**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]

**e.g. increasing brightness of point (100,100) by 30 can be done this way:**

CvPoint pt = {100,100};

((uchar*)(img->imageData + img->widthStep*pt.y))[pt.x*3] += 30;

((uchar*)(img->imageData + img->widthStep*pt.y))[pt.x*3+1] += 30;

((uchar*)(img->imageData + img->widthStep*pt.y))[pt.x*3+2] += 30;

**or more efficiently**

CvPoint pt = {100,100};

uchar* temp_ptr = &((uchar*)(img->imageData + img->widthStep*pt.y))[x*3];

temp_ptr[0] += 30;

temp_ptr[1] += 30;

temp_ptr[2] += 30;

**Suppose, we have 32-bit floating point, 1-channel image I (IplImage* img):**

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

** suppose, we have N-channel image of type T:**

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

or you may use macro CV_IMAGE_ELEM( image_header, elemtype, y, x_Nc )

I(x,y)c ~ CV_IMAGE_ELEM( img, T, y, x*N + c )

There are functions that work with arbitrary (up to 4-channel) images and matrices (cvGet2D, cvSet2D), but they are pretty slow.

((uchar*)(cvImg->imageData+ cvImg->widthStep*j)[i]) is quite messy,

but it is more efficient than elegant looking cvGet2D.

or you can use CV_IMAGE_ELEM(cvImg,uchar,j,i) macro,

but remember j refer to row or related to heigh, and i related to width/column.

CvMat * mat;

mat = ....

cvSave("fn.txt", mMatrix);

cvLoad("fn.txt");

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