Get a bitmap for the DeviceN alpha channel.
int pdfGetDeviceNAlphaBitmap(PDFHandle pdf, int rowPad, int upsideDown, PDFImageHeader *imgHdr, char **bits)
This function retrieves the bitmap for the alpha channel generated by the last call to pdfConvertPageToDeviceNImageWithAlpha.

The bitmap will be an 8-bit grayscale bitmap, indicating the alpha value at each pixel. The bitmap is handled in the same way as for pdfConvertPageToBitmap2 in pdfImageGray mode:

Note: this function is only useful with pdfConvertPageToDeviceNImageWithAlpha and pdfConvertRegionToDeviceNImageWithAlpha.
int nChannels, i; char *channelName; unsigned int channelCMYK; PDFImageHeader hdr; char *bits; int err; err = pdfConvertPageToDeviceNImageWithAlpha(pdf, pageNum, dpi); if (err != pdfOk) { /* handle the error */ ... } nChannels = pdfGetNumDeviceNChannels(pdf); for (i = 0; i < nChannels; ++i) { channelName = pdfGetDeviceNChannelName(pdf, i); channelCMYK = pdfGetDeviceNChannelCMYK(pdf, i); printf("channel %d: %s = %08x\n", channelName, channelCMYK); pdfGetDeviceNBitmap(pdf, i, 1, 0, &hdr, &bits); /* hdr and bits represent a grayscale bitmap -- similar to pdfConvertPageToBitmap2 with pdfImageGray */ /* each channel has an 8-bit bitmap; the bitmaps for all channels will have the same width and height */ ... /* the bitmap memory must be freed when you are finished */ pdfFreeMemory(bits); } pdfGetDeviceNAlphaBitmap(pdf, 1, 0, &hdr, &bits); /* hdr and bits work the same as for the color channel bitmaps */ ... pdfFreeMemory(bits); /* free the internal storage used by pdfConvertPageToDeviceNImage */ pdfClearDeviceNImage(pdf);