How to convert PDF to PNG/TIF along with transparency? How do I set the page backdrop to be transparent?

Q: I need to convert PDF to PNG/TIF along with transparency.

We are using the following functions in PDFTron to Export PDF to PNG/

PDFDraw draw = new PDFDraw();
draw.Export(doc.GetPage(1), "c:\test.png", "PNG");

Is there way to retain the alpha channel in the rendered image?
A: To treat the page as transparent backdrop call
pdfdraw.SetPageTransparent(true) before rendering the page. Note:
SetPageTransparent is only available in PDFNet SDK v.4.0.5 and

The following is the relevant documentation for

* Sets the page color to transparent.
* @default By default, PDFDraw assumes that the page is imposed
directly on an
* opaque white surface. Some applications may need to impose the page
on a different
* backdrop. In this case any pixels that are not covered during
rendering will
* be transparent.
* @param is_transparent Set to 'true' if the page's backdrop should
be transparent;
* If is_transparent is false, the page's backdrop will be a opaque
white surface.
* @note If page transparency is enabled, the alpha channel will be
preserved when
* the image is exported as PNG, TIFF, or RAW.
void SetPageTransparent(bool is_transparent)