Trail: 2D Graphics
Lesson: Working with Images

写/保存图像

This lesson started with an explanation for using the javax.imageio package, to load images from an external image format into the internal BufferedImage format used by Java 2D. Then it explains how to use the Graphics.drawImage() to draw that image, with optional filtering.

The final stage is saving a BufferedImage object into an external image format. This may be an image that was originally loaded by the Image I/O class from an external image format and perhaps modified using the Java 2D APIs, or it may be one that was created by Java 2D.

The Image I/O class provides a simple way to save images in a variety of image formats in the following example:

static boolean ImageIO.write(RenderedImage im, 
                             String formatName,
                             File output)  throws IOException

注意: The BufferedImage class implements the RenderedImage interface.

The formatName parameter selects the image format in which to save the BufferedImage.

try {
    // retrieve image
    BufferedImage bi = getMyImage();
    File outputfile = new File("saved.png");
    ImageIO.write(bi, "png", outputfile);
} catch (IOException e) {
    ...
}

The ImageIO.write method calls the code that implements PNG writing a “PNG writer plug-in”. The term plug-in is used since Image I/O is extensible and can support a wide range of formats.

But the following standard image format plugins : JPEG, PNG, GIF, BMP and WBMP are always be present.

Each image format has its advantages and disadvantages:

Format	Plus	Minus
GIF	Supports animation, and transparent pixels	Supports only 256 colors and no translucency
PNG	Better alternative than GIF or JPG for high colour lossless images, supports translucency	Doesn't support animation
JPG	Great for photographic images	Loss of compression, not good for text, screenshots, or any application where the original image must be preserved exactly

For most applications it is sufficient to use one of these standard plugins. They have the advantage of being readily available. The Image I/O class provides a way to plug in support for additional formats which can be used, and many such plug-ins exist. If you are interested in what file formats are available to load or save in your system, you may use the getReaderFormatNames and getWriterFormatNames methods of the ImageIO class. These methods return an array of strings listing all of the formats supported in this JRE.

String writerNames[] = ImageIO.getWriterFormatNames();

The returned array of names will include any additional plug-ins that are installed and any of these names may be used as a format name to select an image writer. The following code example is a simple version of a complete image edit/touch up program which uses a revised version of the ImageDrawingApplet.java sample program which can be used as follows :

An image is first loaded via Image I/O
The user selects a filter from the drop down list and a new updated image is drawn
The user selects a save format from the drop down list
Next a file chooser appears and the user selects where to save the image
The modified image can now be viewed by other desktop applications

The complete code of this example is represented in SaveImage.java.

In this lesson you have learned just the basics of Image I/O, which provides extensive support for writing images, including working directly with an ImageWriter plug-in to achieve finer control over the encoding process. ImageIO can write multiple images, image metadata, and determine quality vs. size tradeoffs. For more information see Java Image I/O API Guide.