The pixels array is stored in the "data" attribute of cv::Mat. Let's suppose that we have a Mat matrix where each pixel has 3 bytes (CV_8UC3).
For this example, let's draw a RED pixel at position 100x50.
Mat foo;
int x=100, y=50;
Solution 1:
Create a macro function that obtains the pixel from the array.
#define PIXEL(frame, W, x, y) (frame+(y)*3*(W)+(x)*3)
//...
unsigned char * p = PIXEL(foo.data, foo.rols, x, y);
p[0] = 0; // B
p[1] = 0; // G
p[2] = 255; // R
Solution 2:
Get's the pixel using the method ptr.
unsigned char * p = foo.ptr(y, x); // Y first, X after
p[0] = 0; // B
p[1] = 0; // G
p[2] = 255; // R