Release Notes

Open Source Used In AsyncOS 8.8 for Cisco Web Security Appliances

compressor, or to receive raw downsampled data from the decompressor. The

library supports this requirement by allowing the application to write or

read raw data, bypassing the normal preprocessing or postprocessing steps.

The interface is different from the standard one and is somewhat harder to

use. If your interest is merely in bypassing color conversion, we recommend

that you use the standard interface and simply set jpeg_color_space =

in_color_space (or jpeg_color_space = out_color_space for decompression).

The mechanism described in this section is necessary only to supply or

receive downsampled image data, in which not all components have the same

dimensions.

To compress raw data, you must supply the data in the colorspace to be used

in the JPEG file (please read the earlier section on Special color spaces)

and downsampled to the sampling factors specified in the JPEG parameters.

You must supply the data in the format used internally by the JPEG library,

namely a JSAMPIMAGE array. This is an array of pointers to two-dimensional

arrays, each of type JSAMPARRAY. Each 2-D array holds the values for one

color component. This structure is necessary since the components are of

different sizes. If the image dimensions are not a multiple of the MCU size,

you must also pad the data correctly (usually, this is done by replicating

the last column and/or row). The data must be padded to a multiple of a DCT

block in each component: that is, each downsampled row must contain a

multiple of 8 valid samples, and there must be a multiple of 8 sample rows

for each component. (For applications such as conversion of digital TV

images, the standard image size is usually a multiple of the DCT block size,

so that no padding need actually be done.)

The procedure for compression of raw data is basically the same as normal

compression, except that you call jpeg_write_raw_data() in place of

jpeg_write_scanlines(). Before calling jpeg_start_compress(), you must do

the following:

* Set cinfo->raw_data_in to TRUE. (It is set FALSE by jpeg_set_defaults().)

This notifies the library that you will be supplying raw data.

Furthermore, set cinfo->do_fancy_downsampling to FALSE if you want to use

real downsampled data. (It is set TRUE by jpeg_set_defaults().)

* Ensure jpeg_color_space is correct --- an explicit jpeg_set_colorspace()

call is a good idea. Note that since color conversion is bypassed,

in_color_space is ignored, except that jpeg_set_defaults() uses it to

choose the default jpeg_color_space setting.

* Ensure the sampling factors, cinfo->comp_info[i].h_samp_factor and

cinfo->comp_info[i].v_samp_factor, are correct. Since these indicate the

dimensions of the data you are supplying, it's wise to set them

explicitly, rather than assuming the library's defaults are what you want.

To pass raw data to the library, call jpeg_write_raw_data() in place of

jpeg_write_scanlines(). The two routines work similarly except that

jpeg_write_raw_data takes a JSAMPIMAGE data array rather than JSAMPARRAY.