Convert a rectangular region to a DeviceN image with an alpha channel.
convertRegionToDeviceNImageWithAlpha([in] int page, [in] int regionX, [in] int regionY, [in] int regionW, [in] int regionH, [in] double dpi)
This function rasterizes a rectangular region of a PDF page in DeviceN mode, i.e., with separate color channels for each process color (cyan, magenta, yellow, and black), and for each custom (a.k.a., spot) color.

The rectangular region is specified by the x,y coordinates of its upper-left corner and its width and height - in bitmap coordinates, not PDF coordinates. That is, if the whole-page bitmap would have been 1000 pixels wide and 2000 pixels high, and you request a region with (x,y) = (0,0) and (w,h) = (1000,500), the resulting bitmap will be the top fourth of the page.

This function differs from convertRegionToDeviceNImage in that it skips the final page compositing step, and maintains an alpha bitmap. The alpha bitmap can be retrieved with getDeviceNAlphaPicture.

Note: The alpha bitmap is only useful with PDF files that have been constructed with a transparent background.

After calling convertRegionToDeviceNImage:

Finally, call clearDeviceNImage to release the memory used by convertRegionToDeviceNImageWithAlpha.

To enable an overprint preview rasterization, call setConfig to set the overprintPreview parameter before calling convertRegionToDeviceNImage:

pdf.setConfig "overprintPreview yes"
rast.convertRegionToDeviceNImage page, 0, 0, 1000, 500, dpi nChannels = rast.getNumDeviceNChannels For i = 0 To nChannels-1 channelName = rast.getDeviceNChannelName(i) channelCMYK = rast.getDeviceNChannelCMYK(i) pic = rast.getDeviceNPicture(i) ' each channel has an 8-bit grayscale bitmap, similar to what ' is returned by convertPageToPicture in imageGray mode ... Next i pic = rast.getDeviceNAlphaPicture ' the returned image is similar to the color channel images ... rast.clearDeviceNImage