Why my code not correctly split every page in a scanned pdf?

Question

Update: Thanks to stardt whose script works! The pdf is a page of another one. I tried the script on the other one, and it also correctly spit each pdf page, but the order of page numbers is sometimes right and sometimes wrong. For example, in page 25-28 of the pdf file, the printed page numbers are 14, 15, 17, are 16. I was wondering why? The entire pdf can be downloaded from http://download304.mediafire.com/u6ewhjt77lzg/bgf8uzvxatckycn/3.pdf

Original: I have a scanned pdf, where two paper pages sit side by side in a pdf page. I would like to split the pdf page into two, with the original left half becoming the earlier of the two new pdf pages. The pdf looks like .

Here is my Python script named un2up inspired by Gilles:

#!/usr/bin/env python
import copy, sys
from pyPdf import PdfFileWriter, PdfFileReader
input = PdfFileReader(sys.stdin)
output = PdfFileWriter()
for p in [input.getPage(i) for i in range(0,input.getNumPages())]:
    q = copy.copy(p)
    (w, h) = p.mediaBox.upperRight

    p.mediaBox.upperLeft = (0, h/2)
    p.mediaBox.upperRight = (w, h/2)
    p.mediaBox.lowerRight = (w, 0)
    p.mediaBox.lowerLeft = (0, 0)

    q.mediaBox.upperLeft = (0, h)
    q.mediaBox.upperRight = (w, h)
    q.mediaBox.lowerRight = (w, h/2)
    q.mediaBox.lowerLeft = (0, h/2)

    output.addPage(q)
    output.addPage(p)
output.write(sys.stdout)

I tried the script on a pdf in terminal with command being un2up < page.pdf > out.pdf, but the output out.pdf is not correctly split.

I also checked the values of variables w and h, the output of p.mediaBox.upperRight, and they are 514 and 1224 which don't look right based on their actual ratio.

The file can be downloaded from http://download851.mediafire.com/bdr4sv7v5nzg/raci13ct5w4c86j/page.pdf.

Answer 1

Your code assumes that p.mediaBox.lowerLeft is (0,0) but it is actually (0, 497)

This works for the file you provided:

#!/usr/bin/env python
import copy, sys
from pyPdf import PdfFileWriter, PdfFileReader
input = PdfFileReader(sys.stdin)
output = PdfFileWriter()
for i in range(input.getNumPages()):
    p = input.getPage(i)
    q = copy.copy(p)

    bl = p.mediaBox.lowerLeft
    ur = p.mediaBox.upperRight

    print >> sys.stderr, 'splitting page',i
    print >> sys.stderr, '\tlowerLeft:',p.mediaBox.lowerLeft
    print >> sys.stderr, '\tupperRight:',p.mediaBox.upperRight

    p.mediaBox.upperRight = (ur[0], (bl[1]+ur[1])/2)
    p.mediaBox.lowerLeft = bl

    q.mediaBox.upperRight = ur
    q.mediaBox.lowerLeft = (bl[0], (bl[1]+ur[1])/2)
    if i%2==0:
        output.addPage(q)
        output.addPage(p)
    else:
        output.addPage(p)
        output.addPage(q)

output.write(sys.stdout)

Answer 2

@stardt's code was quite useful, but I had problems to split a batch of pdf files with different orientations. Here's a more general function that will work no matter what the page orientation is:

import copy
import math
import pyPdf

def split_pages(src, dst):
    src_f = file(src, 'r+b')
    dst_f = file(dst, 'w+b')

    input = pyPdf.PdfFileReader(src_f)
    output = pyPdf.PdfFileWriter()

    for i in range(input.getNumPages()):
        p = input.getPage(i)
        q = copy.copy(p)
        q.mediaBox = copy.copy(p.mediaBox)

        x1, x2 = p.mediaBox.lowerLeft
        x3, x4 = p.mediaBox.upperRight

        x1, x2 = math.floor(x1), math.floor(x2)
        x3, x4 = math.floor(x3), math.floor(x4)
        x5, x6 = math.floor(x3/2), math.floor(x4/2)

        if x3 > x4:
            # horizontal
            p.mediaBox.upperRight = (x5, x4)
            p.mediaBox.lowerLeft = (x1, x2)

            q.mediaBox.upperRight = (x3, x4)
            q.mediaBox.lowerLeft = (x5, x2)
        else:
            # vertical
            p.mediaBox.upperRight = (x3, x4)
            p.mediaBox.lowerLeft = (x1, x6)

            q.mediaBox.upperRight = (x3, x6)
            q.mediaBox.lowerLeft = (x1, x2)

        output.addPage(p)
        output.addPage(q)

    output.write(dst_f)
    src_f.close()
    dst_f.close()