cl_jpeg(3dm)

NAME
     cl_jpeg - JPEG schemes in the Compression Library

SYNOPSIS
     #include <dmedia/cl.h>


DESCRIPTION
     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
          Type: CL_RANGE_VALUE
          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
          CL_COMPRESSION_RATIO is calculated and when CL_COMPRESSION_RATIO
          is set, the approximate value of CL_QUALITY_FACTOR is calculated.
          When decompressing JPEG, clDecompress() fills this value in.

     CL_JPEG_QUANTIZATION_TABLES
          Type: CL_RANGE_VALUE
          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
          points.



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