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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