How to set a region (roi) ? about opencv4nodejs HOT 5 CLOSED

/**
 * Places a source matrix onto a dest matrix.
 * Note: This replaces pixels entirely, it doesn't merge transparency.
 * Assumes 4 channels or fewer
 * @param {cv.Mat} source_mat matrix being copied
 * @param {cv.Mat} dest_mat matrix being pasted into
 * @param {number} x horizontal offset of source image
 * @param {number} y vertical offset of source image
 */
function overlayOnto(source_mat, dest_mat, x, y){
	if(source_mat.channels != dest_mat.channels) throw new Error('src and dst channel counts must match');
	let source_uint8 = new Uint8Array( source_mat.getData() ); // WARNING 4 CHANNELS
	let dest_uint8 = new Uint8Array( dest_mat.getData() ); // WARNING 4 CHANNELS
	let dest_width = dest_mat.cols;
	let x_count = 0; // set counters
	let y_count = 0; // set counters
	let channel_count = source_mat.channels;
	for (let i = 0; i < source_uint8.length; i += channel_count) { // WARNING 4 CHANNELS
		let dest_x = x_count + x; // add offset
		let dest_y = y_count + y; // add offset
		// only write pixels inside dest mat boundaries
		if( !( (dest_x < 0 || dest_x > dest_mat.cols-1) || (dest_y < 0 || dest_y > dest_mat.rows-1) ) ){
			// get buffer index relative to coordinates
			let dest_i = (dest_x + dest_width * dest_y); // (x + w * h) to get x/y coord in single-dimension array
			let dest_buffer_i = dest_i * channel_count;

			// for some reason, using set is faster for first time function is called, but then slower for each preceeding one???
			// let this_pixel = [];
			// for(let ii=0, l=channel_count; ii<=l; ii++){
			// 	this_pixel[ii] = source_uint8[i+ii]
			// }
			// dest_uint8.set(this_pixel, dest_buffer_i); 

			// fill into buffer
			if(channel_count >= 1)	dest_uint8.fill(source_uint8[i+0], dest_buffer_i+0 , dest_buffer_i+0+1);
			if(channel_count >= 2)	dest_uint8.fill(source_uint8[i+1], dest_buffer_i+1 , dest_buffer_i+1+1);
			if(channel_count >= 3)	dest_uint8.fill(source_uint8[i+2], dest_buffer_i+2 , dest_buffer_i+2+1);
			if(channel_count >= 4)	dest_uint8.fill(source_uint8[i+3], dest_buffer_i+3 , dest_buffer_i+3+1);
		}
		x_count++; // increase col
		x_count = x_count % source_mat.cols; // end of col? move to start
		if(x_count == 0) y_count++; // started new col? increase row 
	}
	return new cv.Mat(dest_uint8, dest_mat.rows, dest_mat.cols, dest_mat.type);
}