WriteableBitmap has two ways to get the visuals of a UIElement onto a bitmap. The first uses one of the constructors:
WriteableBitmap writeableBitmap = new WriteableBitmap(element, transform);
The element argument is of type UIElement and the transform argument is of type Transform. This constructor creates a bitmap based on the size of the UIElement argument as possibly modified by the Transform argument (which you can set to null).
The element and all its visual children are rendered on the bitmap. However, any RenderTransform applied to that element is ignored. Optionally taking account of that transform is the rationale behind the second argument. The resultant bitmap is based on the maximum horizontal and vertical ...