getRGB和getRaster带来不同的结果

Question

If I create an BufferedImage in that way:

    BufferedImage image = new BufferedImage(4, 3, BufferedImage.TYPE_BYTE_GRAY);
    image.getRaster().setPixels(0, 0, image.getWidth(), image.getHeight(), new int[]
                    {
                            0, 255, 192, 183,
                            83, 143, 52, 128,
                            102, 239, 34, 1
                    }
    );

And then when get pixels values using getRGB method:

    for (int y = 0; y < image.getHeight(); y++) {
        for (int x = 0; x < image.getWidth(); x++) {
            System.out.print((image.getRGB(x, y)) & 0xFF);
            System.out.print(" ");
        }
        System.out.println();
    }

I see result that differ from input:

0 255 225 220 
155 197 125 188 
170 248 102 13 

I could get original values using image.getRaster().getDataBuffer() , but why getRGB results are different?

Answer 1

You are directly writing pixels to the Raster, but you go through the Image API for getting them back.

These are very different API's, the Raster works with raw pixel data, while the Image API takes the ColorModel into account. When you call getRGB(), the call is delegated through the ColorModel, so some nonlinear conversion between the sRGB color space and the Raster's color space can be/is performed.

If you try to convert them backwards:

    for (int y = 0; y < image.getHeight(); y++) {
        for (int x = 0; x < image.getWidth(); x++) {
            int rgb = image.getRGB(x, y);
            image.setRGB(x, y, rgb);
        }
    }

in Raster data you will see results that are very close to original:

0 255 192 183 
84 142 52 128 
103 239 34 1

So everything is correct, just assumption that 8-bit grayscale linearly converts to sRGB is wrong.