interpolating_accessor.hxx

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#ifndef VIGRA_INTERPOLATING_ACCESSOR_HXX
#define VIGRA_INTERPOLATING_ACCESSOR_HXX


#include "accessor.hxx"
#include "diff2d.hxx"

namespace vigra {

/** \addtogroup DataAccessors
*/
//@{

/********************************************************/
/*                                                      */
/*            BilinearInterpolatingAccessor             */
/*                                                      */
/********************************************************/

/** \brief Bilinear interpolation at non-integer positions.

    This accessor allows an image be accessed at arbitrary non-integer
    coordinates and performs an bi-linear interpolation to
    obtain a pixel value.
    It uses the given ACCESSOR (which is usually the
    accessor originally associated with the iterator)
    to access data.

    <b>\#include</b> <vigra/accessor.hxx>
    Namespace: vigra

    <b> Required Interface:</b>

    \code
    ITERATOR iter;
    ACCESSOR a;
    VALUETYPE destvalue;
    float s;
    int x, y;

    destvalue = s * a(iter, x, y) + s * a(iter, x, y);

    \endcode
*/
template <class ACCESSOR, class VALUETYPE>
class BilinearInterpolatingAccessor
{
  public:
    /** the iterators' pixel type
    */
    typedef VALUETYPE value_type;

    /** init from given accessor
    */
    BilinearInterpolatingAccessor(ACCESSOR a)
    : a_(a)
    {}

    /** Interpolate the data item at a non-integer position.
        Ensure that no outside pixels are accessed if
        (x, y) is near the image border (as long as
        0 <= x <= width-1, 0 <= y <= height-1).
    */
    template <class ITERATOR>
    value_type operator()(ITERATOR const & i, float x, float y) const
    {
        int ix = int(x);
        int iy = int(y);
        float dx = x - ix;
        float dy = y - iy;

        value_type ret;

        // avoid dereferencing the iterator outside its range
        if(dx == 0.0)
        {
            if(dy == 0.0)
            {
                ret = a_(i, Diff2D(ix, iy));
            }
            else
            {
                ret = detail::RequiresExplicitCast<value_type>::cast(
                  (1.0 - dy) * a_(i, Diff2D(ix, iy)) +
                  dy * a_(i, Diff2D(ix, iy + 1)));
            }
        }
        else
        {
            if(dy == 0.0)
            {
                ret = detail::RequiresExplicitCast<value_type>::cast(
                  (1.0 - dx) * a_(i, Diff2D(ix, iy)) +
                  dx * a_(i, Diff2D(ix + 1, iy)));
            }
            else
            {
                ret = detail::RequiresExplicitCast<value_type>::cast(
                  (1.0 - dx) * (1.0 - dy) * a_(i, Diff2D(ix, iy)) +
                  dx * (1.0 - dy) * a_(i, Diff2D(ix + 1, iy)) +
                  (1.0 - dx) * dy * a_(i, Diff2D(ix, iy + 1)) +
                  dx * dy * a_(i, Diff2D(ix + 1, iy + 1)));
            }
        }

        return ret;
    }

    /** Interpolate the data item at a non-integer position.
        This function works as long as 0 <= x < width-1,
        0 <= y < height-1. It is slightly faster than <TT>operator()</TT>.
    */
    template <class ITERATOR>
    value_type unchecked(ITERATOR const & i, float x, float y) const
    {
        int ix = int(x);
        int iy = int(y);
        float dx = x - ix;
        float dy = y - iy;
        return detail::RequiresExplicitCast<value_type>::cast(
               (1.0 - dx) * (1.0 - dy) * a_(i, Diff2D(ix, iy)) +
               dx * (1.0 - dy) * a_(i, Diff2D(ix + 1, iy)) +
               (1.0 - dx) * dy * a_(i, Diff2D(ix, iy + 1)) +
               dx * dy * a_(i, Diff2D(ix + 1, iy + 1)));
    }

  private:
    ACCESSOR a_;
};

//@}

} // namespace vigra

#endif /* VIGRA_INTERPOLATING_ACCESSOR_HXX */