Programmer's Reference for the BIC Volume IO Library
<<< PreviousNext >>>

Transforms

In dealing with such tasks as inter- and intra-subject registration both within and across imaging modalities, the concept of transformations between different coordinate systems arises. A module is provided to handle both linear (affine) and non-linear transformations, and to provide input and output in a standardized Brain Imaging Centre format, usually to filenames with the extension .xfm.

To support these functions, two structure types are provided. The first (Transform) is a four by four linear (affine) transform, which facilitates rigid transformations, consisting of scaling, rotation, translation, and shearing. The second is a higher level transform (General_transform), which represents either a linear transform, a non-linear transform (thin-plate spline), a smoothly interpolated grid transform, a user definable transform, or a concatenation of two or more of these.

Linear Transforms

The linear transform functions all deal with objects whose type is Transform. 

#define   Transform_elem( transform, i, j )

Is used to set or retrieve the value of the i'th row and j'th column of the transform, where 0<=i, j<=3 

public  void  make_identity_transform(
    Transform   *transform )

Creates a four by four identity transform. 

public  void  transform_point(
    Transform  *transform,
    Real       x,
    Real       y,
    Real       z,
    Real       *x_trans,
    Real       *y_trans,
    Real       *z_trans )

Transforms a three dimensional point by the given transform, passing back the three transformed coordinates. 

public  void  transform_vector(
    Transform  *transform,
    Real       x,
    Real       y,
    Real       z,
    Real       *x_trans,
    Real       *y_trans,
    Real       *z_trans )

Transforms a three dimensional vector by the given transform, passing back the three transformed coordinates. The only difference between transforming a point and a vector is that transforming a vector does not involve the translational component of the transform. 

public  void  inverse_transform_point(
    Transform  *transform,
    Real       x,
    Real       y,
    Real       z,
    Real       *x_trans,
    Real       *y_trans,
    Real       *z_trans )

Assuming the transform is an orthogonal transform (no shear components), the point is transformed by the inverse of the transform. 

public  void  inverse_transform_vector(
    Transform  *transform,
    Real       x,
    Real       y,
    Real       z,
    Real       *x_trans,
    Real       *y_trans,
    Real       *z_trans )

Assuming the transform is an orthogonal transform (no shear components), the vector is transformed by the inverse of the transform. 

public  void   concat_transforms(
    Transform   *result,
    Transform   *t1,
    Transform   *t2 )

Multiplies the transform t2 by the transform t1, storing the product in result. Transforming a point by result is equivalent to transforming the point by t1, then transforming the result by t2.

<<< PreviousHomeNext >>>
Tag Points General Transforms