Converting a SoftwareBitmap or WriteableBitmap to cv::Mat in c++/cx - c++

Question

I tried to convert a WriteableBitmap to a cv::Mat in a c++/cx Microsoft universial App. But when I try to progress with the created Mat, I get the following error:
This is my Code:
void App1::MainPage::processImage(SoftwareBitmap^ bitmap)
{
WriteableBitmap^ wb = ref new WriteableBitmap(bitmap->PixelWidth, bitmap->PixelHeight);
bitmap->CopyToBuffer(wb->PixelBuffer);
Mat img_image(wb->PixelHeight, wb->PixelWidth, CV_8UC3,(void*)wb->PixelBuffer);
//next step results in error
cvtColor(img_image, img_image, CV_BGR2BGRA);
...
}
So my final question:
How to convert the SoftwareBitmap or the WriteableBitmap to a cv::Mat?

Answer 1

The problem is that PixelBuffer is not a void *, it is an IBuffer^.
To get at the raw data, you can either use the IBufferByteAccess interface if you're comfortable with COM programming, or you can initialize a DataReader with an IBuffer if you'd prefer to stay in WinRT (although this technique will make a copy of the data).

Answer 2

I used the DataReader to solve the problem:
void App1::MainPage::processImage(SoftwareBitmap^ bitmap)
{
WriteableBitmap^ wb = ref new WriteableBitmap(bitmap->PixelWidth, bitmap->PixelHeight);
bitmap->CopyToBuffer(wb->PixelBuffer);
IBuffer^ buffer = wb->PixelBuffer;
auto reader = ::Windows::Storage::Streams::DataReader::FromBuffer(buffer);
BYTE *extracted = new BYTE[buffer->Length];
reader->ReadBytes(Platform::ArrayReference<BYTE>(extracted, buffer->Length));
Mat img_image(wb->PixelHeight, wb->PixelWidth, CV_8UC4, extracted);
cvtColor(img_image, img_image, CV_RGBA2BGRA);
...
}
Thx to Peter Torr for the hint.