(imageData[], description).getImage()
        new ImageIcon(filename).getImage()
        new ImageIcon(filename, description).getImage()
        new ImageIcon(url).getImage()
        new ImageIcon(url, description).getImage()

Toolkit:
        createImage(imageData[])
        createImage(imageData[], offset, length)
        createImage(producer)
        createImage(filename)
        createImage(url)
        getImage(filename)
        getImage(url)

There are at least four major things that differentiate these methods:

  • Location of the image data
  • Format of the image data
  • Synchronous or asynchronous behavior of each method
  • Type of image that is created ( Image or BufferedImage)
Location, Location, Location

When I'm talking about location, I'm mainly concerned with whether the file is local or across a network. Also, if it is packed into some resource file, such as a jar file, that also comes into play here.

Loading Across the Network

If you are accessing the data across a network, it is probably easiest to use the URL variations:

        Applet.getImage(url)
        Applet.getImage(url, name)
        ImageIO.read(url)
        new ImageIcon(url).getImage()
        new ImageIcon(url, description).getImage()
        Toolkit.createImage(url)
        Toolkit.getImage(url)

For example, let's say you just have to have a copy of Duke in your application (barring the legal ramifications of shipping someone else's image in your application, of course. I'm no lawyer, but I've seen enough of them on TV to be very, very afraid). You could, of course, copy that image locally and read it from a file. But what if that particular Duke image is modified constantly, preened and pruned to use the very latest imaging technologies and updated to the latest clothing fashions (assuming Duke actually wore clothes, but since he's a California native (I suspect Santa Cruz) he is apparently very comfortable in the buff). Suppose there is always an up-to-date duke.gif file on the Java Sun site. Then you could use something like the following code to load this image into your application:

        URL dukeURL = new URL("#");
        BufferedImage img = ImageIO.read(dukeURL);

Loading from the Local File System

Now suppose you have another image that you have saved locally in a file; just use the filename variation of the above. For example, let us say you loved one particular instantiation of the lovable-yet-quirky duke.gif file above so much that you downloaded and saved it for use in your application (see the above note on scary lawyers). Then you could use the following code to load that file from the directory where the program was launched:

        BufferedImage img = ImageIO.read(new File("duke.gif"));

Loading from JAR Files

It is perhaps more common to bundle up your application and media into JAR files. In this case, the image will be downloaded with your application, but will not be accessible through a simple filename. You will probably want to use a URL in this case also, where you create a URL from the JAR file resource. For example, suppose you took your downloaded version of duke.gif and put him in a subdirectory of your application hierarchy called "media". Then you could load the image with the following code:

        URL dukeURL = getClass().getResource("media/" + "duke.gif");
        BufferedImage img = ImageIO.read(dukeURL);

Image Format

Another consideration is the format of your stored image. The old Toolkit/Applet loaders only understand GIF, JPEG, and PNG format files. (Okay, they also understand XBM and XPM2, old X11 image formats, but those are probably not formats you are terribly concerned about). These loaders works well for most web applications since these image types are traditional web image formats. But what if you have an image in some other format that the Toolkit/Applet loaders do not understand?

ImageIO currently has built-in readers for GIF, JPEG, PNG, BMP, and WBMP. Moreover, there will be more image readers/writers for ImageIO going forward, whereas there are no specific plans to support more formats for the old Toolkit/Applet loaders. And finally, ImageIO has a pluggable reader API, so if you have a custom image format, or some other format not yet supported by the core library, you can write your own loader for that format within ImageIO.

In fact, the JAI team has made available a package with additional ImageIO readers/writers, if you have requirements beyond the current ImageIO defaults.

So ImageIO could also be the right choice if you need to deal with formats beyond the basic web image formats.

Synchronicity

The Applet and Toolkit image loading methods came from the old days of Java 1.0, when Java was seen primarily as a networked application API and image data might come from any source, potentially one on an unreliable or slow network connection. To make networked applications more robust, it is reasonable to put network-dependent operations in separate threads to ensure that an application's main or GUI threads do not hang while waiting for a slow download. Because this was a common pattern for Java GUI applications at that time, the image loading operations were all created to run on a separate image loading thread. Thus when an application calls:

        Applet.getImage(url)

that call will return immediately. That doesn't mean that the image has been loaded; in fact, in most cases the image load may not have even started yet. Internally, getImage() does not load the image at all. Instead, image loading is deferred until some operation requires the image data, at which time a separate Thread processes the image loading.

