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reading image data from the source file is the application's responsibility.
The library emits compressed data by calling a "data destination manager",
which typically will write the data into a file; but the application can
provide its own destination manager to do something else.

Similarly, the rough outline of a JPEG decompression operation is:

	Allocate and initialize a JPEG decompression object
	Specify the source of the compressed data (eg, a file)
	Call jpeg_read_header() to obtain image info
	Set parameters for decompression
	jpeg_start_decompress(...);
	while (scan lines remain to be read)
		jpeg_read_scanlines(...);
	jpeg_finish_decompress(...);
	Release the JPEG decompression object

This is comparable to the compression outline except that reading the
datastream header is a separate step.  This is helpful because information
about the image's size, colorspace, etc is available when the application
selects decompression parameters.  For example, the application can choose an
output scaling ratio that will fit the image into the available screen size.

The decompression library obtains compressed data by calling a data source
manager, which typically will read the data from a file; but other behaviors
can be obtained with a custom source manager.  Decompressed data is delivered
into in-memory buffers passed to jpeg_read_scanlines().

It is possible to abort an incomplete compression or decompression operation
by calling jpeg_abort(); or, if you do not need to retain the JPEG object,
simply release it by calling jpeg_destroy().

JPEG compression and decompression objects are two separate struct types.
However, they share some common fields, and certain routines such as
jpeg_destroy() can work on either type of object.

The JPEG library has no static variables: all state is in the compression
or decompression object.  Therefore it is possible to process multiple
compression and decompression operations concurrently, using multiple JPEG
objects.

Both compression and decompression can be done in an incremental memory-to-
memory fashion, if suitable source/destination managers are used.  See the
section on "I/O suspension" for more details.


BASIC LIBRARY USAGE
===================

Data formats
------------

Before diving into procedural details, it is helpful to understand the
image data format that the JPEG library expects or returns.

The standard input image format is a rectangular array of pixels, with each
pixel having the same number of "component" or "sample" values (color
channels).  You must specify how many components there are and the colorspace
interpretation of the components.  Most applications will use RGB data
(three components per pixel) or grayscale data (one component per pixel).
PLEASE NOTE THAT RGB DATA IS THREE SAMPLES PER PIXEL, GRAYSCALE ONLY ONE.
A remarkable number of people manage to miss this, only to find that their
programs don't work with grayscale JPEG files.

There is no provision for colormapped input.  JPEG files are always full-color
or full grayscale (or sometimes another colorspace such as CMYK).  You can
feed in a colormapped image by expanding it to full-color format.  However
JPEG often doesn't work very well with source data that has been colormapped,
because of dithering noise.  This is discussed in more detail in the JPEG FAQ
and the other references mentioned in the README file.

Pixels are stored by scanlines, with each scanline running from left to
right.  The component values for each pixel are adjacent in the row; for
example, R,G,B,R,G,B,R,G,B,... for 24-bit RGB color.  Each scanline is an
array of data type JSAMPLE --- which is typically "unsigned char", unless
you've changed jmorecfg.h.  (You can also change the RGB pixel layout, say
to B,G,R order, by modifying jmorecfg.h.  But see the restrictions listed in
that file before doing so.)

A 2-D array of pixels is formed by making a list of pointers to the starts of
scanlines; so the scanlines need not be physically adjacent in memory.  Even
if you process just one scanline at a time, you must make a one-element
pointer array to conform to this structure.  Pointers to JSAMPLE rows are of
type JSAMPROW, and the pointer to the pointer array is of type JSAMPARRAY.

The library accepts or supplies one or more complete scanlines per call.
It is not possible to process part of a row at a time.  Scanlines are always
processed top-to-bottom.  You can process an entire image in one call if you
have it all in memory, but usually it's simplest to process one scanline at
a time.

For best results, source data values should have the precision specified by
BITS_IN_JSAMPLE (normally 8 bits).  For instance, if you choose to compress
data that's only 6 bits/channel, you should left-justify each value in a
byte before passing it to the compressor.  If you need to compress data
that has more than 8 bits/channel, compile with BITS_IN_JSAMPLE = 9 to 12.
(See "Library compile-time options", later.)


The data format returned by the decompressor is the same in all details,
except that colormapped output is supported.  (Again, a JPEG file is never
colormapped.  But you can ask the decompressor to perform on-the-fly color
quantization to deliver colormapped output.)  If you request colormapped
output then the returned data array contains a single JSAMPLE per pixel;
its value is an index into a color map.  The color map is represented as
a 2-D JSAMPARRAY in which each row holds the values of one color component,
that is, colormap[i][j] is the value of the i'th color component for pixel
value (map index) j.  Note that since the colormap indexes are stored in
JSAMPLEs, the maximum number of colors is limited by the size of JSAMPLE
(ie, at most 256 colors for an 8-bit JPEG library).


Compression details
-------------------

Here we revisit the JPEG compression outline given in the overview.

1. Allocate and initialize a JPEG compression object.

A JPEG compression object is a "struct jpeg_compress_struct".  (It also has
a bunch of subsidiary structures which are allocated via malloc(), but the