/**

@page transformsAndMaps Transforms and Maps

@section Contents
- @ref sTransforms
  - @ref sLinearTransforms
  - @ref sFrustumTransforms
  - @ref sCellVsVertex
  - @ref sVoxels
  - @ref sStaggered
- @ref sMaps
  - @ref sGettingMat4
  - @ref sCostOfMaps
  - @ref sGradientAndMaps

@section sTransforms Transforms in OpenVDB
The OpenVDB @vdblink::tree::Tree Tree@endlink is a sparse representation
of a three-dimensional array of voxels, each element of which is addressed via
discrete, three-dimensional index space coordinates, typically in the form of
a @vdblink::math::Coord Coord@endlink.
For example, the following code retrieves the floating-point value of
a voxel with index coordinates&nbsp;(1,&nbsp;2,&nbsp;3):
@code
openvdb::FloatGrid grid = ...;
openvdb::FloatGrid::Accessor accessor = grid.getAccessor();
openvdb::Coord ijk(1,2,3);
float value = accessor.getValue(ijk);
@endcode

A @vdblink::math::Transform Transform@endlink relates index space coordinates
to world space coordinates that give a spatial context for the discretized data.Translation from index coordinates @ijk to world space coordinates @xyz is
done with a call to the
@vdblink::math::Transform::indexToWorld() indexToWorld@endlink
method, and from world space coordinates to index space coordinates with
a call to @vdblink::math::Transform::worldToIndex() worldToIndex@endlink:
@code
// Create a linear transform that scales i, j and k by 0.1
openvdb::math::Transform::Ptr linearTransform =
    openvdb::math::Transform::createLinearTransform(0.1);

// Compute the location in world space that is the image of (1,2,3).
// The result will be (0.1, 0.2, 0.3).
openvdb::Coord ijk(1,2,3);
openvdb::Vec3d worldSpacePoint = linearTransform->indexToWorld(ijk);

// Compute the location in index space that is the pre-image of (0.1, 0.2, 0.3).
// The result will be (1.0, 2.0, 3.0).
openvdb::Vec3d indexSpacePoint = linearTransform->worldToIndex(worldSpacePoint);
@endcode
In the above example, there are two things to notice.
First, world space locations are specified as three-dimensional,
double-precision, floating-point vectors, and second, @c worldToIndex
does not return discrete coordinates, but rather a floating-point vector.
This is a reflection of the fact that not every location in a continuous
world space, i.e., not every @xyz, is the image of discrete integral
coordinates @ijk.

@subsection sLinearTransforms Linear Transforms
Currently two different types of transforms are supported: linear and
frustum transforms.
A linear transform can be composed of scale, translation, rotation, and shear;
essentially those things that can be mathematically represented by an
invertible, constant-coefficient, @f$3 \times 3@f$ matrix and a translation
(mathematically, an affine map).
An essential feature of a linear transformation is that it treats all regions
of index space equally: a small box in index space about origin @ijk=(0,0,0)
is mapped (sheared, scaled, rotated, etc.) in just the same way that
a small box about any other index point is mapped.
@code
// Create a linear transform from a 4x4 matrix (identity in this example).
openvdb::math::Mat4d mat = openvdb::math::Mat4d::identity();
openvdb::math::Transform::Ptr linearTransform =
    openvdb::math::Transform::createLinearTransform(mat);

// Rotate the transform by 90 degrees about the X axis.
// As a result the j-index will now map into the -z physical direction,
// and the k-index will map to the +y physical direction.
linearTransform->preRotate(M_PI/2, openvdb::math::X_AXIS);
@endcode

@subsection sFrustumTransforms Frustum Transforms
The frustum transform is a nonlinear transform that treats different
index points differently.
And while the linear transform can be applied to any point in index
or world space, the frustum transform is designed to operate on a subset
of space.  Specifically, it transforms a given box in index space to
a tapered box in world space that is a frustum of a rectangular pyramid.
@code
// Create the bounding box that will be mapped by the transform into
// the shape of a frustum.
// The points (0,0,0), (100,0,0), (0,50,0) and (100,50,0) will map to
// the corners of the near plane of the frustum, while the corners
// of the far plane will be the images of (0,0,120), (100,0,120),
// (0,50,120) and (100,50,120).
const openvdb::math::BBoxd bbox(/*min=*/openvdb::math::Vec3d(0,0,0),
                                /*max=*/openvdb::math::Vec3d(100,50,120));

// The far plane of the frustum will be twice as big as the near plane.
const double taper = 2;

