RenderTargetBitmap seems to be anti-aliasing my geometry drawing

Using C# VS 2008 Sp1 with .NET 3.5 sp1.

When I tried to render some geometries to a bitmap, created an Image and added the bitmap as the Source finally output the Image to a file, I noticed that the line work was anti-aliased (or something equivalent). The PathGeometry objects are solid, single color lines and polygons. I absolutely must NOT have any new colors created for this imaging application.

I hope there is a way to turn off this anti-aliasing; Can anyone point me to where and why this could be happening and how I can turn it off  ? I read that text is always output anti-aliased but please tell me that is not the case for general drawing !