opencv 中对一个像素的rgb值进行操作的几个常用小办法

江流天地外，山色有无中。江间波浪兼天涌，塞上风云接地阴。水至清则无鱼，人至察则无徒。飞流直下三千尺，疑是银河落九天。姑苏城外寒山寺，夜半钟声到客船。You can access the Image pixels in many ways:
1. One using the Inbuilt macro
2. One using the pointer to the image data
3. Getting the raw data from the image.

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Method 1: Using Inbuilt macro:
CV_IMAGE_ELEM( image_header, elemtype, y, x_Nc )

Suppose, we have 8-bit 1-channel image I (IplImage* img):

I(x,y) ~ CV_IMAGE_ELEM( img, uchar, y, x);

Suppose, we have 8-bit 3-channel image I (IplImage* img):

I(x,y)blue ~ CV_IMAGE_ELEM( img, uchar, y, x*3);
I(x,y)green ~ CV_IMAGE_ELEM( img, uchar, y, x*3+1);
I(x,y)red ~ CV_IMAGE_ELEM( img, uchar, y, x*3+2);

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Method 2: Using pointer to the image data:

Suppose, we have 8-bit 1-channel image I (IplImage* img):

I(x,y) ~ ((uchar*)(img->imageData + img->widthStep*y))[x]

Suppose, we have 8-bit 3-channel image I (IplImage* img):

I(x,y)blue ~ ((uchar*)(img->imageData + img->widthStep*y))[x*3]
I(x,y)green ~ ((uchar*)(img->imageData + img->widthStep*y))[x*3+1]
I(x,y)red ~ ((uchar*)(img->imageData + img->widthStep*y))[x*3+2]

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Method 3: Using the raw data:
uchar *data;
cvGetRawData(img, (uchar**)&data);

I(x, y) = data[x*img->Width + y]

对于rgb图像

读入像素RGB

int B = CV_IMAGE_ELEM(img, unsigned char, row, col*3+0);
int G = CV_IMAGE_ELEM(img, unsigned char, row, col*3+1);
int R = CV_IMAGE_ELEM(img, unsigned char, row, col*3+2);



写入像素RGB

CV_IMAGE_ELEM(img, unsigned char, row, col*3+0) = 90;

CV_IMAGE_ELEM(img, unsigned char, row, col*3+1) = 90;

CV_IMAGE_ELEM(img, unsigned char, row, col*3+2) = 90;

修改单个像素，再说得详细点，


1.3通道时：CV_IMAGE_ELEM(image, unsigned char, i, j*3+k) = gray_val; //0<=k<3

2.单通道时：CV_IMAGE_ELEM(image, unsigned char, i, j) = gray_val;

3.通用方法：CvScalar s;
s=cvGet2D(img,i,j); // get the (i,j) pixel value

s.val[0]=111; //单通道就只有这个有效
s.val[1]=111;
s.val[2]=111;
cvSet2D(img,i,j,s);//set the (i,j) pixel value

CV_IMAGE_ELEM的方法比cvGet2D，cvSet2D快了十倍左右（据说）


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