rgb8_planar_image_t construction from rgb8_image_t is not supported

[mloskot]

This issue was reported by @sdebionne on Slack.

I don't know if this is a bug or an undocumented limitation (see below), but there may be not too complex ways to fix it (e.g. planar image copy-construction from interleaved image by zero-filling construction followed by copy_pixels with interleaved to planar pixel conversion; slower than uninitialized_copy though). See more details below.

/cc @stefanseefeld @chhenning in case you know more for background of this limitation (or bug).

Actual behavior

gil::rgb8_image_t            img1(gil::rgb8_planar_image_t{}); // OK
gil::rgb8_planar_image_t img2(gil::rgb8_image_t{});             // KO (fails to compile)

Expected behavior

Successful construction of planar images from interleaved images.

C++ Minimal Working Example

#include <boost/gil.hpp>
namespace gil = boost::gil;
int main()
{
    gil::rgb8_image_t img_i;
    gil::rgb8_planar_image_t img_p(img_i); // fails to compile

    // due to no overload of uninitialized_copy_aux supporting
    // input pair of interleaved iterators and output pair of planar iterators
    using both_planar = std::false_type;
    auto iv = gil::view(img_i);
    auto pv = gil::view(img_p);

    // fails to compile
    gil::detail::uninitialized_copy_aux(iv.begin().x(), iv.end().x(), pv.begin().x(), both_planar{});

    // compiles, becuase this direction is supported
    gil::detail::uninitialized_copy_aux(pv.begin().x(), pv.end().x(), iv.begin().x(), both_planar{});
}

Discussion

Apparently, this comment here is not entirely correct or complete:

gil/include/boost/gil/algorithm.hpp

Lines 764 to 767 in b3f72cc

	/// std::uninitialized_copy for interleaved or mixed iterators
	template <typename It1, typename It2>
	BOOST_FORCEINLINE
	void uninitialized_copy_aux(It1 first1, It1 last1, It2 first2, std::false_type)

and should rather read

/// std::uninitialized_copy for pairs of interleaved iterators or input pair of planar iterators and output pair of interleaved iterators

The gil::detail::uninitialized_copy_aux simply performs https://en.cppreference.com/w/cpp/memory/uninitialized_copy which requires

::new (static_cast<void*>(std::addressof(*current))) Value(*first);

that is not feasible for planar pixels.

References

• Revealed during tests of PR Fix image constructor from other image #477

Environment

• Version (Git ref or <boost/version.hpp>): Boost 1.72, 1.73 (current master and develop)