pdfGetNumMPDeviceNChannels
Get the number of DeviceN color channels used on a page (multi-pass DeviceN API).
int pdfGetNumMPDeviceNChannels(PDFHandle pdf)
This function returns the number of DeviceN color channels in the list
generated by the last call to
pdfLoadMPDeviceNChannels.
The "MP" in the function names refers to multi-pass DeviceN
rasterization support. These functions are different from
pdfGetNumDeviceNChannels and pdfGetDeviceNChannelName, which
look at the list of channels used in the most recent DeviceN
rasterization (via pdfConvertPageToDeviceNImage, etc).
XpdfRasterizer is limited to rasterizing 32 color channels (4 CMYK channels + 28 spot channels) at a time. Normally, any channels beyond those 32 are converted to CMYK. The multi-pass support allows running multiple passes to rasterize any number of channels.
C:
// Print the list of all channels used on <page>.
// This list may contain more than 32 channels.
pdfLoadMPDeviceNChannels(pdf, page);
nChannels = pdfGetNumMPDeviceNChannels(pdf);
for (i = 0; i < nChannels; ++i) {
printf("channel %2d: %s\n", i, pdfGetMPDeviceNChannelName(pdf, i));
}
// All rasterizations include CMYK as the first four channels, and
// the limit is 32 channels, which means we can rasterize 28 spot
// channels per run. For the first run we write the CMYK bitmaps;
// for subsequent runs we skip them (they'll be identical).
i = 4;
do {
// rasterize CMYK (channels 0..3) + spot channels i .. i+28
pdfResetMPDeviceNChannelList(pdf);
for (j = i; j < i + 28 && j < nChannels; ++j) {
pdfAddMPDeviceNChannel(pdf, pdfGetMPDeviceNChannelName(pdf, j));
}
err = pdfConvertPageToDeviceNImage(pdf, page, dpi);
if (err != pdfOk) {
...
}
for (j = (i == 4) ? 0 : 4; j < pdfGetNumDeviceNChannels(pdf); ++j) {
channelName = pdfGetDeviceNChannelName(pdf, j);
pdfGetDeviceNBitmap(pdf, j, 1, 0, &hdr, &bits);
... use the bitmap for <channelName> ...
pdfFreeMemory(bits);
}
pdfClearDeviceNImage(pdf);
i += 28;
} while (i < nChannels);