Your Company
123 Your Street
Your City, ST 12345
(123) 456-7890
Within text strings, characters are shown using character codes (integers) that map to
glyphs in the current font using an encoding. There are several predefined encodings,
including WinAnsi, MacRoman, and many encodings for East Asian languages and a
font can have its own built-in encoding. (Although the WinAnsi and MacRoman
encodings are derived from the historical properties of the Windows and Macintosh
operating systems, fonts using these encodings work equally well on any platform.) PDF
can specify a predefined encoding to use, the font's built-in encoding or provide a
lookup table of differences to a predefined or built-in encoding (not recommended with
[2]
TrueType fonts). The encoding mechanisms in PDF were designed for Type 1 fonts,
and the rules for applying them to TrueType fonts are complex.
For large fonts or fonts with non-standard glyphs, the special encodings Identity-H (for
horizontal writing) and Identity-V (for vertical) are used. With such fonts, it is necessary
to provide a ToUnicode table if semantic information about the characters is to be
preserved.
A text document which is scanned to PDF without the text being recognised by optical
character recognition (OCR) is an image, with no fonts or text properties.
Transparency
The original imaging model of PDF was opaque, similar to PostScript, where each
object drawn on the page completely replaced anything previously marked in the same
location. In PDF 1.4 the imaging model was extended to allow transparency. When
transparency is used, new objects interact with previously marked objects to produce
blending effects. The addition of transparency to PDF was done by means of new
extensions that were designed to be ignored in products written to PDF 1.3 and earlier
specifications. As a result, files that use a small amount of transparency might be
viewed acceptably by older viewers, but files making extensive use of transparency
could be viewed incorrectly by an older viewer.
The transparency extensions are based on the key concepts of transparency groups,
blending modes, shape, and alpha. The model is closely aligned with the features of
Adobe Illustrator version 9. The blend modes were based on those used by Adobe
Photoshop at the time. When the PDF 1.4 specification was published, the formulas for
calculating blend modes were kept secret by Adobe. They have since been
[32]
published.
The concept of a transparency group in PDF specification is independent of existing
notions of "group" or "layer" in applications such as Adobe Illustrator. Those groupings
reflect logical relationships among objects that are meaningful when editing those
objects, but they are not part of the imaging model.
Character Representation and Encoding in PDFs
In PDF documents, characters are represented by numerical codes that correspond to specific
visual glyphs within a chosen font, a process managed by an encoding. Various predefined
encodings exist, such as WinAnsi and MacRoman, which, despite their historical association
with Windows and Macintosh operating systems, function universally across platforms. Fonts
can also possess their own internal encodings.
PDF allows for the specification of a predefined encoding, the font's built-in encoding, or a
lookup table of differences to a predefined or built-in encoding (though the latter is not advised
for TrueType fonts). The encoding mechanisms in PDF were primarily designed for Type 1 fonts,
leading to complex rules when applied to TrueType fonts.
For fonts containing a large number of characters or non-standard glyphs, specialized
encodings like Identity-H (for horizontal text) and Identity-V (for vertical text) are employed.
When using such fonts, a ToUnicode table is essential to preserve the semantic meaning of the
characters.
It's important to note that a text document scanned into a PDF without the application of Optical
Character Recognition (OCR) is treated as an image, lacking any embedded font or text
properties.
Transparency in PDF Imaging
Initially, the PDF imaging model was opaque, similar to PostScript, where each new object
completely obscured anything previously drawn in the same location. However, with PDF 1.4,
the imaging model was enhanced to incorporate transparency. This allows new objects to
interact with existing ones, creating blending effects.
This transparency extension was implemented in a way that older PDF 1.3 and earlier viewers
would ignore it. Consequently, files with minimal transparency might display acceptably in older
viewers, but those heavily utilizing transparency could be rendered incorrectly.
The transparency extensions are built upon the core concepts of transparency groups, blending
modes, shape, and alpha. This model closely aligns with features found in Adobe Illustrator
version 9. The blend modes were derived from those used in Adobe Photoshop at the time.
Although the formulas for calculating blend modes were initially kept proprietary by Adobe when
the PDF 1.4 specification was released, they have since been made public.
It's crucial to understand that the concept of a transparency group within the PDF specification
differs from existing notions of "group" or "layer" in applications like Adobe Illustrator. These
application-level groupings relate to logical relationships among objects for editing purposes and
are not part of the underlying imaging model.
