canny edge detection with two input arrays

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Hi,

I saw a version of the canny function with two input arrays in the samples/cpp/edge.cpp. Here is a code excerpt:

/// Canny detector with scharr
Mat dx,dy;
Scharr(blurImage,dx,CV_16S,1,0);
Scharr(blurImage,dy,CV_16S,0,1);
Canny(dx, dy, edge2, edgeThreshScharr, edgeThreshScharr*3);
/// Using Canny's output as a mask, we display our result
cedge = Scalar::all(0);
image.copyTo(cedge, edge2);
imshow(window_name2, cedge);

All the documentation/API I can find just has the single input array into canny like so:

void Canny(InputArray image, OutputArray edges, double threshold1, double threshold2, int apertureSize=3, bool L2gradient=false )

And so what am I missing?