AWT PixelGrabber API updates - JDK 5 Documentation v1.1.8, Java 2 SDK 英文文档

AWT PixelGrabber API updates

last updated: November 14, 1996

The Issue

The AWT image facility provides ways to retrieve or create images from many sources and to automate the process of displaying those images on the screen. The facility performs all of the required manipulations necessary on the image data behind an opaque external interface which shields users from all but the most abstract of image data information.

The interface provided is so opaque that in the initial development stages there was no direct way to access the pixel data of an image. The data could be accessed if a developer were willing to learn the intricacies of the complex ImageConsumer interface which is used by the image subsystem to communicate pixel data from image decoders to the screen, but most developers wanted an easier mechanism to get this data for the general case.

To satisfy this need a utility class was created to manage the ImageConsumer interface on behalf of the average developer who simply wanted to answer the question "What color is the pixel at location (x, y)?" The PixelGrabber class provided the ability to snapshot a rectangular region of pixels directly from an image's source to a Java integer array.

Three primary oversights have been identified with the interface provided by this class:

The data must be grabbed and stored into the array in the default ARGB (alpha RGB) convenience format Java uses for describing colors. The conversion process to convert the raw pixels into this format can consume a large amount of time especially when the developer only really needs the color information for isolated pixels scattered throughout the region of interest. Since the original ColorModel is discarded as the data is converted to the default representation, there is also no way to figure out what format the original data was in if that happens to be the desired goal of the process.
The developer must know what size array to create even before the data is snapshotted. Since many images do not know their dimensions until they load their data from the source for the first time, there can be a race condition where the developer must track the image loading until the dimensions are delivered and then instantiate a PixelGrabber and point it at the image source before the pixel data is delivered so that no data is lost requiring the image source to reload from scratch. Many developers would not even know how to best achieve this goal since the image progress is not very straightforward to track.
There was no way to start the grab operation without also waiting for the operation to complete and there was no way to stop the grab operation once started.

The new PixelGrabber methods

To enhance the capabilities of the PixelGrabber, new methods and constructors are being added:

	new PixelGrabber(Image img,
			 int x, int y, int w, int h,
			 boolean forceRGB)
	startGrabbing()
	abortGrabbing()
	getWidth()
	getHeight()
	getPixels()
	getColorModel()

Sample Code

Following is sample code showing the use of the new API to grab the pixels of an image of unknown dimensions in its original format:


    import java.awt.*;
    import java.awt.image.*;
    import java.applet.*;

    public class GrabExample extends Applet implements Runnable {
    	Thread grab;
	Image img;
	PixelGrabber pg;

	public void init() {
	    img = getImage(getDocumentBase(), "theImage.gif");
	    pg = new PixelGrabber(img, 0, 0, -1, -1, false);
	    // In reality, the init method shouldn't be kicking off
	    // heavyweight operations like an image download, but
	    // this demonstrates use of the asynchronous grabbing...
	    pg.startGrabbing();
	}

    	public void start() {
	    grab = new Thread(this);
	    grab.start();
	}

	public synchronized void stop() {
	    grab.interrupt();
	    grab = null;
	}

	public synchronized void run() {
	    try {
		pg.grabPixels();
		repaint();
	    } catch (InterruptedException e) {
		System.err.println("grab was interrupted");
		return;
	    }
	}

	public void paint(Graphics g) {
	    // Display some pixel values
	    int status = pg.getStatus();
	    if ((status & ABORT) != 0) {
		g.drawString("Image grab was aborted", 10, 100);
	    } else if ((status & ALLBITS) != 0) {
		g.drawString("Image grab is complete", 10, 100);
	    } else if ((status & FRAMEBITS) != 0) {
		g.drawString("Frame grab is complete", 10, 100);
	    } else if ((status & SOMEBITS) != 0) {
		g.drawString("Image grab is under way", 10, 100);
	    }
	    if ((status & (WIDTH|ALLBITS|FRAMEBITS)) != 0) {
		FontMetrics fm = g.getFontMetrics();
		String caption = "First 3 pixels: ";
		int x = 10;
		int w = pg.getWidth();
		Object pix = pg.getPixels();
		ColorModel cm = pg.getColorModel();

		g.drawString(caption, x, 50);
		x += fm.stringWidth(caption);
		if (pix instanceof byte[]) {
		    for (int i = 0; i < Math.min(3, w); i++) {
			String s = Integer.toString(((byte[])pix)[i]&0xff, 16);
			g.drawString(s, x, 50);
			x += fm.stringWidth(s + " ");
		    }
		} else {
		    for (int i = 0; i < Math.min(3, w); i++) {
			String s = Integer.toString(((int[])pix)[i], 16);
			g.drawString(s, x, 50);
			x += fm.stringWidth(s + " ");
		    }
		}
	    }
	}
    }

Send feedback to:java-awt@java.sun.com