The Photo CD system provides an inexpensive means of participating in the information age with personal digital images. At the heart of the system is a 35 mm, 2000 pixels/inch scanner to acquire image data from a variety of photographic media. The raw data is then processed into a calibrated color metric, decomposed into a hierarchy of 5 resolution, compressed, encoded, and written to a compact disc using a high-speed CD writer. These discs can then be quickly displayed to a consumer television with a low-cost Photo CD player, that is also a high-quality CD-audio player. Additionally, Photo CD discs can be read into a computer using a CD-ROM XA drive, and manipulated on the desktop for importing into documents, or, for hard-copy printing to a variety of devices. The 35 mm-based system described above is only a part of a more generalized architecture. An add-on component capable of scanning larger format negatives and slides will soon be available, and will utilize extensions to the Photo CD format appropriate for these new image modalities. In particular, the formation of a 6th resolution and its relationship to the existing hierarchy will be discussed. Additionally, the structure of future extensions currently under development will be outlined in the context of the specific applications which they are designed to support.
There are an increasing number of digital image processing systems that employ photographic image
capture; that is, a color photographic negative or transparency is digitally scanned, compressed, and stored
or transmitted for further use. To capture the information content that a photographic color negative is
capable of delivering, it must be scanned at a pixel resolution of at least 50 pixels/mm. This type of
high quality imagery presents certain problems and opportunities in image coding that are not present in
lower resolution systems. Firstly, photographic granularity increases the entropy of a scanned negative,
limiting the extent to which entropy encoding can compress the scanned record. Secondly, any MTFrelated
chemical enhancement that is incorporated into a film tends to reduce the pixel-to-pixel correlation
that most compression schemes attempt to exploit. This study examines the effect of noise and MTF
on the compressibility of scanned photographic images by establishing experimental information theoretic
bounds. Images used for this study were corrupted with noise via a computer model of photographic grain
and an MTF model of blur and chemical edge enhancement. The measured bounds are expressed in terms
of the entropy of a variety of decomposed image records (e.g., DPCM predictor error) for a zeroeth-order
Markov-based entropy encoder, and for a context model used by the Q-coder. The resultsshow that the
entropy of the DPCM predictor error is 3-5 bits/pixel, illustrating a 2 bits/pixel difference between an
ideal grain-free case, and a grainy film case. This suggests that an ideal noise filtering algorithm could
lower the bitrate by as much as 50%.
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