SECTION 4.2
203
Coordinate Systems
Other Coordinate Spaces
In addition to device space and user space, PDF uses a variety of other coordinate
spaces for specialized purposes:
•
The coordinates of text are specified in
text space.
The transformation from text
space to user space is defined by a
text matrix
in combination with several text-
related parameters in the graphics state (see Section 5.3.1, “Text-Positioning
•
Character glyphs in a font are defined in
glyph space
(see Section 5.1.3, “Glyph
is defined by the
font matrix.
For most types of fonts, this matrix is predefined
to map 1000 units of glyph space to 1 unit of text space; for Type 3 fonts, the
font matrix is given explicitly in the font dictionary (see Section 5.5.4, “Type 3
•
All sampled images are defined in
image space.
The transformation from image
space to user space is predefined and cannot be changed. All images are 1 unit
wide by 1 unit high in user space, regardless of the number of samples in the
image. To be painted, an image is mapped to a region of the page by temporari-
ly altering the CTM.
Note:
In PostScript, unlike PDF, the relationship between image space and user
space can be specified explicitly. The fixed transformation prescribed in PDF cor-
responds to the convention that is recommended for use in PostScript.
•
A form XObject (discussed in Section 4.9, “Form XObjects”) is a self-contained
content stream that can be treated as a graphical element within another con-
tent stream. The space in which it is defined is called
form space.
The transfor-
mation from form space to user space is specified by a
form matrix
contained
in the form XObject.
•
PDF 1.2 defines a type of color known as a
pattern,
discussed in Section 4.6,
repeatedly to tile an area or by a shading whose color is a function of position.
The space in which a pattern is defined is called
pattern space.
The transforma-
tion from pattern space to user space is specified by a
pattern matrix
contained
in the pattern.
•
PDF 1.6 introduces embedded 3D artwork, which is described in three-dimen-
sional coordinates (see Section 9.5.4, “Coordinate Systems for 3D”) that are