The License

Anti-Grain Geometry is an Open Source, free library released under the terms and conditions of GNU GPL license.

 Anti-Grain Geometry (AGG) - Version 2.5
 A high quality rendering engine for C++
 Copyright (C) 2002-2006 Maxim Shemanarev
 Contact: mcseem@antigrain.com
          mcseemagg@yahoo.com
          http://antigrain.com
 
 AGG is free software; you can redistribute it and/or
 modify it under the terms of the GNU General Public License
 as published by the Free Software Foundation; either version 2
 of the License, or (at your option) any later version.
 
 AGG is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 GNU General Public License for more details.
 
 You should have received a copy of the GNU General Public License
 along with AGG; if not, write to the Free Software
 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, 
 MA 02110-1301, USA.

See the details and full text at http://www.gnu.org/copyleft/gpl.html

The previous versions have more liberal licences that allow for free use in commercial software.

Commercial licensing is also available upon request.

Anti-Grain Geometry - Version 2.4
Copyright (C) 2002-2004 Maxim Shemanarev (McSeem)

Permission to copy, use, modify, sell and distribute this software 
is granted provided this copyright notice appears in all copies. 
This software is provided "as is" without express or implied
warranty, and with no claim as to its suitability for any purpose.

Anti-Grain Geometry - Version 2.4 
Copyright (C) 2002-2005 Maxim Shemanarev (McSeem) 

Redistribution and use in source and binary forms, with or without 
modification, are permitted provided that the following conditions 
are met:

  1. Redistributions of source code must retain the above copyright 
     notice, this list of conditions and the following disclaimer. 

  2. Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in 
     the documentation and/or other materials provided with the 
     distribution. 

  3. The name of the author may not be used to endorse or promote 
     products derived from this software without specific prior 
     written permission. 

THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 
IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED 
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 
ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, 
INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 
SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 
STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING 
IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 
POSSIBILITY OF SUCH DAMAGE.

Please mention the authors in any work derived from Anti-Grain Geometry.

Anti-Grain Geometry distribution package includes General Polygon Clipper (GPC) by Alan Murta. GPC has a different kind of license that allows you to use Alan's work for free only in non-commercial software. Below is the full text of it.

           http://www.cs.man.ac.uk/aig/staff/alan/software/

Author:    Alan Murta (email: gpc@cs.man.ac.uk)
Version:   2.31
Date:      4th June 1999

Copyright: (C) 1997-1999, Advanced Interfaces Group,
           University of Manchester.

           This software is free for non-commercial use. It may be copied,
           modified, and redistributed provided that this copyright notice
           is preserved on all copies. The intellectual property rights of
           the algorithms used reside with the University of Manchester
           Advanced Interfaces Group.

           You may not use this software, in whole or in part, in support
           of any commercial product without the express consent of the
           author.

           There is no warranty or other guarantee of fitness of this
           software for any purpose. It is provided solely "as is".

However GPC is not an obligatory part of AGG and can be easily removed. You just don't copy (or remove) agg2/gpc/* and you can also remove agg2/include/agg_conv_gpc.h in case if the GPC license does not allow you to use it.

GPC is a General Polygon Clipper that performs boolean operations on polygons (poly-polygons to be exact). In AGG you can get the same visual result using Scanline Boolean Algebra. It operates with scanline shapes and in average works 5-10 times faster than GPC. See:
Demo scanline_boolean.cpp
Demo scanline_boolean2.cpp

You need GPC if you really really want to have vector data in the ouput. For screen graphics of any kind Scanline Boolen Algebra is quite a good replacement.