[ VIGRA Homepage | Class Index | Function Index | File Index | Main Page ]

details Resampling Convolution Filters VIGRA


Functions

template<...> void resamplingConvolveX (SrcIter sul, SrcIter slr, SrcAcc src, DestIter dul, DestIter dlr, DestAcc dest, Kernel const &kernel, Rational< int > const &samplingRatio, Rational< int > const &offset)
 Apply a resampling filter in the x-direction.

template<...> void resamplingConvolveY (SrcIter sul, SrcIter slr, SrcAcc src, DestIter dul, DestIter dlr, DestAcc dest, Kernel const &kernel, Rational< int > const &samplingRatio, Rational< int > const &offset)
 Apply a resampling filter in the y-direction.

template<...> void resamplingConvolveImage (SrcIterator sul, SrcIterator slr, SrcAccessor src, DestIterator dul, DestIterator dlr, DestAccessor dest, KernelX const &kx, Rational< int > const &samplingRatioX, Rational< int > const &offsetX, KernelY const &ky, Rational< int > const &samplingRatioY, Rational< int > const &offsetY)
 Apply two separable resampling filters successively, the first in x-direction, the second in y-direction.



Detailed Description


These functions implement the convolution operation when the source and target images have different sizes. This is realized by accessing a continous kernel at the appropriate non-integer positions. The technique is, for example, described in D. Schumacher: General Filtered Image Rescaling, in: Graphics Gems III, Academic Press, 1992.


Function Documentation


  void resamplingConvolveImage (...)
 
 

Apply two separable resampling filters successively, the first in x-direction, the second in y-direction.

This function is a shorthand for the concatenation of a call to resamplingConvolveX() and resamplingConvolveY() with the given kernels. See there for detailed documentation.

Declarations:

pass arguments explicitly:

    namespace vigra {
        template <class SrcIterator, class SrcAccessor,
                  class DestIterator, class DestAccessor,
                  class KernelX, class KernelY>
        void resamplingConvolveImage(SrcIterator sul,SrcIterator slr, SrcAccessor src,
                           DestIterator dul, DestIterator dlr, DestAccessor dest,
                           KernelX const & kx, 
                           Rational<int> const & samplingRatioX, Rational<int> const & offsetX,
                           KernelY const & ky, 
                           Rational<int> const & samplingRatioY, Rational<int> const & offsetY);
    }

use argument objects in conjunction with Argument Object Factories:

    namespace vigra {
        template <class SrcIterator, class SrcAccessor,
                  class DestIterator, class DestAccessor,
                  class KernelX, class KernelY>
        void
        resamplingConvolveImage(triple<SrcIterator, SrcIterator, SrcAccessor> src,
                           triple<DestIterator, DestIterator, DestAccessor> dest,
                           KernelX const & kx, 
                           Rational<int> const & samplingRatioX, Rational<int> const & offsetX,
                           KernelY const & ky, 
                           Rational<int> const & samplingRatioY, Rational<int> const & offsetY);
    }

Usage:

#include "vigra/resampling_convolution.hxx"

    Rational<int> xratio(2), yratio(3), offset(0);

    FImage src(w,h), 
           dest(rational_cast<int>(xratio*w), rational_cast<int>(yratio*h));

    float sigma = 2.0;
    Gaussian<float> smooth(sigma);
    ...

    // simpultaneously enlarge and smooth source image
    resamplingConvolveImage(srcImageRange(src), destImageRange(dest), 
                            smooth, xratio, offset,
                            smooth, yratio, offset);


  void resamplingConvolveX (...)
 
 

Apply a resampling filter in the x-direction.

This function implements a convolution operation in x-direction (i.e. applies a 1D filter to every row) where the width of the source and destination images differ. This is typically used to avoid aliasing if the image is scaled down, or to interpolate smoothly if the image is scaled up. The target coordinates are transformed into source coordinates by

    xsource = (xtarget - offset) / samplingRatio

The samplingRatio and offset must be given as vigra::Rational in order to avoid rounding errors in this transformation. It is required that for all pixels of the target image, xsource remains within the range of the source image (i.e. 0 <= xsource <= sourceWidth-1. Since xsource is in general not an integer, the kernel must be a functor that can be accessed at arbitrary (double) coordinates. It must also provide a member function radius() which specifies the support (non-zero interval) of the kernel. VIGRA already provides a number of suitable functors, e.g. vigra::Gaussian, vigra::BSpline vigra::CatmullRomSpline, and vigra::CoscotFunction. The function resizeImageSplineInterpolation() is implemented by means resamplingConvolveX() and resamplingConvolveY().

