Write a rectangular region to an image file.
convertRegionToBMP2([in] int page, [in] double x0, [in] double y0, [in] double x1, [in] double y1, [in] double dpi, [in] int color, [in] BSTR fileName)
This function converts a rectangular region of a PDF page to an image file on disk, in BMP format. This can be useful if the page is too large to fit in memory at the desired resolution, or if only part of the page is needed for some purpose.

The rectangular region is specified by the x,y coordinates of its upper-left and lower-right corners (in PDF coordinates).

The arguments are:

Note that the color settings are read-only properties on the XpdfViewer control, so you'll need to use viewer.imageMono, etc.
' convert the bottom-left corner of page 5 to a 72 dpi color BMP file viewer.convertRegionToBMP2 5, 0, 0, 144, 144, 72, viewer.imageRGB, "test1.bmp"