[Boost-users] [GIL] Adding base_channel_type metafunction

Subject: [Boost-users] [GIL] Adding base_channel_type metafunction
From: Nathan Crookston (nathan.crookston_at_[hidden])
Date: 2010-11-02 02:31:05

All,

Several months ago I submitted a patch to prevent warnings when
performing certain channel conversions [1].  After some discussion
with Christian a part of the patch was removed in favor of using the
unsigned_integral_max_value metafunction.  After tweaking my
specializations of that metafunction my code compiled fine.

Recently I modified my code to use GIL more extensively.  I'm pleased
with the reduced code footprint (a 5000 line file was reduced to ~150
lines), but unfortunately there are more warnings due to unsafe
conversions with my now-more-extensive testing.

Attached is a file which adds a base_channel_type metafunction which
correctly determines the type which the channel constructor is
expecting.  I believe this is more correct than using
unsigned_integral_max_value as the value_type of *max_value is not the
same as the channel type for, e.g., 8-bit unsigned channels.
Something like this metafunction has also been requested recently [2].

The following demonstrates its use for the built-in channel type
models I'm aware of:
///////////BEGIN CODE////////////////////
#include <boost/cstdint.hpp>
#include <boost/gil/gil_all.hpp>
#include <boost/static_assert.hpp>
#include <boost/type_traits/is_same.hpp>

namespace gil = boost::gil;

typedef gil::packed_image3_type<boost::uint16_t, 7,7,2,
gil::bgr_layout_t>::type bgr772_image_t;

typedef gil::kth_element_type<bgr772_image_t::value_type,0>::type
b7_element_type;

typedef gil::base_channel_type<b7_element_type>::type b7_type;

BOOST_STATIC_ASSERT((boost::is_same<b7_type, boost::uint8_t>::value));

typedef gil::base_channel_type<gil::channel_type<gil::gray32f_pixel_t>::type>::type
gray32f_type;

BOOST_STATIC_ASSERT((boost::is_same<gray32f_type, float>::value));
typedef gil::base_channel_type<gil::channel_type<gil::rgb8_pixel_t>::type>::type
rgb8_type;
BOOST_STATIC_ASSERT((boost::is_same<rgb8_type, boost::uint8_t>::value));

int main()
{}
/////////END CODE/////////////////

Adding the metafunction and using it to cast values prior to channel
construction has removed the warnings from my testing.  The diff file
also contains changes to channel_algorithm.hpp to suppress warnings
using the supplied metafunction.

I'd be happy to write additional test cases, trac tickets, etc.  The
changes have been tested with g++3.4, g++4.3 and VC8 (VS2005).

Thanks,
Nate

[1] <http://lists.boost.org/boost-users/2010/06/60066.php>
[2] <http://lists.boost.org/Archives/boost/2010/07/169159.php>