几种不同的彩色图像转灰度步骤

       最近在分析几个开源图像处理程序时，发现了几种不同的彩色图像转灰度方法，方法都很简单，也非常容易理解，但是效果还是有明显不同。在介绍计算方法前，先要说说亮度（lightness）与照度（luminosity）的区别。亮度（lightness），指的是人在看到光源时，眼睛感觉到的光亮度。照度（luminosity），指的是光源照射到物体上，单位被照射面积上的光通量。可以说，亮度是物体反射光到眼里的强弱，是人的一种主观感觉；而照度是光源照在物体上的强弱，是一种客观的物理量。同样的光源、同样的距离照在白纸和黑纸上，亮度不同，照度相同。下面是简单代码示例：

       1. 基于均值的方法：

uchar *pSrcImg = src.data;
for (int i = 0; i < src.rows; i++)
{
	for (int j = 0; j < src.cols; j++, pSrcImg += 3)
	{
		double average = (pSrcImg[0] + pSrcImg[1] + pSrcImg[2]) / 3;
		pSrcImg[0] = (uchar)MIN(255, average);
		pSrcImg[1] = (uchar)MIN(255, average);
		pSrcImg[2] = (uchar)MIN(255, average);
	}
}

       2. 基于亮度的方法：

uchar *pSrcImg = src.data;
for (int i = 0; i < src.rows; i++)
{
	for (int j = 0; j < src.cols; j++, pSrcImg += 3)
	{
		uchar max_val = MAX(pSrcImg[0], pSrcImg[1]);
		max_val = MAX(max_val, pSrcImg[2]);
		uchar min_val = MIN(pSrcImg[0], pSrcImg[1]);
		min_val = MIN(min_val, pSrcImg[2]);

		double lightness = (max_val + min_val) / 2;
		pSrcImg[0] = (uchar)MIN(255, lightness);
		pSrcImg[1] = (uchar)MIN(255, lightness);
		pSrcImg[2] = (uchar)MIN(255, lightness);
	}
}

       3. 基于照度的方法：

// HDTV with BT.709
uchar *pSrcImg = src.data;
for (int i = 0; i < src.rows; i++)
{
	for (int j = 0; j < src.cols; j++, pSrcImg += 3)
	{
		double luminosity = pSrcImg[0] * 0.0722 + pSrcImg[1] * 0.7152 + pSrcImg[2] * 0.2126;
		pSrcImg[0] = (uchar)MIN(255, luminosity);
		pSrcImg[1] = (uchar)MIN(255, luminosity);
		pSrcImg[2] = (uchar)MIN(255, luminosity);
	}
}

// SDTV with BT.601
uchar *pSrcImg = src3.data;
for (int i = 0; i < src.rows; i++)
{
	for (int j = 0; j < src.cols; j++, pSrcImg += 3)
	{
		double luminosity = pSrcImg[0] * 0.114 + pSrcImg[1] * 0.587 + pSrcImg[2] * 0.299;
		pSrcImg[0] = (uchar)MIN(255, luminosity);
		pSrcImg[1] = (uchar)MIN(255, luminosity);
		pSrcImg[2] = (uchar)MIN(255, luminosity);
	}
}