Declarations:

pass arguments explicitly:

    namespace vigra {
        template <class SrcIter, class SrcAcc,
                  class DestIter, class DestAcc,
                  class Kernel>
        void 
        resamplingConvolveX(SrcIter sul, SrcIter slr, SrcAcc src,
                            DestIter dul, DestIter dlr, DestAcc dest,
                            Kernel const & kernel,
                            Rational<int> const & samplingRatio, Rational<int> const & offset);
    }

use argument objects in conjunction with Argument Object Factories:

    namespace vigra {
        template <class SrcIter, class SrcAcc,
                  class DestIter, class DestAcc,
                  class Kernel>
        void 
        resamplingConvolveX(triple<SrcIter, SrcIter, SrcAcc> src,
                            triple<DestIter, DestIter, DestAcc> dest,
                            Kernel const & kernel,
                            Rational<int> const & samplingRatio, Rational<int> const & offset);
    }

Usage:

#include "vigra/resampling_convolution.hxx"

    Rational<int> ratio(2), offset(0);

    FImage src(w,h), 
           dest(rational_cast<int>(ratio*w), h);

    float sigma = 2.0;
    Gaussian<float> smooth(sigma);
    ...

    // simpultaneously enlarge and smooth source image
    resamplingConvolveX(srcImageRange(src), destImageRange(dest), 
                        smooth, ratio, offset);

Required Interface:

    Kernel kernel;
    int kernelRadius = kernel.radius();
    double x = ...;  // must be <= radius()
    double value = kernel(x);


  void resamplingConvolveY (...)
 
 

Apply a resampling filter in the y-direction.

This function implements a convolution operation in y-direction (i.e. applies a 1D filter to every column) where the height of the source and destination images differ. This is typically used to avoid aliasing if the image is scaled down, or to interpolate smoothly if the image is scaled up. The target coordinates are transformed into source coordinates by

    ysource = (ytarget - offset) / samplingRatio

The samplingRatio and offset must be given as vigra::Rational in order to avoid rounding errors in this transformation. It is required that for all pixels of the target image, ysource remains within the range of the source image (i.e. 0 <= ysource <= sourceHeight-1. Since ysource is in general not an integer, the kernel must be a functor that can be accessed at arbitrary (double) coordinates. It must also provide a member function radius() which specifies the support (non-zero interval) of the kernel. VIGRA already provides a number of suitable functors, e.g. vigra::Gaussian, vigra::BSpline vigra::CatmullRomSpline, and vigra::CoscotFunction. The function resizeImageSplineInterpolation() is implemented by means resamplingConvolveX() and resamplingConvolveY().

Declarations:

pass arguments explicitly:

    namespace vigra {
        template <class SrcIter, class SrcAcc,
                  class DestIter, class DestAcc,
                  class Kernel>
        void 
        resamplingConvolveY(SrcIter sul, SrcIter slr, SrcAcc src,
                            DestIter dul, DestIter dlr, DestAcc dest,
                            Kernel const & kernel,
                            Rational<int> const & samplingRatio, Rational<int> const & offset);
    }

use argument objects in conjunction with Argument Object Factories:

    namespace vigra {
        template <class SrcIter, class SrcAcc,
                  class DestIter, class DestAcc,
                  class Kernel>
        void 
        resamplingConvolveY(triple<SrcIter, SrcIter, SrcAcc> src,
                            triple<DestIter, DestIter, DestAcc> dest,
                            Kernel const & kernel,
                            Rational<int> const & samplingRatio, Rational<int> const & offset);
    }

Usage:

#include "vigra/resampling_convolution.hxx"

    Rational<int> ratio(2), offset(0);

    FImage src(w,h), 
           dest(w, rational_cast<int>(ratio*h));

    float sigma = 2.0;
    Gaussian<float> smooth(sigma);
    ...

    // simpultaneously enlarge and smooth source image
    resamplingConvolveY(srcImageRange(src), destImageRange(dest), 
                        smooth, ratio, offset);

Required Interface:

    Kernel kernel;
    int kernelRadius = kernel.radius();
    double y = ...;  // must be <= radius()
    double value = kernel(y);

© Ullrich Köthe (koethe@informatik.uni-hamburg.de)
Cognitive Systems Group, University of Hamburg, Germany

html generated using doxygen and Python
VIGRA 1.4.0 (21 Dec 2005)