how to get value of pixel from image?

[h4ever]

From one tutorial I have taken this code. I added the second block of loops and I have error on line with alpha * (int) p + beta . I cannot find out how to assign the value from image. I tried this: p = *( image.ptr<uchar>(i) );.
The error is error C2440: '=' : cannot convert from 'uchar' to 'uchar *' on line 50

#include <opencv2/opencv.hpp>
#include <iostream>
using namespace cv;
double alpha; /*< Simple contrast control */
int beta; /*< Simple brightness control */
int main( int argc, char** argv )
{
	cv::Mat image = imread( argv[1] );
	if ( image.empty() )
		{
		std::cout << "image not found" << std::endl;
		return -1;
		}
	if ( image.channels() != 3)
		{
		std::cout << "not 3 channels in image" << std::endl;
		return -2;
		}
	cv::Mat new_image = Mat::zeros( image.size(), image.type() );
	std::cout<<" Basic Linear Transforms "<<std::endl;
	std::cout<<"-------------------------"<<std::endl;
	std::cout<<"* Enter the alpha value [1.0-3.0]: ";std::cin>>alpha;
	std::cout<<"* Enter the beta value [0-100]: "; std::cin>>beta;
	double t = (double) getTickCount();
	for( int y = 0; y < image.rows; y++ ) {
	  for( int x = 0; x < image.cols; x++ ) {
		for( int c = 0; c < 3; c++ ) {
		  new_image.at<Vec3b>(y,x)[c] =
			 saturate_cast<uchar>( 
			   alpha*( image.at<Vec3b>(y,x)[c] ) + beta 
			 );
		}
	  }
	}
	t = (getTickCount()-t)/getTickFrequency();
	std::cout << ".at() time:" << t << "ms" << std::endl;
	/** 0.45ms **/
	
	char p;
	t = (double) getTickCount();
	for( int y = 0; y < image.rows; y++ ) {
	  for( int x = 0; x < image.cols; x++ ) {
		for( int c = 0; c < 3; c++ ) {
			int i = y*x+c;
			p = *( image.ptr<uchar>(i) );
			
			new_image.ptr<uchar>(i) =
			 saturate_cast<uchar>( 
			   alpha * (int) p + beta 
			 );
			
		}
	  }
	}
	t = (getTickCount()-t)/getTickFrequency();
	std::cout << ".at() time:" << t << "ms" << std::endl;

	namedWindow("Original Image", 1);
	namedWindow("New Image", 1);



	imshow("Original Image", image);
	imshow("New Image", new_image);
	waitKey();
return 0;
}