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PDF document structure is a tree structure, the tree's
root node is the Catalog dictionary of the PDF files, and
there are four subtrees below the root node: Pages Tree,
Outline Tree, Article Threads, Named Destination. In the
page tree, all the page object is this tree's leaf node, and
they will inherit the various attribute value of their father
node to take its corresponding attribute default value,
which function is to let the other objects of the PDF
document can also use the the character string name to
express some page region.
IV. RESEARCH THE CONVERSION FROM PDF
DOCUMENTS TO PS FILE
PDF is the document format, while PostScript is the
programming language, so this principled difference
extends the difference of the expression of the document
content inevitably, which cause the difference in
producing and using the PDF document, as well as refer
how to putout the hard copy of the PDF file in the
PostScript printer or the non-PostScript printer.
A. The feasibility of the conversion from PDF documents
to PS file
PDF takes root in PostScript, thus there are many
similarities, which decides to the feasibility of the
document conversion. PDF file format uses the imaging
model of PostScript language to express text, graphics
and other objects, similar to PostScript language
procedure, a PDF page description draws a page through
placing “pigment” In the selected region. Its imaging
characteristics can be summarized as follows:
Colored page objects may be abstract to "map", which
could be the character shape, or be expressed by the
digital sampling of the photo, and the region (graphics)
defined with curves and straight-line.
Imaging operation is relatively flexible, not limited to
the print record or film, which can also on paper and
other record. It could use any color of the pigment when
imaging.
For the page is viewed as “map” when imaging, thus it
could be cut into other shape, with the result that the part
of “map” only in this sharp can be appeared on the page.
Typical PostScript language procedure needs to take
the tag instruction character definition of the PostScript
language to define its set of instruction character, used to
describe the pages and control output. While PDF defines
its own instruction character sets, and the most
instruction characters are very similar to the PostScript
instruction characters.
Although PDF document can not be explained by the
PostScript translator device, but in the conversion
function of the translator device, page description of the
PDF document can transform to the PostScript language
procedure. Basic method is as follows:
The difference between PostScript and PDF mainly
from page description instruction character , so the
solution of this problem is to insert the PostScript
language process definition sets Prosets of the instruction
character s which can realize the PDF page description.
Extract the content on each page of the PDF document,
because it is not required to store by the logical order in
the PDF document format, so it is necessary to describing
each page. Because the description part of the traditional
PostScript language procedure uses the appropriative
process, such as represent “moveto” by “m”, represent
“lineto” by “l” and so on.
To decode the compressed text, image and graphics
data is not a simple work for ordinary users. PostScript
Level 2 printers do not need to decode the compressed
data in the PDF documents, but without the data encoded
by Flate compression algorithm. But now most of PDF
documents use Flate compression algorithm, so it must
first achieve the decompression of Flate compression
algorithm, and then complete the conversion from PS to
PDF.
Insert the resource, such as the font, into PostScript
files, replace the font definition and insert according to
the necessary. The basic is the font specifications of the
PDF files.
Place the information by the correct order, the result
should be, in a usually sense, a PostScript program file,
containing all the visible part of the document, but
hyperlinks, comments, bookmarks and other PDF units
will no longer be included in the documents.
This can obtain the PostScript language document, and
then could printout this document after sending it to the
printer. In view of this, the conversion from the PDF
documents to PS document is entirely feasible, and its
key lies in the realization of translator device.
B. The conversion algorithm from PDF document to PS
file
a) The description of the algorithm
To complete the conversion from PDF files to PS
format, firstly need to extract page information of the
PDF file, which includes resources, content, annotation
and etc. Then express each page by PS language, first
write into the resource referred in this page, and then
write into the content of this page, so it is only necessary
to express with the corresponding PS language in this
part. This part is not so complex as the extraction of the
PDF information above, for most instruction character of
PDF is the same as that of PostScript, thus it is relatively
simple as long as the corresponding of these two
instruction character is achieved in advance. The below is
the algorithm which can constructs the information
extractor of the PDF file.
Achieve the lexical analysis and the syntax analysis of
the PDF document, to analyze the single lexical symbol
and the grammar object separately. The object format has
been introduced above, and we need to achieve its
identification.
Find the location of document rear according to the
keywords, because document rear contains the address of
Catalog and the address of the cross-reference table. then
analyze the content of the cross-reference table, that is the
offset of each indirect object address, and produce
encoding.
Analyze the content of that Catalog, which is the root
object of the PDF files, and obtain the attributes of the
Catalog, such as the Outline attribute, Pages attribute and
so on, that is its indirect object address.
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