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Chapter IV, Image File Format Overview: Lesson IV Image Pac |
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 Lessons in this chapter 
Chapters in this book
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 File Size ![[Indent]](/US/images/en/digital/dlc/book2/chapter4/in.gif){kind=small}
To maintain near-photographic quality for hard-copy output,
it was necessary for the Photo CD system scanner to operate
at a resolution of 2048 X 3072 pixels for each of three colors
(red, green, and blue). The file for a single 35mm image would,
therefore, require 18 MB of storage capacity.
Such file size complicates the requirements for fast
access to television resolution and practical disc capacity.
At the time the system was launched, CD-ROM players accessed
![[PhotoCD]](/US/images/en/digital/dlc/book2/chapter4/pcd.gif)
and transferred data at a rate of 150 KB/sec (1X). At this rate,
it would take two minutes to display one image.
And, with a Photo CD disc capacity of about 540 MB,
only 30 images could be stored on a disc.
To reduce the access time and increase the number of
images that fit on a disc, the Photo CD System compresses Photo CD images.
In simplistic terms, data required for each level of
resolution is recorded only once, so if a user selects
the highest resolution, the system calls up the BASE
resolution and only adds the residual values needed to
produce the higher resolution image.
As a result, the final Image Pac file size ranges between
4.5 and 6.5 MB, which allows storage of up to 100 images
on a single Photo CD disc.Capabilities
The Image Pac -- or PCD -- file is a multi-resolution image
format developed by Kodak as part of the Photo CD System.
Part of the initial strategy for the development and market
positioning of the Photo CD System was to establish the PCD
file format as the industry standard for digital image storage.
Once an imaging professional has a library of
images on one or more Photo CDs, a broad range
of opportunities becomes available:
Pros / Cons of File Type
The PCD file format was designed to reliably perform a specific function.
However, the equipment and software required to scan an image from film
and write it to a PCD file is expensive.
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