PDF generation — How to merge multiple stream objects? - pdf

Question

I'm currently into generating PDF documents without the use of an external library and it has been going well so far. I've written the document exposed below with a text editor (vim) and it renders the expected results using at least two PDF distinct viewers (evince & gsview, running Linux).
This document produces three squares at top of the page, coming in different sizes, widths and colors.
My question is : is there a way to merge two stream objects into a new single one or, in other words, is there a way to compose sophisticated objects starting from simple ones, so we can easily refer to these composite objects, multiple times if needed ?
In the given example, object 5 0 obj is drawing a square, and following ones are just applying colors and coordinates transformations (through a matrix).
The PDF reference manual states that multiple stream contents passed as an array to page object's /Contents parameter are concatenated and processed as a single continuous stream, which totally does the trick… as long as the document remains small and simple!
In this same example, the /Contents array is indirectly passed through object 4 0 obj, which refers three times to 5 0 R, to draw the squares.
The ideal here would be to define three differents objects, each refering to 5 0 R by themselves, then invoke only these objects, a single time each, from the Contents array.
I tried adding subarrays inside it, which could in turn be embedded into dedicated objects and referenced indirectly, but it unfortunately doesn't work. :-(
A lot of thanks to any people that could/try-to help !
PS: I'm doing it because I'm interested in the format itself and would like to produce some autogenerated documents from small scripts. Also, I'll probably embed them into a weakly powered appliance and I cannot afford relying on dozens of megabytes in dependencies.
But before this, I still tried to do that too, using PHP with TCPDF. If there's already some facilities dedicated to this that I would have missed, this is relevant to my interests too. :-)
Small.pdf (hand made PDF file)
%PDF-1.7
1 0 obj
<<
/Type /Catalog
/Pages 2 0 R
>>
endobj
2 0 obj
<<
/Type /Pages
/Count 1
/Kids [ 3 0 R ]
>>
endobj
3 0 obj
<<
/Type /Page
/MediaBox [ 0.000000 0.000000 1000.000000 1414.213562 ]
/Contents 4 0 R
>>
endobj
4 0 obj
% A simple array, just to avoid embedding it directly in /Page object (3 0 R here)
[
6 0 R 5 0 R % Red square
7 0 R 5 0 R % Green square
8 0 R 5 0 R % Blue square (tilted)
]
endobj
5 0 obj
% Draws a square, centered by default on lower left corner
<<
/Length 43
>>
stream
+20 +20 m
+20 -20 l
-20 -20 l
-20 +20 l s Q
endstream
endobj
6 0 obj
<<
/Length 63
>>
stream
/DeviceRGB CS
q
1.0 0.0 0.0 SC
2.0 w
1 0 0 -1 60 1354.213562 cm
endstream
endobj
7 0 obj
<<
/Length 49
>>
stream
q
0.0 1.0 0.0 SC
1.0 w
2 0 0 -2 190 1334.213562 cm
endstream
endobj
8 0 obj
<<
/Length 83
>>
stream
q
0.0 0.0 1.0 SC
5.0 w
0.707106781 0.707106781 -0.707106781 0.707106781 110 1250 cm
endstream
endobj
xref
0 9
0000000000 65535 f
0000000010 00000 n
0000000079 00000 n
0000000168 00000 n
0000000296 00000 n
0000000513 00000 n
0000000674 00000 n
0000000796 00000 n
0000000905 00000 n
trailer
<<
/Size 9
/Root 1 0 R
/ID [ <0000000000> <0000000001> ]
>>
startxref
01047
%%EOF

Answer 1

What you are looking for are form XObjects.
The pdf specification ISO 32000-1 characterizes them like this:
A form XObject is a PDF content stream that is a self-contained description of any sequence of graphics objects. A form XObject may be painted multiple times - either on several pages or at several locations on the same page - and produces the same results each time, subject only to the graphics state at the time it is invoked.
For details please read section 8.10 of the specification.