Apply an arbitrary affine 2D transformation to pixel coordinates.

affine_trans_pixel applies an arbitrary affine 2D transformation, i.e., scaling, rotation, translation, and slant (skewing), to the input pixels (Row,Col) and returns the resulting pixels in (RowTrans,ColTrans); the input and output pixels are subpixel precise coordinates. The affine transformation is described by the homogeneous transformation matrix given in HomMat2D.

The difference between affine_trans_pixel and affine_trans_point_2d lies in the used coordinate system: affine_trans_pixel uses a coordinate system with origin in the upper left corner of the image, while affine_trans_point_2d uses the standard image coordinate system, whose origin lies in the middle of the upper left pixel and which is also used by operators like area_center.

Applying affine_trans_pixel corresponds to the following chain of tansformations (input and output pixels as homogeneous vectors):

  / RowTrans \   / 1 0 -0.5 \              / 1 0 +0.5 \   / Row \
  | ColTrans | = | 0 1 -0.5 | * HomMat2D * | 0 1 +0.5 | * | Col |
  \    1     /   \ 0 0   1  /              \ 0 0   1  /   \  1  /

Hence,

  affine_trans_pixel (HomMat2D, Row, Col, RowTrans, ColTrans)

  affine_trans_point_2d (HomMat2D, Row+0.5, Col+0.5, RowTmp, ColTmp)
  RowTrans := RowTmp-0.5
  ColTrans := ColTmp-0.5

If the matrix HomMat2D represents an affine transformation (i.e., not a projective transformation), affine_trans_pixel returns 2 (H_MSG_TRUE). Otherwise, an exception is raised.

affine_trans_pixel is reentrant and processed without parallelization.

hom_mat2d_translate, hom_mat2d_translate_local, hom_mat2d_scale, hom_mat2d_scale_local, hom_mat2d_rotate, hom_mat2d_rotate_local, hom_mat2d_slant, hom_mat2d_slant_local

affine_trans_point_2d

Foundation