// The depth of the frustum will be 10 times the x-width of the near plane.
cosnt double depth = 10;

// The x-width of the frustum in world space units
const double xWidth = 100;

// Construct a frustum transform that results in a frustum whose
// near plane is centered on the origin in world space.
openvdb::math::Transform::Ptr frustumTransform =
    openvdb::math:::Transform::createFrustumTransform(
        bbox, taper, depth, xWidth);

// The frustum shape can be rotated, scaled, translated and even
// sheared if desired.  For example, the following call translates
// the frustum by 10,15,0 in world space:
frustumTransform->postTranslate(openvdb::math::Vec3d(10,15,0));

// Compute the world space image of a given point within
// the index space bounding box that defines the frustum.
openvdb::Coord ijk(20,10,18);
openvdb::Vec3d worldLocation = frustumTransform->indexToWorld(ijk);
@endcode

@subsection sCellVsVertex Cell-Centered vs. Vertex-Centered Transforms
When partitioning world space with a regular grid, two popular
configurations are cell-centered and vertex-centered grids.  This is really
a question of interpretation and transforms.

The cell-centered interpretation imagines world space as divided
into discrete cells (e.g., cubes) centered on the image of the
index-space lattice points.
So the physical location @xyz that is the image (result of @c indexToWorld)
of a lattice point @ijk is the center of a cube.
In the vertex-centered approach, the images of the lattice points form the
vertices of cells, so the location @xyz would be a corner, not the
center, of a cube.

The link between transforms and cell-centered or vertex-centered
partitioning of world space is tenuous.
It boils down to this: the &ldquo;first&rdquo; cell vertex often is aligned
with the origin.
In the cell-centered case, when the cells are cubes of length @f$\Delta@f$
on a side, the transform
@f$(x,y,z) = (\Delta i + \Delta/2, \Delta j + \Delta/2, \Delta k + \Delta /2 )@f$
will place the center of the first cube (i.e., the image of @f$(0,0,0)@f$)
at the world space location of @f$(\Delta/2, \Delta/2, \Delta/2)@f$,
and the cube will have walls coincident with the @f$x=0@f$, @f$y=0@f$
and @f$z=0@f$ planes.
Using the OpenVDB transforms to create a so-called cell-centered transform
could be done like this:
@code
// -- Constructing a uniform, cell-centered transform --

// The grid spacing
const double delta = 0.1;

// The offset to cell-center points
const openvdb::math::Vec3d offset(delta/2., delta/2., delta/2.);

// A linear transform with the correct spacing
openvdb::math::Transform::Ptr transform =
    openvdb::math:::Transform::createLinearTransform(delta);

// Add the offset.
transform->postTranslate(offset);
@endcode
In contrast, for the vertex-centered partitions of space the first
vertex is just the image of the first lattice point @f$ijk = (0,0,0)@f$,
and the transform would lack the offset used in the cell-centered case;
so it would simply be @f$(x,y,z) = (\Delta i , \Delta j ,\Delta k)@f$
@code
// -- Constructing a uniform, vertex-centered transform --

// The grid spacing
const double delta = 0.1;

// A linear transform with the correct spacing
openvdb::math::Transform::Ptr transform =
    openvdb::math:::Transform::createLinearTransform(delta);
@endcode

@subsection sVoxels Voxel Interpretations
A similar and often related concept to cell- and vertex-centered
partitioning of world space is the idea of a voxel.
A voxel historically refers to the volumetric equivalent of a pixel
and as such implies a small region of world space.
A voxel could, for instance, be the image under transform of a vertex-centered
(or cell-centered) box in index space.
In this way the voxel can be seen as a generalization of regular grid cells.
The interpretation of data stored in a grid can be related to the concept
of a voxel but need not be.
An application might interpret the grid value indexed by @ijk as the
spatial average of a quantity in a defined world-space voxel centered
on the image of that lattice point.
But in other situations the value stored at @ijk might be a discrete sample
taken from a single point in world space.

