pdfLoadMPDeviceNChannels
Get the list of DeviceN color channels used on a page (multi-pass DeviceN API).
void pdfLoadMPDeviceNChannels(PDFHandle pdf, int page)
This function analyzes the specified page and constructs a list of all
DeviceN color channels (CMYK + spot colors) used on that page. The
resulting list can be read with the
pdfGetNumMPDeviceNChannels
and pdfGetMPDeviceNChannelName functions.
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);