visual basic 2010 barcode generator Figure 231: Sample output from SimpleImageLoad in Java

Maker QR Code in Java Figure 231: Sample output from SimpleImageLoad

Figure 231: Sample output from SimpleImageLoad
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When this applet runs, it starts loading img in the init( ) method Onscreen you can see the image as it loads from the network, because Applet's implementation of the ImageObserver interface calls paint( ) every time more image data arrives Seeing the image load is somewhat informative, but it might be better if you use the time it takes to load the image to do other things in parallel That way, the fully formed image can simply appear on the screen in an instant, once it is fully loaded You can use ImageObserver, described next, to monitor loading an image while you paint the screen with other information
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ImageObserver is an interface used to receive notification as an image is being generated ImageObserver defines only one method: imageUpdate( ) Using an image observer allows you to perform other actions, such as show a progress indicator or an attract screen, as you are informed of the progress of the download This kind of notification is very useful when an image is being loaded over the network, where the content designer rarely appreciates that people are often trying to load applets over a slow modem The imageUpdate( ) method has this general form: boolean imageUpdate(Image imgObj, int flags, int left, int top, int width, int height) Here, imgObj is the image being loaded, and flags is an integer that communicates the status of the update report The four integers left, top, width, and height represent a rectangle that contains different values depending on the values passed in flags imageUpdate( ) should return false if it has completed loading, and true if there is more image to process The flags parameter contains one or more bit flags defined as static variables inside the ImageObserver interface These flags and the information they provide are listed in Table 23-1 Table 23-1 Bit Flags of the imageUpdate( ) flags Parameter
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The width parameter is valid and contains the width of the image The height parameter is valid and contains the height of the image The properties associated with the image can now be obtained using imgObjgetProperty( ) More pixels needed to draw the image have been received The parameters left, top, width, and height define the rectangle containing the new pixels A complete frame that is part of a multiframe image, which was previously drawn, has been received This frame can be displayed
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FRAMEBITS
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The left, top, width, and height parameters are not used ALLBITS The image is now complete The left, top, width, and height parameters are not used An error has occurred to an image that was being tracked asynchronously The image is incomplete and cannot be displayed No further image information will be received As a convenience, the ABORT flag will also be set to indicate that the image production was aborted An image that was being tracked asynchronously was aborted before it was complete However, if an error has not occurred, accessing any part of the image's data will restart the production of the image
ERROR
ABORT
The Applet class has an implementation of the imageUpdate( ) method for the ImageObserver interface that is used to repaint images as they are loaded You can override this method in your class to change that behavior Here is a simple example of an imageUpdate( ) method: public boolean imageUpdate(Image img, int flags, int x, int y, int w, int h) { if ((flags & ALLBITS) == 0) { Systemoutprintln("Still processing the image"); return true; } else { Systemoutprintln("Done processing the image"); return false; } }
ImageObserver Example
Now let's look at a practical example that overrides imageUpdate( ) to make a version of the SimpleImageLoad applet that doesn't flicker as much The default implementation of imageUpdate( ) in Applet has several problems First, it repaints the entire image each time any new data arrives This causes flashing between the background color and the image Second, it uses a feature of Appletrepaint( ) to cause the system to only repaint the image every tenth of a second or so This causes a jerky, uneven feel as the image is painting Finally, the default implementation knows nothing about images that may fail to load properly Many beginning Java programmers are frustrated by the fact that getImage( ) always succeeds even when the image specified does not exist You don't find out about missing images until imageUpdate( ) occurs If you use the default implementation of imageUpdate( ), then you'll never know what happened Your paint( ) method will simply do nothing when you call gdrawImage( ) The example that follows fixes all three of these problems in ten lines of code First, it eliminates the flickering with two small changes It overrides update( ) so that it calls paint( ) without painting the background color first The background is set via setBackground( ) in init( ), so the initial color is painted just once Also, it uses a version of repaint( ) that specifies the rectangle in which to paint The system will set the clipping area such that nothing outside of this rectangle is painted This reduces repaint flicker and improves performance Second, it eliminates the jerky, uneven display of the incoming image by painting every time it receives an update These updates occur on a scan line-by-scan line basis, so an
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image that is 100 pixels tall will be "repainted" 100 times as it loads Note that this is not the fastest way to display an image, just the smoothest Finally, it handles the error caused by the desired file not being found by examining the flags parameter for the ABORT bit If it is set, the instance variable error is set to true and then repaint( ) is called The paint( ) method is modified to print an error message over a bright red background if error is true Here is the code /* * <applet code="ObservedImageLoad" width=248 height=146> * <param name="img" value="seattlejpg"> * </applet> */ import javaawt*; import javaapplet*; public class ObservedImageLoad extends Applet { Image img; boolean error = false; String imgname; public void init() { setBackground(Colorblue); imgname = getParameter("img"); img = getImage(getDocumentBase(), imgname); } public void paint(Graphics g) { if (error) { Dimension d = getSize(); gsetColor(Colorred); gfillRect(0, 0, dwidth, dheight); gsetColor(Colorblack); gdrawString("Image not found: " + imgname, 10, dheight/2); } else { gdrawImage(img, 0, 0, this); } } public void update(Graphics g) { paint(g); } public boolean imageUpdate(Image img, int flags, int x, int y, int w, int h) { if ((flags & SOMEBITS) != 0) { // new partial data repaint(x, y, w, h); // paint new pixels } else if ((flags & ABORT) != 0) { error = true; // file not found repaint(); // paint whole applet } return (flags & (ALLBITS|ABORT)) == 0; }
Figure 23-2 shows two separate screens of this applet running The top screen shows the image half loaded, and the bottom screen displays a filename that has been mistyped in the applet tag
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