The @vdblink::math::Transform Transform@endlink class does include
methods such as @vdblink::math::Transform::voxelSize() voxelSize@endlink
and @vdblink::math::Transform::voxelVolume() voxelVolume@endlink that suppose
a particular interpretation of a voxel.
They assume a voxel that is the image of a vertex-centered cube in index space,
so the @c voxelSize methods return the lengths of line segments in world space
that connect vertices:
@code
openvdb::Coord ijk(0,0,0);
openvdb::Coord tmp0(1,0,0), tmp1(0,1,0), tmp2(0,0,1);

openvdb::math::Vec3d size;
size.x() = (xform.indexToWorld(ijk + tmp0) - xform.indexToWorld(ijk)).length();
size.y() = (xform.indexToWorld(ijk + tmp1) - xform.indexToWorld(ijk)).length();
size.z() = (xform.indexToWorld(ijk + tmp2) - xform.indexToWorld(ijk)).length();

// The voxelSize() for the voxel at (0,0,0) is consistent with
// the computation above.
assert(xform.voxelSize(ijk) == size);
@endcode
In the case where the transform is linear, the result of @c voxelSize
will be independent of the actual location @ijk, but the voxel size for a
nonlinear transform such as a frustum will be spatially varying.
The related @c voxelVolume can not in general be computed from the
@c voxelSize, because there is no guarantee that a general transform
will convert a cube-shaped voxel into another cube.
As a result, the @c voxelVolume actually returns the determinant of the
transform, which will be a correct measure of volume even if the voxel
is sheared into a general parallelepiped.

@subsection sStaggered Staggered Velocity Grids
Staggered velocity data is often used in fluid simulations, and the
relationship between data interpretation and transforms is
somewhat peculiar when using a vector grid to hold
staggered velocity data.
In this case the lattice point @ijk identifies a cell in world space
by mapping to the cell&rsquo;s center, but each element of the velocity
vector is identified with a different face of the cell.
The first element of the vector is identified with the image of the
@f$(i-1/2,j,k)@f$ face, the second element with @f$(i,j-1/2,k)@f$,
and the third element with @f$(i,j,k-1/2)@f$.


@section sMaps Maps in OpenVDB Transforms
The actual work of a @vdblink::math::Transform Transform@endlink is performed
by an implementation object called a @vdblink::math::MapBase Map@endlink.
The map in turn is a polymorphic object whose derived types are designed to
optimally represent the most common transformations.
Specifically, the @vdblink::math::Transform Transform@endlink holds a
@vdblink::math::MapBase MapBase@endlink pointer to a derived type that
has been simplified to minimize calculations.
When the transform is updated by prepending or appending a linear operation
(e.g., with @vdblink::math::Transform::preRotate() preRotate@endlink),
the implementation map is recomputed and simplified if possible.
For example, in many level-set-oriented applications the transform
between index space and world space is simply a uniform scaling of the
index points, i.e., @f$(x,y,z) = (\Delta i, \Delta j, \Delta k)@f$,
where @f$\Delta@f$ has some world space units.
For transforms such as this, the implementation is a
@vdblink::math::UniformScaleMap UniformScaleMap@endlink.
@code
// Create a linear transform that scales i, j and k by 0.1
openvdb::math::Transform::Ptr linearTransform =
    openvdb::math::Transform::createLinearTransform(0.1);

// Create an equivalent map.
openvdb::math::UniformScaleMap uniformScale(0.1);

// At this time the map holds a openvdb::math::UniformScaleMap.
assert(linearTransform->mapType() == openvdb::math::UniformScaleMap::type());

openvdb::Coord ijk(1,2,3);

// Applying the transform...
openvdb::math::Vec3d transformResult = linearTransform->indexToWorld(ijk);

// ...is equivalent to applying the map.
openvdb::math::Vec3d mapResult = uniformScale.applyMap(ijk);

assert(mapResult == transformResult);
@endcode

There are specialized maps for a variety of common linear transforms:
pure translation (@vdblink::math::TranslationMap TranslationMap@endlink),
uniform scale (@vdblink::math::UniformScaleMap UniformScaleMap@endlink),
uniform scale and translation
(@vdblink::math::UniformScaleTranslateMap UniformScaleTranslateMap@endlink),
non-uniform scale (@vdblink::math::ScaleMap ScaleMap@endlink), and
non-uniform scale and translation
(@vdblink::math::ScaleTranslateMap ScaleTranslateMap@endlink).
A general affine map (@vdblink::math::AffineMap AffineMap@endlink) is used
for all other cases (those that include non-degenerate rotation or shear).