Note that this model of asynchronous loading does not apply solely to networked applications, or even to image loading specifically; any operation that takes a significant amount of time should not be done on the GUI thread, lest you run the chance of making your application appear hung while the operation is taking place. So, for example, if you are loading in a huge image from a local file, you may want that non-networked operation to happen in a separate worker thread to ensure that your GUI has no pauses during image loading.

This model works well enough for applications that create their images early for later use. The application may simply need to check whether the image has been loaded whenever it is required in the application.

When applications do need the data (for example, if they need image sizes in order to determine layout correctly, or if they need to display images in their final form), they may need to synchronize on the image loader and wait until the image loading is done. For example, an application may want to load local image data and be willing to wait for that data to load before proceeding (knowing that a local load will usually not take very long). In that case, the application might do something similar to the following:

        public Image loadImageSynchronously(Component comp, String filename) {
          URL url = getClass().getResource("media/" + filename);
          Image image = Toolkit.getDefaultToolkit().getImage(url);
          MediaTracker tracker = new MediaTracker(comp);
          int id = 0;
          tracker.addImage(image, id);
          try {
                tracker.waitForID(id);
          } catch (InterruptedException e) {
                System.out.println("INTERRUPTED while loading Image");
          }
          return image;
        }

Note that you can use ImageIcon to load the image synchronously; you could rewrite the above method as follows:

        public Image loadImageSynchronously(String filename) {
          URL url = getClass().getResource("media/" + filename);
          Image image = new ImageIcon(url).getImage();
          return image;
        }

ImageIcon is simply a wrapper around this functionality (in fact, I stole part of the above sample code directly from the ImageIcon implementation: don't tell the Swing team!). But if your intention is to load an image synchronously as quickly as possible, why cause us to go through the process of spawning a separate thread and then synchronizing on that thread?

Meanwhile, ImageIO has synchronous loading methods that do not return until the image has been loaded and is ready to go. There are no extra threads spawned, no MediaTracker usage, no existential "it is through yet?" wonderings; just call one of the ImageIO loading methods and use the resulting BufferedImage object when it returns.

Note that some applications and situations may need asynchronous loading behavior (for long image loads or to more efficiently multitask). For example, it does not take a huge amount of time to affect perceived GUI performance, so if an image load will take even as long as a tenth of a second, you may want to avoid loading that image synchronously on the Event Dispatch Thread (so don't load it in your paint() method). You can always spawn a new Thread yourself to call the ImageIO loading methods if necessary.

Resulting Java Image Type

Part of the decision over which image creation API you use is in which image type you want to get back from the creation method. In particular, do you want a Toolkit Image or a BufferedImage?

Toolkit Images are created by the Applet, Toolkit, and ImageIcon methods listed above. The resulting images are easy to use for display purposes (just call drawImage(...) from this image into a Graphics destination and the image will be copied appropriately), but they lack the power of BufferedImage objects for manipulation of the data.

BufferedImage objects are created by the ImageIO methods listed above. These objects offer a more powerful API, albeit with potentially more work involved to do some operations (such as displaying an animating GIF image).

Image or BufferedImage: What's in a Name?

Although both Image and BufferedImage have similar properties in terms of being displayable, BufferedImage has many more capabilities. For one thing, the Image objects created by the Toolkit, Applet, and ImageIcon load methods are read-only; you cannot get the Graphics of those Images and render to them. So if you want to modify the image data, you will need to do more work (such as creating another image that is modifiable and copying the loaded Image into that new image). Image has some very simple methods and is mostly intended to be a simple object that holds image data. But BufferedImage has many methods for modifying and extracting all kinds of data from an image; color models, pixel data, and more. Given a choice between the two, I would always opt for the one that gave me more power and flexibility.

But doesn't that increased capability mean increased overhead? Not at all; there is no extra processing involved in BufferedImages when these other powerful methods are not used. If all you do is load and image and display it, BufferedImage can do this just as easily as the more streamlined Image object.

So go ahead and use BufferedImage. It is, after all, better than butter.

