I was using opencv 2.3 o my project two years ago with the cvSet2D, but i use the command with opencv 3.4 i receive the message undeclared identifier

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assuming, Activerows, Activecols actually mean, what they say,
try to stick with conventional x:cols, y:rows indexing

	Mat image(Activerows, Activecols, CV_8UC3);  // Mat is 'row major'

	for (int x = 0; x<Activecols; x++)
		for (y = 0; y<Activerows; y++)
		{
            // get a *reference* to the pixel,
            // make sure to get the type right, CV_8UC3 <--> Vec3b
			Vec3b &pixel_val = image.at<Vec3b>(y,x);//toutes les composantes YCrCb
			pixel_val[0] = matrice[y][x];//Y
			pixel_val[1] = ????;//Cr
			pixel_val[2] = ????;//Cb
            // no need to 'set' anything, it's a reference
		}

for further reference, please read tutorials