Function overlay

jni/venus/region_operation.cpp:1200–1257

}

Mat overlay(const Mat& src, Mat& dst, const Point& origin, int alpha)
{
	assert(src.channels() == 4 && dst.channels() == 4);  // use alpha channel to blend
	assert(0 <= alpha && alpha < 256);

	Size size(src.cols, src.rows);
	Rect rect1(origin, size);
	Rect rect2(0, 0, dst.cols, dst.rows);
	
	// http://docs.opencv.org/2.4/modules/core/doc/basic_structures.html?highlight=rect#Rect_
	rect1 &= rect2;
	if(rect1.area() <= 0)  // not overlap
		return dst;
	
	// https://en.wikipedia.org/wiki/Alpha_compositing
	// if dst is BGR format, treat A as 255, namely opaque.
	Mat roi = dst(rect1);
	for(int r = 0; r < roi.rows; ++r)
	for(int c = 0; c < roi.cols; ++c)
	{
		const cv::Vec4b& src_bgra = src.at<cv::Vec4b>(r, c);
		cv::Vec4b& dst_bgra = roi.at<cv::Vec4b>(r, c);
//		cv::Vec3b& dst_bgr = dst_has_alpha? *reinterpret_cast<cv::Vec3b*>(&roi.at<cv::Vec4b>(r, c)) : roi.at<cv::Vec3b>(r, c);
		
//		cv::Vec3i src_bgr(src_bgra[0], src_bgra[1], src_bgra[2]);

		const uchar& src_a = src_bgra[3];

		//(src_bgr * src_a + dst_bgr * 255 * (1 - src_a)) / (src_a + 255 * (1 - src_a));
		//dst_color = dst_color * 255 +
		// https://docs.gimp.org/en/gimp-concepts-layer-modes.html
		// overlay
		if(src_a == 0)
			continue;

		float t = static_cast<float>(alpha * src_a)/(255 * 255);
		float one_minuse_t = 1.0f - t;

        dst_bgra[0] = cvRound((one_minuse_t * dst_bgra[0] + t * src_bgra[0]));
        dst_bgra[1] = cvRound((one_minuse_t * dst_bgra[1] + t * src_bgra[1]));
        dst_bgra[2] = cvRound((one_minuse_t * dst_bgra[2] + t * src_bgra[2]));
//		for(int i = 0; i < 3; ++i)
//		{
//			int value = cvRound((1.0f - t) * dst_bgra[i] + t * src_bgra[i]);
			
//			float value = dst_bgra[i]/255.0f * (dst_bgra[i] + 2* src_bgra[i]/255.0f*(255.0f - dst_bgra[i]));
//			assert(0 <= value && value <= 255.0f);
//			dst_bgr[i] = static_cast<uchar>(value);
//			int value = ((255 - src_a) * dst_bgra[i] + src_a * src_bgra[i]) / 255;
//			float value = (1.0f - alpha) * dst_bgra[i] + alpha * src_bgra[i];
//			assert(0 <= value && value <= 255);
//			dst_bgra[i] = static_cast<uchar>(value);
//		}

	}

	return dst;
}