图像处理之调整亮度与对比度
很多时候，一张图像被过度曝光显得很白，或者光线不足显得很暗，有时候背景跟图像人物
也观察不清楚，这个时候可以通过调节图像的两个基本属性-亮度与对比度来获得整体效果
的提升，从而得到质量更高的图片。
基本原理：
图像亮度本质上图像中每个像素的亮度，每个像素的亮度本质上RGB值的大小，RGB值为0
是像素点为黑色，RGB都为255时像素点最亮，为白色。对比度则是不同像素点之间的差值，
差值越大，对比度越明显。从直方图分析的观点来看，对比度越好的图片，直方图曲线会越
明显，分布也越显得均匀。
算法流程：
调整图像亮度与对比度算法主要由以下几个步骤组成：
1. 计算图像的RGB像素均值– M
2. 对图像的每个像素点Remove平均值-M
3. 对去掉平均值以后的像素点 P乘以对比度系数
4. 对步骤上处理以后的像素P加上 M乘以亮度系统
5. 对像素点RGB值完成重新赋值
算法系数
对比度 contrast的最佳取值范围在[0 ~ 4],
亮度 brightness的最佳取值范围在[0~ 2]之间
算法的源程序代码见最后源代码部分
程序效果：
调整亮度与对比度的滤镜源代码如下：
package com.process.blur.study; import java.awt.image.BufferedImage; /** * this filter illustrate the brightness and contrast of the image * and demo how to change the both attribute of the image. * * @author gloomy fish * */ public class ConBriFilter extends AbstractBufferedImageOp { private float contrast = 1.5f; // default value; private float brightness = 1.0f; // default value; public ConBriFilter() { // do stuff here if you need...... } @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]; src.getRGB( 0, 0, width, height, inPixels, 0, width ); // calculate RED, GREEN, BLUE means of pixel int index = 0; int[] rgbmeans = new int[3]; double redSum = 0, greenSum = 0, blueSum = 0; double total = height * width; 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; redSum += tr; greenSum += tg; blueSum +=tb; } } rgbmeans[0] = (int)(redSum / total); rgbmeans[1] = (int)(greenSum / total); rgbmeans[2] = (int)(blueSum / total); // adjust contrast and brightness algorithm, here 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; // remove means tr -=rgbmeans[0]; tg -=rgbmeans[1]; tb -=rgbmeans[2]; // adjust contrast now !!! tr = (int)(tr * getContrast()); tg = (int)(tg * getContrast()); tb = (int)(tb * getContrast()); // adjust brightness tr += (int)(rgbmeans[0] * getBrightness()); tg += (int)(rgbmeans[1] * getBrightness()); tb += (int)(rgbmeans[2] * getBrightness()); outPixels[index] = (ta << 24) | (clamp(tr) << 16) | (clamp(tg) << 8) | clamp(tb); } } setRGB( dest, 0, 0, width, height, outPixels ); return dest; } public int clamp(int value) { return value > 255 ? 255 :(value < 0 ? 0 : value); } public float getContrast() { return contrast; } public void setContrast(float contrast) { this.contrast = contrast; } public float getBrightness() { return brightness; } public void setBrightness(float brightness) { this.brightness = brightness; } }
