[Java] Retrieve pixel values from image

用來抓單一個點的pixel value
int BufferedImage.getRGB(int x, int y)
用來抓一個區域的pixel value
int [] BufferedImage.getRGB(int startX, int startY, int w, int h,
int[] rgbArray, int offset, int scansize)

sample code:

import java.awt.image;
import javax.imageio;

BufferedImage source = ImageIO.read(inputfile);

int imageWidth = source.getWidth();
int imageHeight = source.getHeight();

int [][] pixelValue = new int[height][width];

for(int i = 0; i < pixelValue.length; i++) {
    for(int j = 0; j < pixelValue[i].length; j++) {
        pixelValue[i][j] = source.getRGB(j, i);
    }
}

Note:
1. 使用BufferedImage.getRGB(int x, int y)讀進來的是pixel value，並不能
直接使用，還要透過下面補充的函數[1]才可以得到RGB value。

2. 影像中的座標值剛好與記憶體中Array存放的位置為transpose，需特別注意。

補充:
1. 將使用BufferedImage.getRGB(int x, int y)讀進來的pixel value轉成RGB值

       private int[] getRGBFromPixel(int pixelvalue) {
            int[] RGB = new int[4];

            /* alpha value denotes the transparent level of pixel */
            int alpha = (pixelvalue >> 24) & 0xff;
            int red = (pixelvalue >> 16) & 0xff;
            int green = (pixelvalue >> 8 ) & 0xff;
            int blue = pixelvalue & 0xff;

            RGB[0] = alpha;
            RGB[1] = red;
            RGB[2] = green;
            RGB[3] = blue;

            return RGB;
        }

2. 將RGB value轉成BufferedImage中儲存的pixel value

       private int returnToRGB(int R, int G, int B) {
            int RGB = 0x00000000;

            /* process alpha value */
            int alpha = (0xff << 24);

            /* process Red value */
            int Red = (R & 0xff) << 16;

            /* process Green value */
            int Green = (G & 0xff) << 8;

            /* process Blue value */
            int Blue = (B & 0xff);

            RGB = alpha | Red | Green | Blue;

            return RGB;
       }

可以用 void BufferedImage.setRGB(int x, int y, int RGB) 設定RGB值