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unsigned int scale_num, scale_denom
    Scale the image by the fraction scale_num/scale_denom.  Default is
    1/1, or no scaling.  Currently, the only supported scaling ratios
    are 1/1, 1/2, 1/4, and 1/8.  (The library design allows for arbitrary
    scaling ratios but this is not likely to be implemented any time soon.)
    Smaller scaling ratios permit significantly faster decoding since
    fewer pixels need be processed and a simpler IDCT method can be used.   

6. while (scan lines remain to be read)
    jpeg_read_scanlines(...);

Now you can read the decompressed image data by calling jpeg_read_scanlines()
one or more times.  At each call, you pass in the maximum number of scanlines
to be read (ie, the height of your working buffer); jpeg_read_scanlines()
will return up to that many lines.  The return value is the number of lines
actually read.  The format of the returned data is discussed under "Data
formats", above.  Don't forget that grayscale and color JPEGs will return
different data formats!

Image data is returned in top-to-bottom scanline order.  If you must write
out the image in bottom-to-top order, you can use the JPEG library's virtual
array mechanism to invert the data efficiently.  Examples of this can be
found in the sample application djpeg.

The library maintains a count of the number of scanlines returned so far
in the output_scanline field of the JPEG object.  Usually you can just use
this variable as the loop counter, so that the loop test looks like
"while (cinfo.output_scanline < cinfo.output_height)".  (Note that the test
should NOT be against image_height, unless you never use scaling.  The
image_height field is the height of the original unscaled image.)
The return value always equals the change in the value of output_scanline.

If you don't use a suspending data source, it is safe to assume that
jpeg_read_scanlines() reads at least one scanline per call, until the
bottom of the image has been reached.

If you use a buffer larger than one scanline, it is NOT safe to assume that
jpeg_read_scanlines() fills it.  (The current implementation returns only a
few scanlines per call, no matter how large a buffer you pass.)  So you must
always provide a loop that calls jpeg_read_scanlines() repeatedly until the
whole image has been read.

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.)