how do I downsample the output of kinect?

See more:

C#

Kinect

Hello everyone!
I am trying to downsample the RGB output of kinect to 40x40 using C#. I have been using writeablebitmap to get the RGB output however, I could not find a good way to convert writeablebitmap to bitmap where I can downsample.
I tried a couple ways including saving the writeablebitmap frame as a JPEG then downsampling the JPEG but after the first frame, it would not work.

I was wondering if there is any better way to downsample the output of kinect.