Release Notes

Open Source Used In AsyncOS 8.8 for Cisco Web Security Appliances

Selected by cinfo.two_pass_quantize = TRUE and cinfo.colormap = NULL.

(This is the default setting selected by jpeg_read_header, but it is

probably NOT what you want for the first pass of progressive display!)

These methods offer successively better quality and lesser speed. However,

only the first method is available for quantizing in non-RGB color spaces.

IMPORTANT: because the different quantizer methods have very different

working-storage requirements, the library requires you to indicate which

one(s) you intend to use before you call jpeg_start_decompress(). (If we did

not require this, the max_memory_to_use setting would be a complete fiction.)

You do this by setting one or more of these three cinfo fields to TRUE:

enable_1pass_quantFixed color cube colormap

enable_external_quantExternally-supplied colormap

enable_2pass_quantTwo-pass custom colormap

All three are initialized FALSE by jpeg_read_header(). But

jpeg_start_decompress() automatically sets TRUE the one selected by the

current two_pass_quantize and colormap settings, so you only need to set the

enable flags for any other quantization methods you plan to change to later.

After setting the enable flags correctly at jpeg_start_decompress() time, you

can change to any enabled quantization method by setting two_pass_quantize

and colormap properly just before calling jpeg_start_output(). The following

special rules apply:

1. You must explicitly set cinfo.colormap to NULL when switching to 1-pass

or 2-pass mode from a different mode, or when you want the 2-pass

quantizer to be re-run to generate a new colormap.

2. To switch to an external colormap, or to change to a different external

colormap than was used on the prior pass, you must call

jpeg_new_colormap() after setting cinfo.colormap.

NOTE: if you want to use the same colormap as was used in the prior pass,

you should not do either of these things. This will save some nontrivial

switchover costs.

(These requirements exist because cinfo.colormap will always be non-NULL

after completing a prior output pass, since both the 1-pass and 2-pass

quantizers set it to point to their output colormaps. Thus you have to

do one of these two things to notify the library that something has changed.

Yup, it's a bit klugy, but it's necessary to do it this way for backwards

compatibility.)

Note that in buffered-image mode, the library generates any requested colormap

during jpeg_start_output(), not during jpeg_start_decompress().

When using two-pass quantization, jpeg_start_output() makes a pass over the

buffered image to determine the optimum color map; it therefore may take a

significant amount of time, whereas ordinarily it does little work. The

progress monitor hook is called during this pass, if defined. It is also

important to realize that if the specified target scan number is greater than

or equal to the current input scan number, jpeg_start_output() will attempt