## Friday, January 28, 2005

### OpenCV: How to access image pixels

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);