2011

If you enter only two points, the program automatically computes an
orthogonal transformation.
If you enter three or more points, the program computes the transformation
in each of the three transformation types (Orthogonal, Affine, and Projective)
to determine which best fits the calibration points. If you enter more than
four points, computing the best-fitting projective transformation can take a
long time. You can cancel the process by pressing Esc.
When the computations are complete, the program displays a table with the
number of calibration points and a column for each transformation type.
If there have been no failures of projection transformation, the program
prompts you to choose a transformation type.
Only transformation types for which the outcome was Success, Exact, or
Canceled are included in this prompt. A projective transformation can be
specified even if it was canceled. The program uses the result computed at the
time you canceled.
Orthogonal Specifies translation, uniform scaling, and rotation with two
calibration points.
Use Orthogonal for dimensionally accurate paper drawings and paper drawings
in which the portion to be digitized is long and narrow, with most points
confined to single lines.
NOTE You must specify the lower-left point location before specifying the
upper-right point location.
Affine Specifies arbitrary linear transformation in two dimensions consisting
of translation, independent X- and Y-scaling, rotation, and skewing with three
calibration points.
Use Affine when horizontal dimensions in a paper drawing are stretched with
respect to vertical dimensions, and lines that are supposed to be parallel
actually are parallel.
The RMS (root mean square) error reported after calibration measures how
close the program has come to making a perfect fit. Affine should be used if
the RMS is small.
Projective Specifies a transformation equivalent to a perspective projection
of one plane in space onto another plane with four calibration points. A
projective transformation provides a limited form of what cartographers call
rubber sheeting, in which different portions of the tablet surface are stretched
by varying amounts. Straight lines map into straight lines. Parallel lines do
not necessarily stay parallel.
Projective transformation corrects parallel lines that appear to converge.
TABLET | 1923