C++ opencv edit pixel

#include<opencv2/core/core.hpp>
#include<opencv2/highgui/highgui.hpp>
#include<opencv2/imgproc/imgproc.hpp>

#include<iostream>
#include<conio.h>         

using namespace std;
using namespace cv;

void procFrame(cv::Mat &frame);
int main(int argc,char** argv)
{
VideoCapture cap(0);
if (!cap.isOpened())
{
cout << "Could not open camera" << endl;
return -1;
}
cap.set(CV_CAP_PROP_FRAME_WIDTH, 640);
cap.set(CV_CAP_PROP_FRAME_HEIGHT, 480);

namedWindow("Exam", WINDOW_AUTOSIZE);
while (true)
{
Mat frame;
if (cap.read(frame))
{
procFrame(frame);
imshow("Exam", frame);
}
if (waitKey(30) == 27)
{
break;
}
}
return 0;
}

void procFrame(cv::Mat & frame)
{
static float alpha = 1.0f;
for (unsigned int z = 0; z<frame.rows; z++)
{
for (unsigned int i = 0; i < frame.cols; i++)
{
Vec3b &cur_pixel = frame.at<Vec3b>(z, i);
cur_pixel[0] *= (-alpha);
cur_pixel[1] *= (-alpha);
cur_pixel[2] *= alpha;
}
}
alpha += 0.1f;
}