如何在java中将8位灰度位图转换为4位？

Question

I want to read an gray level image file, for example, lena.bmp, then reduce the gray level resolution in java, I used PixelGrabber to get the pixels data of this image, how to convert this 8 bit gray scale bitmap to 4 bit? Thanks!

Answer 1

Not shure if they can help you here, but maybe you can take a look at JAI (Java Advanced Imaging) , Apache Commons Sanselan or Image4j .

Edit:

Just tested the Image4J Library:

Converting 8 Bit -> 4 Bit is very simple:

final File inFile = new File("test.bmp");
final File outFile = new File("test2.bmp");

BufferedImage inImage = ImageIO.read(inFile);
BufferedImage outImage = ConvertUtil.convert4(inImage); // Converts to 4 Bit

ImageIO.write(outImage, "bmp", outFile);

See Image4J - ConvertUtil Documentation for more Informations.

Answer 2

This does reduce the number of bits to 4:

public static void main(String[] args) throws Exception {
    BufferedImage in = ImageIO.read(new File(args[0]));
    int w = in.getWidth(), h = in.getHeight();
    int[] bits = { 4 };
    ColorSpace cs = ColorSpace.getInstance(ColorSpace.CS_GRAY);
    int dt = DataBuffer.TYPE_BYTE;
    ColorModel cm = new ComponentColorModel
        (cs, bits, false, false, Transparency.OPAQUE, dt);
    WritableRaster wr = cm.createCompatibleWritableRaster(w, h);
    BufferedImage out = new BufferedImage(cm, wr, false, null);
    Graphics2D g = out.createGraphics();
    g.drawImage(in, 0, 0, null);
    g.dispose();
    ImageIO.write(out, "png", new File(args[1]));
}

The resulting file will look too dark on all viewer applications I've tried so far. But if you're only interested in the bits, then perhaps operating on the wr raster after the above transformation is good enough for you.

If not, then perhaps you should set up an IndexedColorModel containing the 2 ⁴ gray levels you want. You can simply multiply the index by 17 to obtain evenly spaced intensities, from 0x0 * 17 = 0x00 through 0xf * 17 = 0xff .

public static void main(String[] args) throws Exception {
    BufferedImage in = ImageIO.read(new File(args[0]));
    int w = in.getWidth(), h = in.getHeight();
    byte[] v = new byte[1 << 4];
    for (int i = 0; i < v.length; ++i)
        v[i] = (byte)(i*17);
    ColorModel cm = new IndexColorModel(4, v.length, v, v, v);
    WritableRaster wr = cm.createCompatibleWritableRaster(w, h);
    BufferedImage out = new BufferedImage(cm, wr, false, null);
    Graphics2D g = out.createGraphics();
    g.drawImage(in, 0, 0, null);
    g.dispose();
    ImageIO.write(out, "png", new File(args[1]));
}