DrawImage with NearestNeighbor-Interpolation

[C# Programming]

Hello,
does anyone know where I can find the function which maps the pixel coordinates from the source to the destination?

I draw a (stretched) bitmap using

InterpolationMode::NearestNeighbor

and

PixelOffsetMode::Half

onto a canvas and now want to know which pixel was mapped to a certain position - so that if I click with the mouse on that stretched pixel, I get the original coordinates.

Thank you in advance!

Alex