Release Notes

Open Source Used In AsyncOS 8.8 for Cisco Web Security Appliances

For program-supplied data, use an APPn marker, and be sure to begin it with an

identifying string so that you can tell whether the marker is actually yours.

It's probably best to avoid using APP0 or APP14 for any private markers.

(NOTE: the upcoming SPIFF standard will use APP8 markers; we recommend you

not use APP8 markers for any private purposes, either.)

Keep in mind that at most 65533 bytes can be put into one marker, but you

can have as many markers as you like.

By default, the IJG compression library will write a JFIF APP0 marker if the

selected JPEG colorspace is grayscale or YCbCr, or an Adobe APP14 marker if

the selected colorspace is RGB, CMYK, or YCCK. You can disable this, but

we don't recommend it. The decompression library will recognize JFIF and

Adobe markers and will set the JPEG colorspace properly when one is found.

You can write special markers immediately following the datastream header by

calling jpeg_write_marker() after jpeg_start_compress() and before the first

call to jpeg_write_scanlines(). When you do this, the markers appear after

the SOI and the JFIF APP0 and Adobe APP14 markers (if written), but before

all else. Specify the marker type parameter as "JPEG_COM" for COM or

"JPEG_APP0 + n" for APPn. (Actually, jpeg_write_marker will let you write

any marker type, but we don't recommend writing any other kinds of marker.)

For example, to write a user comment string pointed to by comment_text:

jpeg_write_marker(cinfo, JPEG_COM, comment_text, strlen(comment_text));

If it's not convenient to store all the marker data in memory at once,

you can instead call jpeg_write_m_header() followed by multiple calls to

jpeg_write_m_byte(). If you do it this way, it's your responsibility to

call jpeg_write_m_byte() exactly the number of times given in the length

parameter to jpeg_write_m_header(). (This method lets you empty the

output buffer partway through a marker, which might be important when

using a suspending data destination module. In any case, if you are using

a suspending destination, you should flush its buffer after inserting

any special markers. See "I/O suspension".)

Or, if you prefer to synthesize the marker byte sequence yourself,

you can just cram it straight into the data destination module.

If you are writing JFIF 1.02 extension markers (thumbnail images), don't

forget to set cinfo.JFIF_minor_version = 2 so that the encoder will write the

correct JFIF version number in the JFIF header marker. The library's default

is to write version 1.01, but that's wrong if you insert any 1.02 extension

markers. (We could probably get away with just defaulting to 1.02, but there

used to be broken decoders that would complain about unknown minor version

numbers. To reduce compatibility risks it's safest not to write 1.02 unless

you are actually using 1.02 extensions.)