It has just been brought to my attention that there was a MIME BOF at
last week's IETF meeting, and that one minor change was proposed in that
BOF that will make image/g3fax work better. This is the addition of an
optional parameter for "non-basic parameters", in FAX terminology. What
follows, therefore, is a revised version of Section 7.5 of the MIME
document that includes this addition. Please send any comments or
objections to the list as a whole.
If you FTP the BodyFormats document after approximately 14:15 EST on
Tuesday, 3/24/92, you will get a version that has this section already
in it. -- Nathaniel
7.5 The Image Content-Type
A Content-Type of "image" indicates that the bodycontains an image. The
subtype names the specific image format. These names are case
insensitive. Three initial subtypes are "G3Fax" for Group Three Fax,
"jpeg" for the JPEG format, JFIF encoding, and "gif" for GIF format
[GIF].
A Content-Type of "image/g3fax" indicates a Group 3 (G3) Facsimile
image. The encoding format is defined in the CCITT T.4 Recommendation.
NOTE: The T.4 Recommendation defines two major encoding schemes:
one-dimensional and two-dimensional. Practical experience shows that
most implementations of g3fax encoding/decoding software implement one,
but not both of these schemes. As such, use of this subtype is strongly
discouraged. Instead, it is recommended that the sender should convert
image/g3fax parts to another format. If "image/g3fax" is used, there
is an additional optional parameter that may be included in the
Content-type header field. The "non-basic-parameter" parameter
specifies what are called "g3 non-basic parameters" in fax terminology.
These are a collection of attributes that are associated with a g3fax
image. If none of these attributes are specified in the
"non-basic-parameter" parameter, the default is "oneDimensional,
a4Length". The optional values that may be specified are:
twoDimensional - a 2-d encoding is used
fineResolution
unlimitedLength
b4Length
a3Width
b4Width
uncompressed
Note that "unlimitedLength" and "b4Length" are mutually exclusive, as
are "a3width" and "b4Width". Thus, for example, the following is a
valid header field for a g3fax entity:
Content-type: image/g3fax; non-basic-parameter="b4Length";
non-basic-parameter="a3Width";
non-basic-parameter="twodimensional"
The list of image subtypes given here is neither exclusive nor
exhaustive, and is expected to grow as more types are registered with
IANA, as described in Appendix F.