图像处理之距离变换

图像处理之距离变换

概述

距离变换是二值图像处理与操作中常用手段，在骨架提取，图像窄化中常有应用。距离

变换的结果是得到一张与输入图像类似的灰度图像，但是灰度值只出现在前景区域。并

且越远离背景边缘的像素灰度值越大。

基本思想

根据度量距离的方法不同，距离变换有几种不同的方法，假设像素点p1(x1, y1), 

p2(x2, y2)计算距离的方法常见的有：

1.      欧几里德距离，Distance =

2.      曼哈顿距离(City Block Distance)，公式如下：Distance = |x2-x1|+|y2-y1|

3.      象棋格距离(Chessboard Distance)，公式如下：Distance = max(|x2-x1|,|y2-y1|)

一旦距离度量公式选择，就可以在二值图像的距离变换中使用。一个最常见的距离变换

算法就是通过连续的腐蚀操作来实现，腐蚀操作的停止条件是所有前景像素都被完全

腐蚀。这样根据腐蚀的先后顺序，我们就得到各个前景像素点到前景中心骨架像素点的

距离。根据各个像素点的距离值，设置为不同的灰度值。这样就完成了二值图像的距离

变换。

注意点：

腐蚀操作结构体的选取会影响距离变换的效果，例子使用3*3的矩阵完成。有很多快速

的距离变换算法，感兴趣的可以自己研究。

运行结果：

关键代码解析：

初始化二值图像，读取像素，获取前景边缘像素与背景边缘像素

	public DistanceTransform(float scaleValue, float offsetValue, BufferedImage src)
	{
		this.scaleValue = scaleValue;
		this.offsetValue = offsetValue;
		this.inputImage = src;
		this.width = src.getWidth();
		this.height = src.getHeight();
        int[] inPixels = new int[width*height];
        getRGB( src, 0, 0, width, height, inPixels );
        int index = 0;
        pixels2D = new int[height][width]; // row, column
        greyLevel = new int[height][width];
        for(int row=0; row < height; row++)
        {
        	for(int col=0; col<width; col++) 
        	{
        		index = row * width + col;
        		int grayValue = (inPixels[index] >> 16) & 0xff;
        		pixels2D[row][col] = grayValue;
        		greyLevel[row][col] = 0;
        	}
        }
        
        generateForegroundEdge();
        generateBackgroundEdgeFromForegroundEdge();
        
	}

@Override
public BufferedImage filter(BufferedImage src, BufferedImage dest) {
	
	// calculate the distance here!!
	int index = 1;
    while (foregroundEdgePixels.size() > 0) {
    	distanceSingleIteration(index);
        ++index;
    }
    
    // loop the each pixel and assign the color value according to distance value
	for (int row = 0; row < inputImage.getHeight(); row++) {
	      for (int col = 0; col < inputImage.getWidth(); col++) {
	    	  if(greyLevel[row][col] > 0) {
		    	  int colorValue = (int)Math.round(greyLevel[row][col] * scaleValue + offsetValue);
		    	  colorValue = colorValue > 255 ? 255 : ((colorValue < 0) ? 0 : colorValue);
		    	  this.pixels2D[row][col] = colorValue;
	    	  }
	    	  
	      }
	}
	
	// build the result pixel data at here !!!
    if ( dest == null )
        dest = createCompatibleDestImage(inputImage, null );
    
    index = 0;
    int[] outPixels = new int[width*height];
    for(int row=0; row<height; row++) {
    	int ta = 0, tr = 0, tg = 0, tb = 0;
    	for(int col=0; col<width; col++) {
    		if(row == 75 && col > 60) {
    			System.out.println("ddddd");
    		}
    		index = row * width + col;
    		tr = tg = tb = this.pixels2D[row][col];
    		ta = 255;
    		outPixels[index] = (ta << 24) | (tr << 16) | (tg << 8) | tb;
    	}
    }
    setRGB( dest, 0, 0, width, height, outPixels );
	return dest;
}

  private void generateForegroundEdge()
  {
    foregroundEdgePixels.clear();

    for (int row = 0; row < height; row++)
      for (int col = 0; col < width; col++)
        if (this.pixels2D[row][col] == foreground) {
          Point localPoint = new Point(col, row);
          for (int k = -1; k < 2; ++k) // 3*3 matrix
        	  for (int l = -1; l < 2; ++l) {
              if ((localPoint.x + l < 0) || (localPoint.x + l >= this.width) || (localPoint.y + k < 0) || (localPoint.y + k >= this.height) || 
                (this.pixels2D[(localPoint.y + k)][(localPoint.x + l)] != background) || (this.foregroundEdgePixels.contains(localPoint)))
                continue;
              this.foregroundEdgePixels.add(localPoint);
            }
        }
  }
  
  private void generateBackgroundEdgeFromForegroundEdge()
  {
    this.backgroundEdgePixels.clear();

    Iterator<Point> localIterator = this.foregroundEdgePixels.iterator();
    while (localIterator.hasNext()) {
      Point localPoint1 = new Point((Point)localIterator.next());
      for (int i = -1; i < 2; ++i)
        for (int j = -1; j < 2; ++j)
          if ((localPoint1.x + j >= 0) && (localPoint1.x + j < this.width) && (localPoint1.y + i >= 0) && (localPoint1.y + i < this.height)) {
            Point localPoint2 = new Point(localPoint1.x + j, localPoint1.y + i);
            if (this.pixels2D[localPoint2.y][localPoint2.x] == background)
              this.backgroundEdgePixels.add(localPoint2);
          }
    }
  }