Added very limited conversion for cv::Mat to and from Matlab

Can return 4-chan uchar matrices (isomaps) back to Matlab, and convert 3-chan uchar matrices (images) from Matlab to C++.

matlab/include/mexplus_opencv.hpp

+/*
+ * eos - A 3D Morphable Model fitting library written in modern C++11/14.
+ *
+ * File: matlab/include/mexplus_opencv.hpp
+ *
+ * Copyright 2017 Patrik Huber
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#pragma once
+
+#ifndef MEXPLUS_OPENCV_HPP_
+#define MEXPLUS_OPENCV_HPP_
+
+#include "mexplus/mxarray.h"
+
+#include "opencv2/core/core.hpp"
+
+#include "mex.h"
+
+#include <cstdint>
+#include <vector>
+
+namespace mexplus {
+
+// Note/Todo: Currently only works for 4-chan U8 images
+template <typename InputScalar, typename OutputScalar>
+void deepcopy_and_transpose(const cv::Mat& in, MxArray& out)
+{
+    if (in.channels() != 4)
+    {
+        mexErrMsgIdAndTxt("eos:matlab", "Currently only works for 4-channel U8 images.");
+    }
+
+    const std::size_t num_rows = in.rows;
+    const std::size_t num_cols = in.cols;
+    const std::size_t num_chans = in.channels();
+    out = MxArray::Numeric<std::uint8_t>({num_rows, num_cols, num_chans});
+
+    for (std::size_t c = 0; c < num_cols; ++c) {
+        for (std::size_t r = 0; r < num_rows; ++r) {
+            for (std::size_t chan = 0; chan < num_chans; ++chan) {
+                out.set(std::vector<mwIndex>{r, c, chan}, in.at<cv::Vec4b>(r, c)[chan]);
+            }
+        }
+    }
+};
+
+// Note/Todo: Currently only works for 3-chan U8 images
+template <typename InputScalar, typename OutputScalar>
+void deepcopy_and_transpose(const MxArray& in, cv::Mat& out)
+{
+	auto r = in.rows();
+	auto c = in.cols(); // if dimensionSize() == 3, this returns cols * third_dim
+	auto d = in.dimensions();
+	auto ds = in.dimensionSize();
+	auto e = in.elementSize(); // Number of bytes required to store each data element
+	auto s = in.size();
+	auto n = in.isNumeric();
+	
+	const auto num_channels = [num_dims = in.dimensionSize()]() {
+		if (num_dims == 1 || num_dims == 2)
+		{
+			return 1;
+		}
+		else if (num_dims == 3) {
+			return 3;
+		}
+		// What about 4-channel images, what dimensionSize() will they have?
+	}();
+
+	const auto actual_cols = [&num_channels, num_cols = in.cols()]() {
+		if (num_channels == 3)
+		{
+			return num_cols / num_channels;
+		}
+		else {
+			return num_cols;
+		}
+	}();
+
+    std::vector<cv::Mat> channels;
+    for (size_t c = 0; c < num_channels; ++c)
+    {
+        cv::Mat outmat;
+		// Note: I think this doesn't actually copy the data, does it?
+        cv::Mat inmat(actual_cols, in.rows(), cv::DataType<InputScalar>::type,
+            static_cast<void*>(const_cast<InputScalar*>(in.getData<InputScalar>() + actual_cols * in.rows() * c)));
+        inmat.convertTo(outmat, cv::DataType<OutputScalar>::type);
+        cv::transpose(outmat, outmat);
+        channels.push_back(outmat);
+    }
+    cv::merge(channels, out);
+};
+
+/**
+ * @brief Convert a 4-channel uchar cv::Mat to a Matlab matrix.
+ *
+ * Currently, only CV_8UC4 matrices are supported. Don't try to convert anything else.
+ * Todo: Add some error detection, it will silently fail or crash when the type/channels are
+ * not correct.
+ */
+template <>
+mxArray* MxArray::from(const cv::Mat& opencv_matrix)
+{
+    MxArray out_array;
+    deepcopy_and_transpose<std::uint8_t, std::uint8_t>(opencv_matrix, out_array);
+    return out_array.release();
+};
+
+/**
+ * @brief Convert a 3-channel uint8 Matlab matrix to a cv::Mat.
+ *
+ * Currently only works for 3-channel uint8 Matlab matrices (i.e. images)!
+ * Todo: Add some error detection, it will silently fail or crash when the type/channels are
+ * not correct.
+ */
+template <>
+void MxArray::to(const mxArray* in_array, cv::Mat* opencv_matrix)
+{
+    MxArray array(in_array);
+    deepcopy_and_transpose<std::uint8_t, std::uint8_t>(array, *opencv_matrix);
+};
+
+} /* namespace mexplus */
+
+#endif /* MEXPLUS_OPENCV_HPP_ */