Specifications

Print2CAD OCR 2013- 11
Print2CAD
OCR 2013
PDF´s adoption in the early days of the format‘s history was slow.
Adobe Acrobat, Adobe‘s suite for reading and creating PDFs,
was not freely available; early versions of PDF had no support
for external hyperlinks, reducing its usefulness on the Internet;
the additional size of the PDF document compared to plain
text meant signicantly longer download times over the slower
modems common at the time, and rendering the les was slow
on less powerful machines. Additionally, there were competing
formats such as Envoy, Common Ground Digital Paper, Farallon
Replica and even Adobe‘s own PostScript format (.ps); in those early years, the PDF le
was mainly popular in desktop publishing workow. In 1995, AT&T Labs commenced
work on another electronic document standard targeted at libraries and archives for pre-
serving their books and documents, DjVu. This standard has evolved into the .djv/ .djvu
format, which has had growing success and penetration in the online world for eBooks,
catalogs, and image-sharing.
The original imaging model of PDF was, like PostScript‘s, opaque: 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 the PDF 1.3 and earlier specications. As
a result, les that use a small amount of transparency might view acceptably in older
viewers, but les making extensive use of transparency could view completely wrongly
in an older viewer without warning.
The transparency extensions are based on the key concepts of transparency groups, blend-
ing 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.
The concept of a transparency group in PDF specication is independent of existing
notions of “group” or “layer” in applications such as Adobe Illustrator. Those groupings
reect logical relationships among objects that are meaningful when editing those objects,
but they are not part of the imaging model.
Source: Wikipedia under the subject PDF
License Agreement: http://creativecommons.org/licenses/by-sa/3.0/deed.de