Dirty Laundry

One good (and not entirely obvious) reason for using the ImageIO API for loading images is the unfortunate reality that the code is simply newer, cleaner, and more maintained (both now and in the future). Much of the old Applet and Toolkit image code was written years ago and has many assumptions and situations that it must account for and is therefore tricky to maintain and upgrade.

Our future image reading/writing direction is with ImageIO; yours should be too, because that is where the focus of our efforts will be in the future.

Having said all that wonderful stuff about ImageIO, there could be situations in which the old Toolkit/Applet/ImageIcon approach makes more sense for your particular application, including:

  • Performance: Ideally, ImageIO would be more performant than the old code. But some code paths in ImageIO have not yet been optimized to the extent that the old code was, so you may find some situations (such as loading images in a particular format) which perform better using the old APIs than they do with ImageIO. The problems are (mostly) known, the bugs have (mostly) been filed, and these performance gaps will (definitely) be fixed. But if performance in these situations is important for your application, by all means use the old methods in the meantime if they work better for you.
  • Ease of Use: One of the goals of ImageIO was to expose more capabilities to the developer. There is much information, such as metadata, in an image that is not exposed through the old image loading methods or the old Image class. The flipside of this approach (at least in the current API) is that it can be more work to do trivial tasks. A good example of this is loading and displaying an animated GIF.
    In the old image loading APIs, if you loaded an animated GIF file, you did not need to worry about the details of animating that image. Instead, you could just load the image and display it and we would automatically schedule the animations of that image appropriately. You could not access the information about the animation (how many frames? what animation rate? loop? halt?), but making it animate was quite simple.
    ImageIO, on the other hand, exposes all of that animation information to you through the API, but you must actually use that information to manage the image animation yourself; you cannot just tell us to draw the image and have us manage it for you. This is a known issue and we will do something about it Real Soon Now.
  • Compatibility: Sometimes applications need to be as backward-compatible as possible across most or all JDK releases. In particular, some applications need to be able to run on old JDK 1.1 or earlier releases. For example, applet games tend to need ubiquity of the runtime environment and cannot count on browsers having any later version of Java than the 1.1 APIs. In this case, you may need to use the older APIs simply so that they will run across all of these platforms.

Certainly, if you need the kind of functionality that the old APIs give you, go ahead and use those APIs. That is what they're (still) there for...

Note also that if you need to use the old APIs for some reason but you still want the power and flexibility of BufferedImage, it is easy enough to load the images in through whatever methods are appropriate, create a new BufferedImage object, and then simply copy the loaded images into the BufferedImage. For example:

        Image img = new ImageIcon(filename).getImage();
        BufferedImage bImg = new BufferedImage(img.getWidth(null), 
        img.getHeight(null).
        BufferedImage.TYPE_INT_RGB);
        Graphics g = bImg.getGraphics();
        g.drawImage(img, 0, 0, null);

Hey! What About the Other Loading Methods Above?

The approaches above cover most of the loading methods I listed, but some are notably skipped. The *Stream methods of ImageIO are simply variations on a theme; if you happen to have your data in that format (versus a URL or file), go for it; it's just a convenience to use these alternatives.

As for the other skipped methods (one using an ImageProducer and some using data arrays), I hoped you wouldn't notice....
These other loading approaches are somewhat dated and come from the old days of Java image processing. There are still some situations which might require the ImageProducer/ImageConsumer approach, but in general it should be easier, more straightforward, and more performant to use the newer APIs such as ImageIO and BufferedImage. The uses and misuses of the older Consumer/Producer/PixelGrabber APIs could stand a separate article all by themselves. I'll try to tackle this one in the future.

As far as reading the image data from an array of data (see the methods above with the imageData[] parameter), this is really only appropriate if you have already read the data into the array to begin with. This could be necessary if you have some custom image storage mechanism, such as a database. But if the image exists in a regular file/URL/stream format, you should probably be using one of the other loading methods instead.

Image Creation

What if you do not have an existing image on the network or file system? What if you just want a buffer of pixel data that you can use in your application? This could be for creating sprites or icons with rendering calls instead of loaded image information (perhaps you've found this to be faster in your situation than reading image files). Or it could be a buffer that you can use for caching intermediate results or for providing double-buffered rendering for an animation.

