= Building to use Intel IPP libjpeg = 

== Objective ==

Intel produces a custom optimized version of libjpeg which takes advantage of various particular chip features of some intel chips for faster access.  

== The Dilemma ==

However, there are a few tricky aspects to using it as a drop in replacement for normal libjpeg "6b" builds.  In particular in non-progressive mode the IPP libjpeg does not always fully fill or drain the io buffers while the generic libjpeg does.  This causes problems for application level code (normally io redirection code) that assumes the libjpeg 6b behavior. 

== The solutions ==

The work around is to patch the libjpeg interfaces in places like the GDAL jpeg driver, and the GDAL GTiff driver (for jpeg compressed tiff).  

Unfortunately, the patch turns out not to be compatible with libjpeg 6b, which does not properly set some state information before calling the flush/fill functions.  This puts us in a situation where it is ''nearly'' impossible to have a version of GDAL that works with either the IPP or non IPP versions of libjpeg without recompiling.  However, if we also fix libjpeg 6b to properly set the state information as shown in r15507 then the other patches ''are'' safe.   Note that r15507 should not break any application using libjpeg.  So it is safe to apply to libjpeg 6b regardless of who is using it.

== How this plays out in OSGeo4W ==

We need to:

 * Rebuild the libjpeg package (6b) with r15507, r16363 and r16601 (done). 
 * We need to rebuild GDAL with the IPP support in the JPEG (r15508) driver enabled (done). 
 * We need to rebuild the libtiff package with IPP support enabled in tif_jpeg.c (done). 
 * We need to create a libjpeg-ipp package for OSGeo4W that replaces the standard jpeg_osgeo.dll (done). 
 * We need to rebuild MapServer with an adjusted GD (is this right?) (not done)


Outstanding:

 * MapServer GD not yet updated.
 * libtiff with libjpeg-ipp sometimes produces spurious "Premature end of file" messages. 

== Building ==

In order to build GDAL and libtiff in "IPP compatible" form, it is necessary to define the IPPJ_HUFF macro for the build.  This can be manually accomplished for each.

=== libtiff ===

Add the following in libtiff\libtiff\tiffconf.h:

{{{
#define IPPJ_HUFF
}}}

=== GDAL ===

On unix edit GDALmake.opt and add a "USER_DEFS=-DIPPJ_HUFF" macro at the top of the file.

On windows add /DIPPJ_HUFF in the OPTFLAGS macro.

== Related Pages ==

 * OSGeo4W libjpeg-ipp package: http://trac.osgeo.org/osgeo4w/wiki/pkg-libjpeg-ipp
 * IPP GDAL ticket: http://trac.osgeo.org/gdal/ticket/2606
 * IPP Libtiff ticket: http://bugzilla.maptools.org/show_bug.cgi?id=1951
 * IPP Related GDAL problem ticket: http://trac.osgeo.org/gdal/ticket/2845