Release Notes

Open Source Used In AsyncOS 8.8 for Cisco Web Security Appliances
73
For decompression, the JPEG file's color space is given in jpeg_color_space,
and this is transformed to the output color space out_color_space.
jpeg_read_header's setting of jpeg_color_space can be relied on if the file
conforms to JFIF or Adobe conventions, but otherwise it is no better than a
guess. If you know the JPEG file's color space for certain, you can override
jpeg_read_header's guess by setting jpeg_color_space. jpeg_read_header also
selects a default output color space based on (its guess of) jpeg_color_space;
set out_color_space to override this. Again, you must select a supported
transformation. jdcolor.c currently supports
YCbCr => GRAYSCALE
YCbCr => RGB
GRAYSCALE => RGB
YCCK => CMYK
as well as the null transforms. (Since GRAYSCALE=>RGB is provided, an
application can force grayscale JPEGs to look like color JPEGs if it only
wants to handle one case.)
The two-pass color quantizer, jquant2.c, is specialized to handle RGB data
(it weights distances appropriately for RGB colors). You'll need to modify
the code if you want to use it for non-RGB output color spaces. Note that
jquant2.c is used to map to an application-supplied colormap as well as for
the normal two-pass colormap selection process.
CAUTION: it appears that Adobe Photoshop writes inverted data in CMYK JPEG
files: 0 represents 100% ink coverage, rather than 0% ink as you'd expect.
This is arguably a bug in Photoshop, but if you need to work with Photoshop
CMYK files, you will have to deal with it in your application. We cannot
"fix" this in the library by inverting the data during the CMYK<=>YCCK
transform, because that would break other applications, notably Ghostscript.
Photoshop versions prior to 3.0 write EPS files containing JPEG-encoded CMYK
data in the same inverted-YCCK representation used in bare JPEG files, but
the surrounding PostScript code performs an inversion using the PS image
operator. I am told that Photoshop 3.0 will write uninverted YCCK in
EPS/JPEG files, and will omit the PS-level inversion. (But the data
polarity used in bare JPEG files will not change in 3.0.) In either case,
the JPEG library must not invert the data itself, or else Ghostscript would
read these EPS files incorrectly.
Error handling
--------------
When the default error handler is used, any error detected inside the JPEG
routines will cause a message to be printed on stderr, followed by exit().
You can supply your own error handling routines to override this behavior
and to control the treatment of nonfatal warnings and trace/debug messages.
The file example.c illustrates the most common case, which is to have the