Converting Caffe caffe::Datum to OpenCV cv::Mat in C++ - c++

Question

I'm doing some debugging and so I'm dumping image files to look at the predictions and transformations.
I can create a caffe::Datum from cv::Mat:
cv::Mat matrix;
// ... initialize matrix
caffe::Datum datum;
caffe::CVMatToDatum(matrix, &datum)
but how do I create a cv::Mat from caffe::Datum? The following code gives the fatal exception "Datum not encoded":
caffe::Datum datum;
// ... initialize datum
cv::Mat matrix;
matrix = DecodeDatumToCVMat(datum, true);

Answer 1

You can use following functions.
cv::Mat DatumToCVMat(const Datum& datum){
int datum_channels = datum.channels();
int datum_height = datum.height();
int datum_width = datum.width();
string strData = datum.data();
cv::Mat cv_img;
if (strData.size() != 0)
{
cv_img.create(datum_height, datum_width, CV_8UC(datum_channels));
const string& data = datum.data();
std::vector<char> vec_data(data.c_str(), data.c_str() + data.size());
for (int h = 0; h < datum_height; ++h) {
uchar* ptr = cv_img.ptr<uchar>(h);
int img_index = 0;
for (int w = 0; w < datum_width; ++w) {
for (int c = 0; c < datum_channels; ++c) {
int datum_index = (c * datum_height + h) * datum_width + w;
ptr[img_index++] = static_cast<uchar>(vec_data[datum_index]);
}
}
}
}
else
{
cv_img.create(datum_height, datum_width, CV_32FC(datum_channels));
for (int h = 0; h < datum_height; ++h) {
float* ptr = cv_img.ptr<float>(h);
int img_index = 0;
for (int w = 0; w < datum_width; ++w) {
for (int c = 0; c < datum_channels; ++c) {
ptr[img_index++] = static_cast<float>(datum.float_data(img_index));
}
}
}
}
return cv_img;
}
all code :
http://lab.deepaivision.com/2017/06/opencv-mat-caffe-datum-datum-mat.html

Answer 2

Addition to the answer by #ttagu99 , in the float section there is a slight bug:
The line:
ptr[img_index++] = static_cast<float>(datum.float_data(img_index));
Should be in fact:
int datum_index = (c * datum_height + h) * datum_width + w;
ptr[img_index++] = static_cast<float>(datum->float_data(datum_index));
This fixes the order of writing the data to the matrix. Otherwise, you only see lines of color.