For the purposes of this discussion, I'll break down this category of images into three types:

  • Static Images: These are images that you render to infrequently (perhaps just once, when you create the image). The images are mainly used to copy from.
  • Dynamic Images: These are images that you render to often, like an animating sprite.
  • Back Buffers: These are like dynamic images, in that they are rendered to frequently (usually at least once per frame), but they are specifically intended to provide buffering support for the onscreen window, and are thus usually copied from often as well, like once per frame.
Static Images

Static images are ones that are created and rendered to once (or infrequently) but probably copied from often. Examples of this type of image include icons for a GUI or sprites for a game.

The best approach for this type of image is to create an image that is in the same format as the image or window that the image will be copied to; this ensures the most straightforward copy mechanism since the underlying software will not have to perform a conversion on the image data while copying to the destination.

You could, of course, create a BufferedImage object manually through one of its constructors; you could query the GraphicsDevice for its display information and then create a BufferedImage of the appropriate type:

        new BufferedImage(colorModel, raster, premultiplied, properties)
        new BufferedImage(width, height, type)
        new BufferedImage(width, height, type, colorModel)

But why go to the hassle of all of that when there are convenience mechanisms that do all of this for you? Specifically, take a look at:

        Component.createImage(width, height)
        GraphicsConfiguration.createCompatibleImage(width, height)
        GraphicsConfiguration.createCompatibleImage(width, height, transparency)

These methods examine the display resolution for the given Component or GraphicsConfiguration and create an image of an appropriate type. The Component variation is specified to return an object of type Image, but currently it is actually a BufferedImage internally. If you need a BufferedImage object instead (you won't necessarily need one; you can get a Graphics object from an Image and render to it just the same as you can to a BufferedImage), then you can perform an instanceof and cast to a BufferedImage in your code, or simply call the GraphicsConfiguration variation).

The best part about static images is that you can use very simple means to create the images and then we will try very hard internally to see that you get any available hardware acceleration for these images when they get copied around. We call these "managed images", because we manage the acceleration details for you. For more information on managed images, please see my blog on BufferedImage performance .

Note that as of JDK 5.0 we manage nearly all types of images and take advantage of hardware acceleration if it exists. So go ahead and create the type of image that is most convenient for you and we will try to do the right thing under the hood.

Dynamic Images

This kind of image may be rendered to quite often, as in an animating icon, or a sprite that is modified on a frequent basis. You could certainly use the same image-creation APIs listed above for static images; these will work fine in most situations and are certainly the easiest way to go in general.

However, some developers interested in maximizing performance may want to know more about image management and how dynamic images can affect it.

We manage images by detecting when the application is copying from an image to a destination (either another image or an onscreen window) that lives in accelerated memory. If this copy is done successively when the source image has not changed, then we may decide to cache a copy of that image in accelerated memory and perform future copies from this cached version. For an illustration of how managed images work under the hood, see Figure 1

 
Figure 1: Managed images
Figure 1: Managed images
Click to enlarge

In the case of a dynamic image, if that image is being updated one or more times for every copy to the destination, then we will never create an accelerated version of it, and thus the image will never benefit from any hardware acceleration that we could otherwise provide.

( Aside: For the insatiably curious, the reason for this oddity in acceleration comes from "surface loss", where an accelerated version of an image may simply go away at any time due to operating-system or user-caused situations. To keep the original image data intact, we store the main image data (that which is modified by the application) in an unaccelerated location, and only accelerate a mirror copy of that image. That way, if the accelerated version gets wiped out, we still have the original data from which we can create a new accelerated copy. The problem here, in terms of performance, is that an "unaccelerated image" means that all rendering to and from that image is unaccelerated. And if an application is constantly modifying the image, all of that rendering will be unaccelerated and it is never appropriate for us to create and use an accelerated version of that image.)

Developers that care about top performance for these types of images may want to look into using VolatileImages instead. These images store the data in accelerated memory (when possible) and thus rendering to and from that image may be accelerated automatically. The downside is that these images require a bit more care and feeding, due to "surface loss" issues that arise with current video memory architectures and operating systems. Note that not all types of rendering to these images is accelerated, either, but simple types of rendering like lines and rectangular fills and copies can usually be accelerated, depending on the platform configuration.

