AWT image scaling API

last updated: October 30, 1996

The Issue

Java has provided a number of mechanisms to scale an image automatically at the time that the drawImage() is called. These mechanisms have caused a bit of confusion since the exact behavior was not well documented and the particular implementation chosen for the reference port behaved in a way that differed from most programmer's expectations.

In particular, when images are displayed on the screen, the Image data is converted to a format which matches the color capabilities of the screen. The original image data is not stored persistently, only the converted screen data is stored by the screen representations. If the data for the image is needed for some other purpose (such as filtering the data for a new image), then the raw data has to be reloaded from the original source.

Note that the original data is stored in a cache in case it is needed immediately after the first time it is loaded for a secondary purpose. If a single image is run through several filters, then the subsequent filtered images will often be able to use the cached converted data without having to reload the image file from the source, but this cached data cannot be relied on in general.
When the image is scaled using one of the drawImage method variants that allows the specification of a width and height, a separate screen representation is created for each size that is drawn. Furthermore, since scaling an image after it has been dithered for output on an 8-bit screen results in very poor quality output, the scaling is always done from the original image data. Even if the screen is 24 bits deep and thus the screen representation contains the original image data with no conversion loss the implementation is not smart enough to use the local copy and contacts the original source of the image anyway.

Another drawback of storing a separate image representation for each size drawn is the amount of memory consumed. The strategy of keeping a separate representation for each size drawn is meant to optimize rendering speed since the data is all ready to be copied to the screen byte for byte. This strategy ends up backfiring since the speed gain by having the scaled size precalculated often is more than offset by the pain and inconvenience of having to wait while the scaled variant is loaded and converted, especially if the image data needs to be reloaded over a slow network. Also, if the image is being drawn at many different sizes which will never be repeated the memory used to cache the scaled representations is wasted.

In short, there is no programmer control over whether or not the scaled version of the image is cached for subsequent rendering and whether or not the image scaling is done immediately or in the background.

Another issue which has bothered developers is that there is no way to create a new object which represents a scaled view of an existing image. An Image object can be scaled by supplying new width and height parameters to the drawImage call, but there is no way to encapsulate that information into an Image object so that by passing around that Image object, the desired size image will be drawn.

The new image scaling API and behavior

To resolve this problem a new strategy will be adopted for the drawImage methods that scale an image. In 1.1, all scaling by drawImage will be done "on the fly" at rendering time from the full sized screen representation.

There are two primary drawbacks of this change.

  1. Using the new drawImage to scale the image will take slightly longer than before since the pixel reordering will have to be performed at render time.
  2. The quality of the scaled images will be reduced on 8-bit screens due to the fact that the pixel reordering will be performed after the dithering process.

As it turns out, the first problem is much less dramatic than was originally feared. In the past couple of years processors have become very fast especially in comparison to the speeds of memory systems. As a result image scaling performance is mostly limited by memory access speed and the calculations necessary to determine which is the next pixel to copy to the screen for the particular scaling desired are not very noticeable compared to the work of moving the data to the screen.

The quality issue could have been a major problem. With the 8-bit color conversion code used in JDK implementations prior to 1.1 images degraded dramatically if they were scaled after dithering. Fortunately, new color conversion algorithms have been developed for 1.1 which increase the quality of the resulting dithered image so much that the results of scaling the dithered representations are typically as good, if not better than the results of the 1.0.2 images which were dithered after scaling.

Still, even though the implementation of image scaling was beefed up in 1.1 enough to make render-time on-the-fly scaling possible, there will be cases where the programmer wants every possible measure taken to improve the quality of the scaled image. To allow the programmer to choose when explicit scaling should be performed up front instead of at rendering time, two new filter classes will be available and a new convenience method will be added to the AWT Image class in 1.1. These same APIs can be used by programmers to create a new Image object representing a scaled version of that image encapsulated into its own Image object. The new image scaling classes are: java.awt.image.ReplicateScaleFilter java.awt.image.AreaAveragingScaleFilter The new method in the Image class is: getScaledInstance(int width, int height, int hints) The hints parameter controls what kind of algorithm is used to scale the image. There are hints which refer to specific algorithms to be used and there are more abstract hints which indicate whether or not to optimize the operation for speed or quality. The current set of legal values for the hints parameter are: Image.SCALE_DEFAULT

Use the default scaling algorithm (which may be chosen based on the user's configuration)
Choose a scaling algorithm to optimize speed more than smoothness of the scaled image
Choose a scaling algorithm to optimize smoothness of the scaled image more than speed
Use the algorithm implemented by the ReplicateScaleFilter
Use the algorithm implemented by the AreaAveragingScaleFilter

Sample Code

Following is sample code showing the use of the new API to scale an image: import java.awt.*; import java.applet.*; public class ImgScaleExample extends Applet { Image img, img2; public void init() { img = getImage(getDocumentBase(), "foo.gif"); img2 = img.getScaledInstance(100, 100, Image.SCALE_DEFAULT); } public void paint(Graphics g) { // Draw the full size image g.drawImage(img, 0, 0, this); // Draw the scaled version of the image g.drawImage(img2, 10, 10, this); // Now draw a scaled version of the scaled image g.drawImage(img2, 110, 10, 50, 50, this); } }

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