在java中,如何将java.awt.image.BufferedImage写入8位png文件？

Question

我试图从java.awt.image.BufferedImage写出一个png文件.一切正常,但生成的png是一个32位文件.

有没有办法让png文件成为8位？图像是灰度的,但我确实需要透明度,因为这是一个叠加图像.我正在使用java 6,我宁愿返回一个OutputStream,这样我就可以让调用类处理将文件写入磁盘/数据库.

以下是代码的相关部分:

 public static ByteArrayOutputStream createImage(InputStream originalStream)
            throws IOException {

        ByteArrayOutputStream oStream = null;

        java.awt.Image newImg = javax.imageio.ImageIO.read(originalStream);
        int imgWidth = newImg.getWidth(null);
        int imgHeight = newImg.getHeight(null);
        java.awt.image.BufferedImage bim = new java.awt.image.BufferedImage(imgWidth,
                imgHeight, java.awt.image.BufferedImage.TYPE_INT_ARGB);

        Color bckgrndColor = new Color(0x80, 0x80, 0x80);

        Graphics2D gf = (Graphics2D)bim.getGraphics();

        // set transparency for fill image
        gf.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.3f));
        gf.setColor(bckgrndColor);
        gf.fillRect(0, 0, imgWidth, imgHeight);

        oStream = new ByteArrayOutputStream();
        javax.imageio.ImageIO.write(bim, "png", oStream);
        oStream.close();

        return oStream;
    }

Answer 1

这是一个有趣的问题...现在很晚了，我明天实验一下。我将首先尝试使用 BufferedImage.TYPE_BYTE_INDEXED （可能在绘制之后）来查看 Java 是否足够智能来生成 8 位 PNG。
或者也许某些图像库可以允许这样做。

[编辑]几年后...实际上，我当时编写了代码，但忘记更新此线程...我使用了Kat 指出的代码，对透明度的处理进行了一些细化，并保存在PNG 格式而不是 Gif 格式。它适用于制作具有全有或全无透明度的 8 位 PNG 文件。

您可以使用我的ImageUtil类在http://bazaar.launchpad.net/~philho/+junk/Java/view/head:/Tests/src/org/philhosoft/tests/image/AddTransparency.java找到工作测试文件。

由于代码不是那么大，为了后代的缘故，我将其发布在这里，没有 JavaDoc 来节省一些行。

public class ImageUtil
{
  public static int ALPHA_BIT_MASK = 0xFF000000;

  public static BufferedImage imageToBufferedImage(Image image, int width, int height)
  {
    return imageToBufferedImage(image, width, height, BufferedImage.TYPE_INT_ARGB);
  }

  public static BufferedImage imageToBufferedImage(Image image, int width, int height, int type)
  {
    BufferedImage dest = new BufferedImage(width, height, type);
    Graphics2D g2 = dest.createGraphics();
    g2.drawImage(image, 0, 0, null);
    g2.dispose();
    return dest;
  }

  public static BufferedImage convertRGBAToIndexed(BufferedImage srcImage)
  {
    // Create a non-transparent palletized image
    Image flattenedImage = transformTransparencyToMagenta(srcImage);
    BufferedImage flatImage = imageToBufferedImage(flattenedImage,
        srcImage.getWidth(), srcImage.getHeight(), BufferedImage.TYPE_BYTE_INDEXED);
    BufferedImage destImage = makeColorTransparent(flatImage, 0, 0);
    return destImage;
  }

  private static Image transformTransparencyToMagenta(BufferedImage image)
  {
    ImageFilter filter = new RGBImageFilter()
    {
      @Override
      public final int filterRGB(int x, int y, int rgb)
      {
        int pixelValue = 0;
        int opacity = (rgb & ALPHA_BIT_MASK) >>> 24;
        if (opacity < 128)
        {
          // Quite transparent: replace color with transparent magenta
          // (traditional color for binary transparency)
          pixelValue = 0x00FF00FF;
        }
        else
        {
          // Quite opaque: get pure color
          pixelValue = (rgb & 0xFFFFFF) | ALPHA_BIT_MASK;
        }
        return pixelValue;
      }
    };

    ImageProducer ip = new FilteredImageSource(image.getSource(), filter);
      return Toolkit.getDefaultToolkit().createImage(ip);
  }

  public static BufferedImage makeColorTransparent(BufferedImage image, int x, int y)
  {
    ColorModel cm = image.getColorModel();
    if (!(cm instanceof IndexColorModel))
      return image; // No transparency added as we don't have an indexed image

    IndexColorModel originalICM = (IndexColorModel) cm;
    WritableRaster raster = image.getRaster();
    int colorIndex = raster.getSample(x, y, 0); // colorIndex is an offset in the palette of the ICM'
    // Number of indexed colors
    int size = originalICM.getMapSize();
    byte[] reds = new byte[size];
    byte[] greens = new byte[size];
    byte[] blues = new byte[size];
    originalICM.getReds(reds);
    originalICM.getGreens(greens);
    originalICM.getBlues(blues);
    IndexColorModel newICM = new IndexColorModel(8, size, reds, greens, blues, colorIndex);
    return new BufferedImage(newICM, raster, image.isAlphaPremultiplied(), null);
  }
}