Apply crystal filter when saving PDF in Mac OS X 10.6.3

Using the Mac OS X API, I'm trying to save a PDF file with the Quartz filter applied, as possible in the Save As dialog in the Preview application. So far I have written the following code (using Python and pyObjC, but it doesn't matter to me):

- filter-pdf.py: begin

from Foundation import *
from Quartz import *
import objc

page_rect = CGRectMake (0, 0, 612, 792)
fdict = NSDictionary.dictionaryWithContentsOfFile_("/System/Library/Filters/Blue
\ Tone.qfilter")
in_pdf = CGPDFDocumentCreateWithProvider(CGDataProviderCreateWithFilename ("test
.pdf"))
url = CFURLCreateWithFileSystemPath(None, "test_out.pdf", kCFURLPOSIXPathStyle, 
False)
c = CGPDFContextCreateWithURL(url, page_rect, fdict)

np = CGPDFDocumentGetNumberOfPages(in_pdf)
for ip in range (1, np+1):
        page = CGPDFDocumentGetPage(in_pdf, ip)
        r = CGPDFPageGetBoxRect(page, kCGPDFMediaBox)
        CGContextBeginPage(c, r)
        CGContextDrawPDFPage(c, page)
        CGContextEndPage(c)

      

        
        
        
      

    

- filter-pdf.py: end

Unfortunalte, no "Blue Tone" filter is applied, the output PDF looks exactly like the input PDF.

Question: what am I missing? How do I apply a filter?

Well, the documentation doesn't promise that this way of creating and using "fdict" should cause the filter to be applied. But I just rewrote (as far as I can) the example code / Developer / Examples / Quartz / Python / filter -pdf.py that was common on older Mac versions (meanwhile this code doesn't work either):

----- filter-pdf-old.py: begin

from CoreGraphics import *
import sys, os, math, getopt, string

def usage ():
  print '''
usage: python filter-pdf.py FILTER INPUT-PDF OUTPUT-PDF

Apply a ColorSync Filter to a PDF document.
'''

def main ():

  page_rect = CGRectMake (0, 0, 612, 792)

  try:
    opts,args = getopt.getopt (sys.argv[1:], '', [])
  except getopt.GetoptError:
    usage ()
    sys.exit (1)

  if len (args) != 3:
    usage ()
    sys.exit (1)

  filter = CGContextFilterCreateDictionary (args[0])
  if not filter:
    print 'Unable to create context filter'
    sys.exit (1)
  pdf = CGPDFDocumentCreateWithProvider (CGDataProviderCreateWithFilename (args[1]))
  if not pdf:
    print 'Unable to open input file'
    sys.exit (1)

  c = CGPDFContextCreateWithFilename (args[2], page_rect, filter)
  if not c:
    print 'Unable to create output context'
    sys.exit (1)

  for p in range (1, pdf.getNumberOfPages () + 1):
    #r = pdf.getMediaBox (p)
    r = pdf.getPage(p).getBoxRect(p)
    c.beginPage (r)
    c.drawPDFDocument (r, pdf, p)
    c.endPage ()

  c.finish ()

if __name__ == '__main__':
  main ()

      

        
        
        
      

    

----- filter-pdf-old.py: end

=============================================== === ======================

Working code based on answer:

from Foundation import *
from Quartz import *

pdf_url = NSURL.fileURLWithPath_("test.pdf")
pdf_doc = PDFDocument.alloc().initWithURL_(pdf_url)

furl = NSURL.fileURLWithPath_("/System/Library/Filters/Blue Tone.qfilter")
fobj = QuartzFilter.quartzFilterWithURL_(furl)
fdict = { 'QuartzFilter': fobj }
pdf_doc.writeToFile_withOptions_("test_out.pdf", fdict)