@subsection sGettingMat4 An Equivalent Matrix Representation
The matrix representation used within OpenVDB adheres to the minority
convention of right-multiplication of the matrix against a vector:
@code
// Example matrix transform that scales, next translates,
// and finally rotates an incoming vector
openvdb::math::Mat4d transform = openvdb::math::Mat4d::identity();
transform.preScale(openvdb::math::Vec3d(2,3,2));
transform.postTranslate(openvdb::math::Vec3d(1,0,0));
transform.postRotate(openvdb::math::X_AXIS, M_PI/3.0);

// Location of a point in index space
openvdb::math::Vec3d indexSpace(1,2,3);

// Apply the transform by right-multiplying the matrix.
openvdb::math::Vec3d worldSpace = indexSpace * transform;
@endcode
Any linear map can produce an equivalent
@vdblink::math::AffineMap AffineMap@endlink, which in turn can produce
an equivalent @f$4 \times 4@f$ matrix.
Starting with a linear transform one can produce a consistent matrix as follows:
@code
openvdb::math::Mat4d matrix;
if (transform->isLinear()) {
   // Get the equivalent 4x4 matrix.
   matrix = transform->getBaseMap()->getAffineMap()->getMat4();
}
@endcode

This could be used as an intermediate form when constructing new linear
transforms by combining old ones.
@code
// Get the matrix equivalent to linearTransformA.
openvdb::math::Mat4d matrixA =
    linearTransformA->getBaseMap()->getAffineMap()->getMat4();

// Invert the matrix equivalent to linearTransformB.
openvdb::math::Mat4d matrixBinv =
    (linearTransformB->getBaseMap()->getAffineMap()->getMat4()).inverse();

// Create a new transform that maps the index space of linearTransformA
// to the index space of linearTransformB.
openvdb::math::Transform::Ptr linearTransformAtoB =
    openvdb::math::Trasform::createLinearTransform(matrixA * matrixBinv);
@endcode
Notice that in the above example, the internal representation used by
the transform will be simplified if possible to use one of the various
map types.

@subsection sCostOfMaps Working Directly with Maps
Accessing a transform&rsquo;s map through virtual function calls
introduces some overhead and precludes compile-time optimizations
such as inlining.
For this reason, the more computationally intensive OpenVDB tools are
templated to work directly with any underlying map.
This allows the tools direct access to the map&rsquo;s simplified
representation and gives the compiler a free hand to inline.

Maps themselves know nothing of index space and world space, but are
simply functions @f$x_{range} = f(x_{domain})@f$ that map 3-vectors from
one space (the domain) to another (the range), or from the range back to
the domain (@f$x_{domain} = f^{-1}(x_{range})@f$), by means of the methods
@vdblink::math::MapBase::applyMap() applyMap@endlink and
@vdblink::math::MapBase::applyInverseMap() applyInverseMap@endlink.

@code
// Create a simple uniform scale map that scales by 10.
openvdb::math::UniformScaleMap usm(10.0);

// A point in the domain
openvdb::math::Vec3d domainPoint(0,1,3);

// The resulting range point
openvdb::math::Vec3d rangePoint = usm.applyMap(domainPoint);

// The map is inverted thus:
assert(domainPoint == usm.applyInverseMap(rangePoint));
@endcode

In many tools, the actual map type is not known a priori and
must be deduced at runtime prior to calling the appropriate
map-specific or map-templated code.  The type of map currently being
used by a transform can be determined using the
@vdblink::math::Transform::mapType() mapType@endlink method:
@code
// Test for a uniform scale map.
if (transform->mapType() == openvdb::math::UniformScaleMap::type()) {

   // This call would return a null pointer in the case of a map type mismatch.
   openvdb::math::UniformScaleMap::ConstPtr usm =
       transform->map<openvdb::math::UniformScaleMap>();

   // Call a function that accepts UniformScaleMaps.
   dofoo(*usm)
}
@endcode

To simplify this process, the function
@vdblink::math::processTypedMap processTypedMap@endlink has been provided.
It accepts a transform and a functor templated on the map type.

@subsection sGradientAndMaps Maps and Mathematical Operations
In addition to supporting the mapping of a point from one space to another,
maps also support mapping of local gradients.
This results from use of the chain rule of calculus in computing the
Jacobian of the map.
Essentially, the gradient calculated in the domain of a map can be converted
to the gradient in the range of the map, allowing one to compute a gradient
in index space and then transform it to a gradient in world space.
@code
// Compute the gradient at a point in index space in a
// floating-point grid using the second-order central difference.
openvdb::FloatGrid grid = ...;
openvdb::Coord ijk(2,3,5)
openvdb::math::Vec3d isGrad =
    openvdb::math::ISGradient<CD_2D>::result(grid, ijk);

// Apply the inverse Jacobian transform to convert the result to the
// gradient in the world space defined by a map that scales index space
// to create voxels of size 0.1 x 0.2 x 0.1
openvdb::math::ScaleMap scalemap(0.1, 0.2, 0.1);
openvdb::math::Vec3d wsGrad = scalemap.applyIJT(isGrad);
@endcode

*/
