SECTION 4.2
201
Coordinate Systems
The user space coordinate system is initialized to a default state for each page of a
document. The
CropBox
entry in the page dictionary specifies the rectangle of
user space corresponding to the visible area of the intended output medium (dis-
play window or printed page). The positive
x
axis extends horizontally to the
right and the positive
y
axis vertically upward, as in standard mathematical prac-
tice (subject to alteration by the
Rotate
entry in the page dictionary). The length
of a unit along both the
x
and
y
axes is set by the
UserUnit
entry
(PDF 1.6)
in the
page dictionary (see Table 3.27). If that entry is not present or supported, the de-
fault value of 1
⁄
72 inch is used. This coordinate system is called
default user space.
Note:
In PostScript, the origin of default user space always corresponds to the lower-
left corner of the output medium. While this convention is common in PDF docu-
ments as well, it is not required; the page dictionary’s
CropBox
entry can specify any
rectangle of default user space to be made visible on the medium.
Note:
The default for the size of the unit in default user space (1
⁄
72 inch) is approx-
imately the same as a
point,
a unit widely used in the printing industry. It is not ex-
actly the same, however; there is no universal definition of a point.
Conceptually, user space is an infinite plane. Only a small portion of this plane
corresponds to the imageable area of the output device: a rectangular region de-
fined by the
CropBox
entry in the page dictionary. The region of default user
space that is viewed or printed can be different for each page and is described in
Note:
Because coordinates in user space (as in any other coordinate space) may be
specified as either integers or real numbers, the unit size in default user space does
not constrain positions to any arbitrary grid. The resolution of coordinates in user
space is not related in any way to the resolution of pixels in device space.
The transformation from user space to device space is defined by the
current
transformation matrix
(CTM), an element of the PDF graphics state (see Section
native resolution of a particular output device, maintaining the device-
independence of the PDF page description. Figure 4.3 shows how this allows an
object specified in user space to appear the same regardless of the device on
which it is rendered.
The default user space provides a consistent, dependable starting place for PDF
page descriptions regardless of the output device used. If necessary, a PDF con-