PDFlib Text Extraction Toolkit (TET) Manual

6.2 Page and Text Geometry
Default coordinate system. By default TET represents all page and text metrics in the
standard coordinate system of PDF. However, the origin of the coordinate system
(which could be located outside the page) is adjusted to the lower left corner of the visible
page. More precisely, the origin is located in the lower left corner of the CropBox if it
is present, or the MediaBox otherwise. Page rotation is applied if the page has a Rotate
key. The coordinate system uses the DTP point as unit:
1 pt = 1 inch / 72 = 25.4 mm / 72 = 0.3528 mm
The first coordinate increases to the right, the second coordinate increases upwards. All
coordinates expected or returned by TET are interpreted in this coordinate system, regardless
of their representation in the underlying PDF document. See the pCOS Path
Reference to learn how to determine the size of a PDF page.
Top-down coordinate system. Unlike PDF’s bottom-up coordinate system some graphics
environments use top-down coordinates which may be preferred by some developers.
In order to facilitate the use of top-down coordinates TET supports an alternative
coordinate system in which all relevant coordinates are interpreted relative to the upper
left corner of the page instead of the lower left corner, with y coordinates increasing
downwards. This topdown feature has been designed to make it quite natural for TET users
to work in a top-down coordinate system. As an additional advantage, top-down coordinates
are identical to the coordinate values displayed in Acrobat (see below). The
top-down coordinate system for a page can be activated with the page option topdown=
{output}.
Visualizing coordinates in Acrobat. You can visualize page coordinates in Acrobat as
follows (see Figure 6.2):
> To display cursor coordinates in Acrobat X/XI use View, Show/Hide, Cursor Coordinates.
> The coordinates are displayed in the unit which is currently selected in Acrobat. To
change the display units to points (as used in TET) in Acrobat X/XI proceed as follows:
go to Edit, Preferences, [General...], Units & Guides, Units and select Points.
Note that the coordinates displayed refer to an origin in the top left corner of the page,
and not the default coordinate system of PDF and TET with an origin in the lower left
corner. See the previous section for details on selecting a top-down coordinate system
which aligns with Acrobat’s coordinate display.
Area of text extraction. By default, TET will extract all text from the visible page area.
Using the clippingarea option of TET_open_page( ) (see Table 10.10, page 169) you can
change this to any of the PDF page box entries (e.g. TrimBox). With the keyword unlimited
all text regardless of any page boxes can be extracted. The default value cropbox instructs
TET to extract text within the area which is visible in Acrobat.
The area of text extraction can be specified in more detail by providing an arbitrary
number of rectangular areas in the includebox and excludebox options of TET_open_
page( ). This is useful for extracting partial page content (e.g. selected columns), or for
excluding irrelevant parts (e.g. margins, headers and footers). The final clipping area is
6.2 Page and Text Geometry 73