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Creating, Saving, and Reading Images
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Creating a image directly rather than loading a file is often useful. Obviously, saving an image object to a file is also quite handy. Here we show ways to do both with BufferedImage objects. We discussed creation of Image objects in the Chapter 11: Java section. The BufferedImage class provides a constructor in which you pass the desired dimensions and a parameter indicating the type of object.
The overridden getGraphics() method for BufferedImage returns a Graphics2D object typed as its superclass Graphics type and so must be cast to Graphics2D to use its methods. This method is present for backward compatibility. The preferred alternative is to use createGraphics(), which explicitly returns a Graphics2D type. The Java2D API offers many image tools that work primarily with the BufferedImage class. You might encounter a situation where you get an Image object but you want to apply Java 2D operations to the image. You can obtain a BufferedImage object from an Image object by drawing the image onto the graphics context of the BufferedImage.
Another approach to image creation is to build them directly from pixel arrays. We discussed this for Image objects in Chapter 11: Java : Pixel Handling. We also discuss in this section how to build BufferedImage objects from pixel arrays. Although the core Java packages always allowed for reading GIF and JPEG files, not until Java 1.4 did it provide for saving images to disk files in JPEG format. (Java 1.4 also provided for reading and writing images in the PNG format.) Previously, Sun had provided the package com.sun.image.codec.jpeg for writing to JPEG files but it belonged to the category of extension package, which meant that it was only available for a limited number of platforms. With Java 5.0 came BMP/WBMP image reading and writing. GIF files can be read but not saved. The GIF encoder involves the patented LZW compression algorithm so GIF encoding was not included in the standard Java packages. GIF encoders can be obtained from third party sources (see below). JavaTM
Image I/O includes the javax.imageio.*
packages. The javax.imageio.ImageIO
class provides a number of static methods for image reading and
writing with several encoding formats. For example, The second argument of write is a string that corresponds to a supported format. A list of supported formats for output can be obtained with String[] formatNames = ImageIO.getWriterFormatNames(); The ImageIO class similarly offers several read() methods to obtain images from files via a URL, File, and input streams. For example, this code reads an image file via a File object:
Note that the read method will return only after the image data is completely read. There is no image input monitoring such as with a MediaTracker involved. If you want to use an ImageIO read methods to obtain a file over the network that may take considerable time to download, then you might want to do this in a thread process since the read() will block until it completes the operation. A list of the supported image formats for input can be obtained with
The Image I/O framework also provide for more sophisticated image handling such as creating plug-ins for reading and writing custom image formats. See the Image I/O API Guide and Java API Specifications for more about these classes. Other Java Image Processing Tools & Techniques The Java Advanced Imaging (JAI) API also provides for I/O for a number of formats including JPEG, BMP, PNG, PNM, and uncompressed TIFF. (Includes a reader but no writer for GIF). See the JAI API Specifications. for the javax.media.jai.* and com.sun.media.jai.* packages. This API, however, is an extension and not part of standard Java, though it should run correctly with a Java. 1.2 or later JVM. (A new JAI release will only be compatible with 1.3) Some platforms provide lower level native code libraries that accelerate the JAI processing. If the JAI code does not find these libraries, the programs will run with purely Java classes. Other third party classes in Java, and thus portable, can be found for saving images in various formats:
A crude approach for saving/reading images is to use the file I/O techniques discussed in a Chapter 9 : Java : Binary file. You can simply write an image pixel array directly to a local disk file. Then you would read the pixel array and recreate the images as discussed above. To take up less space, the Java Zip or Gzip I/O can provide some compression. References & Web Resources
Last update: Dec. 5, 2004 |
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