I've already written about VolatileImages in past blogs ( Part I and Part II), so I will not go into the details of their usage here; please check out those other articles for more information. But it is worth covering the APIs used to create the images, just for consistency's sake in this article:

        Component.createVolatileImage(width, height)
        Component.createVolatileImage(width, height, capabilities)
        GraphicsConfiguration.createCompatibleVolatileImage(width, height)
        GraphicsConfiguration.createCompatibleVolatileImage(width, height, capabilities)
        GraphicsConfiguration.createCompatibleVolatileImage(width, height, transparency)
        GraphicsConfiguration.createCompatibleVolatileImage(width, height, capabilities, transparency)

Note that some of these methods are duplicated in GraphicsConfiguration and Component just for consistency with the pre-existing createImage() and createCompatibleImage() methods. Calling Component.createVolatileImage(w,h) is exactly like calling Component.getGraphicsConfiguration().createCompatibleVolatileImage(w,h).

The use of the ImageCapabilities object in these methods gives you the ability to require certain attributes (such as hardware acceleration) from any image created with that method. In general, you probably will not need to use that variation, although as we enable more hardware acceleration features in our platform, we may expand the ImageCapabilities API to be more powerful and useful. (Note, too, that ImageCapabilities can be used effectively as a means of inquiring what capabilities an existing image has).

Back Buffers

By "back buffer" I mean an arbitrary offscreen image that is created for use in a double-buffering situation. Typically, an application that wishes to have smooth graphics, especially animations, will draw to a back buffer and then copy that buffer onto the screen instead of drawing directly to the screen. Swing does this by default, so that you do not see the various GUI elements in a Swing app flash as they are drawn to the screen. The buffer copy in these applications typically happens so fast that the graphics in the application are perceptibly smoother than if they were drawn one-by-one directly to the screen.

A developer could use any of the above static or dynamic image APIs that I listed for creating a back buffer. However, the following things should be taken into account when doing so:

  • Use of managed images for buffers has the same performance implications as dynamic images; we will be unable to manage the images effectively, thus you will not be able to take advantage of any hardware acceleration either for rendering to or copying from the buffer. Since the copy from the back buffer to the front buffer involves a lot of pixels, being able to accelerate that operation can have a huge performance impact on your application.
  • Use of VolatileImages for buffers is a better way to go in terms of hardware acceleration potential. But why go to all the bother of managing your VolatileImages when there is an easier route?
BufferStrategy: Better Buffering for Java

In JDK 1.4, we introduced the BufferStrategy API, which is a wrapper around VolatileImages. This API allows you to ask for an accelerated back buffer and avoid having to manage the details of surface loss associated with VolatileImages. It also ensures that you will get a buffer of the optimal type for your application. In particular, you will get either a FlipBuffer (which can only be used in fullscreen-exclusive mode on Windows) or a BltBuffer (which is used by default for windowed applications). A FlipBuffer performs a swap of the front and back buffers in video memory when you request BufferStrategy.show(). A BltBuffer copies the contents from the back buffer to the front (just as you would if you called drawImage() from a VolatileImage back buffer to the front buffer).

With this API, there is little need to create and manage VolatileImages directly; just let us manage the details for you inside the BufferStrategy implementation.

For more information on BufferStrategy, check out the javadocs; they're pretty clear on how the system works.

The APIs you will need when creating a BufferStrategy are:

        Canvas.createBufferStrategy(numBuffers)
        Canvas.createBufferStrategy(numBuffers, capabilities)
        Frame.createBufferStrategy(numBuffers)
        Frame.createBufferStrategy(numBuffers, capabilities)

For the most part, you will do this on a Frame (or JFrame, for the Swing developers). I know the API says that the methods are in Window, but a Window is really just a pseudo-frame and Frames are more useful in general. Take my advice; use Frame/JFrame.

Wrap-Up

So that's pretty much it. You have image loading methods and image (or buffer) creation methods. And in each category, you have various flavors depending on the location and type of the data, and the type of image you want returned to you. So even though there are a lot of methods listed at the top of this article, they all break down into just a few comprehensible categories and can be used effectively, once you understand the implications of each variation.

