ISP自动白平衡：完美反射算法

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前言

之前学习了ISP自动白平衡 - 灰度世界算法，这里继续跟大家分享下第二个经典算法 - 完美反射算法。

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1. 完美反射算法介绍

完美反射算法是选取图像中R/G/B三通道中像素值最大的点作为白点，以此来更新图像，实现图像白平衡。
算法步骤：

1. 创建一个一维数组用来保存R/G/B三通道像素点的和，数组大小为766（每通道像素值范围在0-255， 三个通道像素值的和在0-765，所以定义数组大小为766）；
2. 遍历图像，填充步骤1定义的数组，并统计图像中最大像素值MaxVal；
3. 查找阈值：按照从大到小的索引顺序遍历步骤1中的数组，并指定一个像素点数比率（比如0.1），当步骤1中数组累积像素点数大于图像像素点数和定义的比率之积时，此时的索引值即为阈值；
4. 遍历图像，当每一像素点R/G/B三通道值的和大于步骤三定义的阈值时，分别统计R/G/B三通道像素值的和以及满足阈值的像素点数；
5. 根据步骤4计算得到的R/G/B三通道像素值的和与统计的像素点数，分别计算R/G/B三通道像素均值，记为$R_{mean},G_{mean},B_{mean}$；
6. 遍历图像， 根据步骤5计算的R/G/B像素均值和步骤2获得的最大像素值，对图像R/G/B三通道像素值进行更新，更新公式：
   $R=\frac{R\ast MaxVal}{R_{mean}},B=\frac{B\ast MaxVal}{B_{mean}},G=\frac{G\ast MaxVal}{G_{mean}}$

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2. 完美反射算法C++ Opencv实现

#include 
#include 
#include 
#include 
#include 
#include 

using namespace cv;

// Auto White Balance - Gray World Algorithm
int AWB_GrayWorld(InputArray src, OutputArray dst)
{
	CV_Assert(src.channels() == 3, "AWB_GrayWorld() input image must be 3 channels!");

	Mat mSrc = src.getMat();
	if (mSrc.empty())
	{
		std::cout << "AWB_GrayWorld() input image is empty!" << std::endl;
		return -1;
	}
	
	dst.create(mSrc.size(), mSrc.type());
	Mat mDst = dst.getMat();

	if (mDst.empty())
	{
		std::cout << "AWB_GrayWorld() create dst image failed!" << std::endl;
		return -1;
	}

	//对输入src图像进行RGB分离
	std::vector<Mat> splitedBGR;
	splitedBGR.reserve(3);

	split(mSrc, splitedBGR);

	//分别计算R/G/B图像像素值均值
	double meanR = 0, meanG = 0, meanB = 0;
	meanB = mean(splitedBGR[0])[0];
	meanG = mean(splitedBGR[1])[0];
	meanR = mean(splitedBGR[2])[0];

	//计算R/G/B图像的增益
	double gainR = 0, gainG = 0, gainB = 0;
	gainR = (meanR + meanG + meanB) / (3 * meanR);
	gainG = (meanR + meanG + meanB) / (3 * meanG);
	gainB = (meanR + meanG + meanB) / (3 * meanB);

	//计算增益后R/G/B图像
	splitedBGR[0] = splitedBGR[0] * gainB;
	splitedBGR[1] = splitedBGR[1] * gainG;
	splitedBGR[2] = splitedBGR[2] * gainR;

	//将三个单通道图像合成一个三通道图像
	merge(splitedBGR, mDst);

	return 0;
}

int AWB_PerfectReflect(InputArray src, OutputArray dst)
{
	CV_Assert_2(src.channels() == 3, "AWB_PerfectReflect() src image must has 3 channels!");

	Mat mSrc = src.getMat();
	if (mSrc.empty())
	{
		std::cout << "AWB_PerfectReflect() src image can't be empty!" << std::endl;
		return -1;
	}

	dst.create(mSrc.size(), mSrc.type());
	Mat mDst = dst.getMat();

	int sumHist[766] = { 0 };//max(R+G+B) = 255*3 = 765, 0~765->766
	int maxVal = 0;

	for (int i = 0; i < mSrc.rows; i++)
	{
		for (int j = 0; j < mSrc.cols; j++)
		{
			Vec3b p = mSrc.at<Vec3b>(i, j);
			int sum = p[0] + p[1] + p[2];
			sumHist[sum]++;
			maxVal = maxVal > p[0] ? maxVal : p[0];
			maxVal = maxVal > p[1] ? maxVal : p[1];
			maxVal = maxVal > p[2] ? maxVal : p[2];
		}
	}

	int totalPixels = 0;
	for (int i = 765; i >= 0; i--)
	{
		totalPixels += sumHist[i];
	}

	CV_Assert_2(totalPixels == mSrc.rows*mSrc.cols, "sumHist pixels number isn't equal with image size!");

	float ratio = 0.1;
	int cumPixel = 0;
	int threshold = 0;
	for (int i = 765; i >= 0; i--)
	{
		cumPixel += sumHist[i];
		if (cumPixel >= ratio * mSrc.rows* mSrc.cols)
		{
			threshold = i;
			break;
		}
	}

	int avgB = 0, avgG = 0, avgR = 0;
	int countPixels = 0;
	for (int i = 0; i < mSrc.rows; i++)
	{
		for (int j = 0; j < mSrc.cols; j++)
		{
			Vec3b p = mSrc.at<Vec3b>(i, j);
			int sum = p[0] + p[1] + p[2];
			if (sum > threshold)
			{
				countPixels++;
				avgB += p[0];
				avgG += p[1];
				avgR += p[2];
			}
		}
	}

	avgB /= countPixels;
	avgG /= countPixels;
	avgR /= countPixels;

	for (int i = 0; i < mSrc.rows; i++)
	{
		for (int j = 0; j < mSrc.cols; j++)
		{
			Vec3b p = mSrc.at<Vec3b>(i, j);
			int B = p[0] * maxVal / avgB;
			B = B > 255 ? 255 : B;
			mDst.at<Vec3b>(i, j)[0] = (uchar)B;

			int G = p[1] * maxVal / avgG;
			G = G > 255 ? 255 : G;
			mDst.at<Vec3b>(i, j)[1] = (uchar)G;

			int R = p[2] * maxVal / avgR;
			R = R > 255 ? 255 : R;
			mDst.at<Vec3b>(i, j)[2] = (uchar)R;
		}
	}

	return 0;
}

int main()
{
	std::string imgPath = "C:\\Temp\\common\\Workspace\\Opencv\\images\\awb_grayworld.jpg";
	Mat src = imread(imgPath);
	Mat dstGW;
	int status = AWB_GrayWorld(src, dstGW);
	if (status != 0)
		goto EXIT;

	imshow("src", src);
	imshow("AWB GrayWorld", dstGW);
	waitKey(0);

	{
		Mat dstPR;
		status = AWB_PerfectReflect(src, dstPR);
		if (status != 0)
			goto EXIT;

		imshow("AWB PerfectReflect", dstPR);
		waitKey(0);
	}

EXIT:
	system("pause");
	destroyAllWindows();

	return 0;
}

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3. 执行结果

原图：

灰度世界算法结果：

完美反射算法结果：

总结

从结果来看，完美反射算法结果要好一些，但是如果图像最亮点不是白点的话，效果不佳。

参考

https://www.cnblogs.com/Imageshop/archive/2013/04/20/3032062.html