Just An Application

August 23, 2014

Anatomy Of A PDF: Part Four — Beware Of Objects Bearing Gifts Or Containing Forms

Filed under: Document Format, PDF, PDF Objects — Tags: , , , — Simon Lewis @ 7:07 pm

The start of Object 1 looks like this.

    1 0 obj
    <</Filter [/Fl /Fl] /Length 13178 >>
    stream
    
    ...

and the end of it looks like this

    ...
    
    endstream
    endobj
    
    ...

and in between there are 13178 bytes of assorted binary sludge.

This is beacause the contents of the object have been filtered before being written to the file.

This is what the presence of the Filter entry in the object’s dictionary is telling us.

The associated value is an array which specifies the filters that have been applied.

    Fl

is an abbreviation for

    FlateDecode

There are two entries, so the FlateDecode filter has been applied twice, first to the original object, and then to the output produced by the first pass.

The FlateDecode filter produces data in the DEFLATE compressed data format.

To find out what is in Object 1 we are going to have to inflate it twice.

Seconds Out, Round One

Inflating it once gives us 408521 bytes of compressed XML up from 13178 so its obviously got a lot of zeroes in it or something.

Seconds Out, Round Two

Inflating our collection of 408521 newly minted bytes we end up with a decidedly non-trivial 91044256 bytes of XML which is a whole lot of angle brackets.

Its going to take forever to fill in all the fields in this thing.


Copyright (c) 2014 By Simon Lewis. All Rights Reserved.

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Excerpts and links may be used, provided that full and clear credit is given to Simon Lewis and justanapplication.wordpress.com with appropriate and specific direction to the original content.

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