CHAPTER 8
588
Interactive Features
The thumbnail image for a page is an image XObject specified by the
Thumb
entry in the page object (see “Page Objects” on page 144). It has the usual struc-
ture for an image dictionary (Section 4.8.4, “Image Dictionaries”), but only the
Width
,
Height
,
ColorSpace
,
BitsPerComponent
, and
Decode
entries are signifi-
cant; all of the other entries listed in Table 4.39 on page 340 are ignored if present.
(If a
Subtype
entry is specified, its value must be
Image
.) The image’s color space
must be either
DeviceGray
or
DeviceRGB
, or an
Indexed
space based on one of
these. Example 8.2 shows a typical thumbnail image definition.
Example 8.2
12 0 obj
<< /Width 76
/Height 99
/ColorSpace /DeviceRGB
/BitsPerComponent 8
/Length 13 0 R
/Filter [ /ASCII85Decode /DCTDecode ]
>>
stream
s4IA>!"M;*Ddm8XA,lT0!!3,S!/(=R!<E3%!<N<(!WrK*!WrN,
…
Omitted data
…
endstream
endobj
13 0 obj
…
endobj
% Length of stream
8.2.4 Collections
Beginning with PDF 1.7, PDF documents can specify how a viewer application’s
user interface presents collections of file attachments, where the attachments are
related in structure or content. Such a presentation is called a portable collection.
The intent of
portable collections
is to present, sort, and search collections of relat-
ed documents, such as email archives, photo collections, and engineering bid
sets. There is no requirement that files in a collection have an implicit relation-
ship or even a similarity; however, showing differentiating characteristics of relat-
ed documents can be helpful for document navigation.