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cl_jpeg(3dm)							  cl_jpeg(3dm)

NAME    [Toc]    [Back]

     cl_jpeg - JPEG schemes in the Compression Library

SYNOPSIS    [Toc]    [Back]

     #include <dmedia/cl.h>

DESCRIPTION    [Toc]    [Back]

     The JPEG compression standard, jpeg(4), is	supported in the Compression
     Library, CLintro(3dm), with two schemes

     1.	CL_JPEG_SOFTWARE    software implementation
     2.	CL_JPEG_COSMO	    real-time hardware device

     Each scheme is accessed using the basic CL	API and	parameter set.	This
     man page describes	the additional,	JPEG-specific, parameters that are
     common to both schemes.

     Note that there are two methods of	controlling the	quantization tables.
     An	overall	quality	rating can be specified	that will be used to scale a
     default set of quantization tables, or alternatively, the quantization
     tables may	be manually specified.

     CL_JPEG_QUALITY_FACTOR    [Toc]    [Back]
	  This is one method to	control	the image loss and therefore the
	  compression ratio.

	  The quality factor controls the scaling of the quantization
	  tables and thereby represents	a rough	percentage of image detail
	  preservation.	 Each time the quality factor is set, the reference
	  quantization tables are scaled and downloaded	into the codec.

	  The formula used to obtain the scale factor is
	       scalefactor = 50/quality		 (quality < 50)
	       scalefactor = 2 - 2*quality/100;	 (otherwise)

	  A value of 1 causes very coarse quantization and results in a	high
	  compression ratio, the image quality is practically useless.

	  A value of 100 causes	the codec to quantize as finely	as possible,
	  often	resulting in image expansion and near-perfect quality.

	  A value of CL_JPEG_QUALITY_NO_SCALE will bypass any scaling.
	  The default quality is CL_JPEG_QUALITY_DEFAULT and represents
	  a good quality compressed image.

	  A typical useful range is 25-95.

	  When CL_QUALITY_FACTOR is set, the approximate value of
	  is set, the approximate value	of CL_QUALITY_FACTOR is	calculated.

									Page 1

cl_jpeg(3dm)							  cl_jpeg(3dm)

	  When decompressing JPEG, clDecompress() fills	this value in.

	  Sets or gets the quantization	tables to be used.  For	setting,
	  an unsigned short *qtables[4]	argument is specified as the
	  parameter value.  For	each j,	qtables[j] must	either be NULL or
	  point	to a unsigned short[64]	area of	memory which represents
	  a JPEG baseline (values from 0-255) quantization table in natural
	  scan order.  The user	specified tables are stored as reference
	  tables and scaled versions of	them based on the current
	  CL_JPEG_QUALITY_FACTOR are downloaded	into the codec and become
	  the table associated with id j.   For	getting, the library returns
	  a pointer to static memory containing	a similar array	of four

     The CL programming	interface is supported on the following	platforms:
     Cosmo Compress for	Indy and Indigo2 for use with Indy Video, Galileo
     Video, Indigo2 Video, Impact Video	and Octane Compression.

     The CL is NOT supported on	O2.  Refer to dm_jpeg for information on
     programming the built-in compression hardware on O2.

SEE ALSO    [Toc]    [Back]

     jpeg(4), CLintro(3dm), cl_cosmo(3dm)

									PPPPaaaaggggeeee 2222
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