Cover Sheet for the GIF89a Specification
DEFERRED CLEAR CODE IN LZW COMPRESSION
There has been confusion about where clear codes can be found in the
data stream. As the specification says, they may appear at anytime.
There
is not a requirement to send a clear code when the string table is
full.
It is the encoder's decision as to when the table should be cleared.
When
the table is full, the encoder can chose to use the table as is,
making no
changes to it until the encoder chooses to clear it. The encoder
during
this time sends out codes that are of the maximum Code Size.
As we can see from the above, when the decoder's table is full, it
must
not change the table until a clear code is received. The Code Size is
that
of the maximum Code Size. Processing other than this is done
normally.
Because of a large base of decoders that do not handle the
decompression in
this manner, we ask developers of GIF encoding software to NOT
implement
this feature until at least January 1991 and later if they see that
their
particular market is not ready for it. This will give developers of
GIF
decoding software time to implement this feature and to get it into
the
hands of their clients before the decoders start "breaking" on the new
GIF's. It is not required that encoders change their software to take
advantage of the deferred clear code, but it is for decoders.
APPLICATION EXTENSION BLOCK - APPLICATION IDENTIFIER
There will be a Courtesy Directory file located on CompuServe in the
PICS
forum. This directory will contain Application Identifiers for
Application
Extension Blocks that have been used by developers of GIF
applications.
This file is intended to help keep developers that wish to create
