The file twain.h may be copied under the TWAIN License as specified at
http://www.twain.org/license.shtm

The TWAIN License

The TWAIN Working Group grants customer ("Customer") the worldwide,
royalty-free, non-exclusive license to reproduce and distribute the
software and documentation of the TWAIN toolkit ("TWAIN Toolkit"). The
TWAIN Toolkit was designed to be used by third parties to assist them
in becoming compliant with the TWAIN standard, but it has not been
developed to the standards of a commercial product. Consequently, the
TWAIN toolkit is provided AS IS without any warranty. THE TWAIN
Working Group disclaims all warranties in the TWAIN toolkit whether
implied, express or statutory, including, without limitation, the
implied warranties of merchantability, noninfringement of third party
rights and fitness for a particular purpose. The TWAIN Working Group
disclaims all liability for damages, whether direct, indirect,
special, incidental, or consequential, arising from the reproduction,
distribution, modification, or other use of the TWAIN Toolkit.

As a condition of this license, Customer agrees to include in software
programs based in whole or in part on the TWAIN Toolkit the following
providions in (i) the header or similar file in such software and (ii)
prominently in its documentation and to require its sublicensees to
include these provisions in similar locations: The TWAIN Toolkit is
distributed as is. The developer and distributors of the TWAIN Toolkit
expressly disclaim all implied, express or statutory warranties
including, without limitation, the implied warranties of
merchantability, noninfringement of third party rights and fitness for
a particular purpose. Neither the developers nor the distributors will
be liable for damages, whether direct, indirect, special, incidental,
or consequential, as a result of the reproduction, modification,
distribution or other use of the TWAIN Toolkit.