Although there is certainly more complexity here than we can cover with a simple table, it might help to break down some of the basic attributes of the image types we have talked about and the reasons to consider one type over another when writing your application:

  • Toolkit Image:
    • Creation methods: Applet.getImage(...), Toolkit.createImage(...), Toolkit.getImage(...), ImageIcon.getImage()
    • Most useful for: Simple loading and display of image data, applications that need to use 1.x APIs, asynchronous usage models (except for ImageIcon), loading performance (versus some ImageIO loading operations in the current releases).
    • Less useful for: Image data manipulation, reading/writing individual pixel data within the image, synchronous loading operations (except for ImageIcon), more varied or custom source image formats..
  • BufferedImage:
    • Creation methods: new BufferedImage(...), Component.createImage(...), GraphicsConfiguration.createCompatibleImage(...), ImageIO.read(...)
    • Most useful for: Image data manipulation, read/write access to individual pixel data, synchronous loading or creation, various image data formats (both source data that ImageIO can read and Java formats (the BufferedImage types TYPE_INT_RGB, TYPE_INT_RGBA, and so on).
    • Less useful for: Applications that require JDK 1.x compatibility, easy display of animating GIF images.
  • VolatileImage:
    • Creation methods: Component.createVolatileImage(...), GraphicsConfiguration.createVolatileImage(...)
    • Most useful for: Hardware acceleration of dynamic images, more fine-grained control over acceleration properties.
    • Less useful for: Simple image creation and display (applications must use the VolatileImage API to handle surface loss issues)
  • BufferStrategy:
    • Creation methods: Canvas.createBufferStrategy(), Window.createBufferStrategy()
    • Most useful for: Easy creation and management of window buffering schemes
    • Less useful for: Access to actual image data of buffer
Hey! You Forgot Some Methods!

There are still a couple of the creation methods up top that I have not covered yet:

BufferStrategy:
        new BufferStrategy()
Image:
        new Image()

These classes are abstract base classes and you cannot create an instances of them directly (and couldn't do anything useful with it if you could). BufferStrategy objects must be created on the Component which they will buffering and Image objects are created from one of the load/create methods explained above.

What About Performance?

It is difficult or impossible for me to write a long block of text or code without thinking about performance. And since some of the users of the APIs above, and image operations in general, care a great deal about performance, I should spend a few words discussing some performance issues to be aware of.

Again, check out my blogs on BufferedImage ( Part I and Part II); I go into much more detail on image management there. Some important things to keep in mind with respect to managed images (and making sure they are benefiting from available acceleration):

  • If you want copies from an image to be accelerated but you will be modifying the image frequently, think about VolatileImage instead (see the above discussion on Dynamic Images).
  • Careful with that Raster, Eugene! This is a hidden gotcha in the current implementation. Part of the BufferedImage API is a method that allows you to get the Raster of the image ( getRaster()). From that Raster, you can create a writable child raster ( WritableRaster.createWritableChild()) or get the internal data array for the raster ( Raster.getDataBuffer()). Both of these methods end up giving you a copy of the data that we have no control over. Specifically, we cannot tell when you have updated the contents of the image, so we cannot tell whether our cached/accelerated version of the image needs to be updated. So, when you call these Raster methods, we throw up our hands and give up on acceleration entirely.
    If you need to access the pixel data but you still want us to accelerate image copies, call methods which manipulate the data for you without giving you a reference to the data; for example, BufferedImage.setRGB() and WritableRaster.setPixels() for setting arbitrary pixel values, or any of the Graphics.draw*() or Graphics.fill*() methods for setting blocks of pixel values.
  • If you are transforming the image while copying it around (say, scaling it to the destination, or rotating it), we will not be able to do that using hardware acceleration with the current implementation. If you anticipate using a particular transformation of an image (like a zoomed-in view or several rotations of a sprite) multiple times, then it might be worth creating a temporary version of that transformed image and letting us manage (accelerate) that transformed version for you. This technique is covered in detail in my Intermediate Images article.
For More Information
  • Image Articles: I've written a few blogs trying to come to grips with the various image types we have in Java and the performance implications of different approaches. If this interests you, you might check out some of the articles on my blog site.
  • Java 2D homepage : This site has links to performance whitepapers, FAQs, and other information helpful to graphics programmers.
  • http://javadesktop.org : This community site has forums, projects, blogs, and articles aimed at the Java desktop client developer.
About the Author
Chet Haase is a member of the Java 2D team at Sun Microsystems. He has been involved with graphics software technologies for the past 15 years and currently spends his time worrying about performance and hardware acceleration issues for the Java graphics libraries.
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