[FFmpeg-devel] [PATCH 1/3] Add clew.c & clew.h to libavutil

[Gupta, Maneesh]

> -----Original Message-----
> From: ffmpeg-devel-bounces at ffmpeg.org [mailto:ffmpeg-devel-
> bounces at ffmpeg.org] On Behalf Of wm4
> Sent: Friday, February 06, 2015 4:58 PM
> To: ffmpeg-devel at ffmpeg.org
> Subject: Re: [FFmpeg-devel] [PATCH 1/3] Add clew.c & clew.h to libavutil
> 
> On Fri, 6 Feb 2015 11:06:28 +0000
> "Gupta, Maneesh" <Maneesh.Gupta at amd.com> wrote:
> 
> >
> >
> > > -----Original Message-----
> > > From: ffmpeg-devel-bounces at ffmpeg.org [mailto:ffmpeg-devel-
> > > bounces at ffmpeg.org] On Behalf Of wm4
> > > Sent: Friday, February 06, 2015 4:31 PM
> > > To: ffmpeg-devel at ffmpeg.org
> > > Subject: Re: [FFmpeg-devel] [PATCH 1/3] Add clew.c & clew.h to
> > > libavutil
> > >
> > > On Fri, 6 Feb 2015 11:14:21 +0100
> > > Hendrik Leppkes <h.leppkes at gmail.com> wrote:
> > >
> > > > On Fri, Feb 6, 2015 at 11:09 AM, Gupta, Maneesh
> > > <Maneesh.Gupta at amd.com> wrote:
> > > > > Hi All,
> > > > >
> > > > > There are several issues with the way ffmpeg compilation works
> > > > > when
> > > OpenCL is enabled using --enable-opencl. Chief among them are:
> > > > > 1. One needs to also use --extra-cflags, --extra-ldflags &
> > > > > -extra-libs to
> > > specify the path to the OpenCL header and library files. Otherwise
> > > configure fails.
> > > > > 2. ffmpeg currently requires OpenCL 1.2, but the build system
> > > > > may have
> > > another version installed (such as the newer OpenCL 2.0 which has
> > > deprecated some 1.2 APIs).
> > > > >
> > > > > There are a couple of ways to address this.
> > > > >
> > > > > * One way is the x264 approach which is to bundle cl.h,
> > > > > cl_platform.h and
> > > a wrapper c file which relies on dynamically loading OpenCL rather
> > > than using static linking. This requires modifications to the entire
> > > OpenCL based code since we cannot use the OpenCL APIs directly.
> > > > > * Another approach taken by LibreOffice and Blender for example
> > > > > is to
> > > use clew (i.e. The OpenCL Extension Wrangler Library). This has the
> > > advantage of letting us use the OpenCL APIs directly yet at the same
> > > time relying on dynamically loading OpenCL. The clew project is
> > > hosted at https://github.com/OpenCLWrangler/clew. This is maintained
> > > by two people (Sergey and Martijn).
> > > > > Usage of clew in Libreoffice can be seen @
> > > http://cgit.freedesktop.org/libreoffice/core/tree/clew/source and in
> > > Blender @
> > > https://git.blender.org/gitweb/gitweb.cgi/blender.git/tree/HEAD:/ext
> > > ern/cl
> > > ew.
> > > > >
> > > > > This patch is for adding clew to the ffmpeg tree. The subsequent
> > > > > patches
> > > switch the OpenCL code in ffmpeg from using cl.h to using clew.
> > > > >
> > > >
> > > >
> > > > Using such a library is fine, if its deemed the best solution
> > > > available, but flat-out importing its code into avutil is not.
> > >
> > > +1
> > >
> > > Also look at all these awful "portability" hacks in the added code.
> > > It's not a good idea to agree to maintaining this terrible mess.
> >
> > Alright. Can you suggest an alternate way of building ffmpeg using clew?
> Clew has lot of advantages and essentially consists of a single source and
> header file. I will modify and resubmit the patch accordingly.
> 
> Convince upstream to package it as real library.

Clew can already be built as a static library. It ships with a  cmake file. However I am a little confused here. My intention for using clew was to remove the pain of having to specify multiple parameters to enable the OpenCL feature. If clew is used as a library, instead of having use --extra-cflags, --extra-ldflags to specific the path to standard OpenCL bits I would have to still use them to point ffmpeg to the path to clew. Or am I missing something here?

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