opencv_grabber should use YARP_cv utilities

[Nicogene]

opencv_grabber converts the cv::Mat to a yarp::sig::image with a memcpy.

yarp/src/devices/opencv/OpenCVGrabber.cpp

Lines 196 to 200 in 0ac66ed

	cv::Mat frame_rgb;
	cv::cvtColor(frame, frame_rgb, cv::COLOR_BGR2RGB);
	// Copy the captured image to the output image
	memcpy(image.getRawImage(), frame_rgb.data, sizeof(unsigned char) * frame_rgb.rows * frame_rgb.cols * frame_rgb.channels());

We should use instead fromCvMat (introduced in #1932) that does not involves copies:

yarp/src/libYARP_cv/src/yarp/cv/Cv-inl.h

Lines 90 to 112 in 0ac66ed

	template<typename T>
	yarp::sig::ImageOf<T> yarp::cv::fromCvMat(::cv::Mat& cvImage)
	{
	constexpr size_t align_8_bytes = 8;
	constexpr size_t align_4_bytes = 4;
	yarp::sig::ImageOf<T> outImg;
	// Checking cv::Mat::type() compatibility with the T PixelType
	assert(yarp::cv::type_code<T>::value == cvImage.type());
	if (convert_code_from_cv<T>::value >= 0)
	{
	::cv::cvtColor(cvImage, cvImage, convert_code_from_cv<T>::value);
	}
	// Check the cv::Mat alignment
	if (cvImage.step % align_8_bytes == 0) {
	outImg.setQuantum(align_8_bytes);
	}
	else if (cvImage.step % align_4_bytes == 0) {
	outImg.setQuantum(align_4_bytes);
	}
	outImg.setExternal(cvImage.data, cvImage.cols, cvImage.rows);
	return outImg;
	}

[PeterBowman]

I'm afraid it is not viable, or at least not as straightforward as a simple invocation of yarp::cv::fromCvMat. Since no copies are involved, but rather yarp::sig::Image holds a pointer to a memory location managed by an instance of cv::Mat, the lifetime of the latter is critical. However, this cv::Mat dies as soon as the getImage method is exited, therefore the wrapping grabberdual device attempts to forward a bogus yarp::sig::Image over the network (the memory location of the desired image may be erased at that point).

[drdanz]

Fixed by @PeterBowman in #2713