基本思想：

RGB像素的亮度是由RGB各个分量值的大小决定的，分量越大，亮度越大。看上去

好像光照效果越明显，光源退化效果是模拟光照在图像的中心点上，慢慢扩散到周

围，越靠近中心点像素，图像越亮，越远离图像越暗。原理可以说是非常的简单，

只要计算图像中每个像素到中心像素的欧几里德距离，归一化以后得到scale值(0

到1之间)乘以原来的RGB像素值即得到每个像素处理以后的RGB像素值。

效果如下：

关键代码解释：

中心像素点坐标取得：

int centerX = width/2;

int centerY = height/2;

 

任意一个像素点到中心像素的距离计算：

double xx = (centerX - px)*(centerX - px);

double yy = (centerY - py)*(centerY - py);

return (int)Math.sqrt(xx + yy);

 

距离归一化以及衰减因子考虑：

double scale = 1.0 - getDistance(centerX, centerY, col,row)/maxDistance;

for(int i=0; i<factor; i++) {

scale = scale * scale;

}

计算每个像素点的新RGB值：

tr = (int)(scale * tr);

tg = (int)(scale * tg);

tb = (int)(scale * tb);

滤镜源代码如下：

package com.gloomyfish.filter.study;

import java.awt.image.BufferedImage;

public class SpotlightFilter extends AbstractBufferedImageOp {
	// attenuation coefficient, default is 1 means line decrease...
	private int factor;
	public SpotlightFilter() {
		factor = 1;
	}
	
	public void setFactor(int coefficient) {
		this.factor = coefficient;
	}

	@Override
	public BufferedImage filter(BufferedImage src, BufferedImage dest) {
		int width = src.getWidth();
        int height = src.getHeight();

        if ( dest == null )
            dest = createCompatibleDestImage( src, null );

        int[] inPixels = new int[width*height];
        int[] outPixels = new int[width*height];
        getRGB( src, 0, 0, width, height, inPixels );
        int index = 0;
        int centerX = width/2;
        int centerY = height/2;
        double maxDistance = Math.sqrt(centerX * centerX + centerY * centerY);
        for(int row=0; row<height; row++) {
        	int ta = 0, tr = 0, tg = 0, tb = 0;
        	for(int col=0; col<width; col++) {
        		index = row * width + col;
        		ta = (inPixels[index] >> 24) & 0xff;
                tr = (inPixels[index] >> 16) & 0xff;
                tg = (inPixels[index] >> 8) & 0xff;
                tb = inPixels[index] & 0xff;
                double scale = 1.0 - getDistance(centerX, centerY, col, row)/maxDistance;
                for(int i=0; i<factor; i++) {
                	scale = scale * scale;
                }

            	tr = (int)(scale * tr);
            	tg = (int)(scale * tg);
            	tb = (int)(scale * tb);
                
                outPixels[index] = (ta << 24) | (tr << 16) | (tg << 8) | tb;
                
        	}
        }
        setRGB( dest, 0, 0, width, height, outPixels );
        return dest;
	}
	
	private double getDistance(int centerX, int centerY, int px, int py) {
		double xx = (centerX - px)*(centerX - px);
		double yy = (centerY - py)*(centerY - py);
		return (int)Math.sqrt(xx + yy);
	}

}

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