# Subject: Understanding transformation types

Depending on the number of points specified, you have a choice of transformation types to use, along with the type recommended for CADdirect.

Transformation refers to the calculation of the points on the screen that correspond to points you digitize on the tablet.

Orthogonal Preserves all angles between lines, preserves all relative distances, and, in general, preserves shapes. If you specify only two points, an orthogonal transformation is automatically generated. The orthogonal transformation type yields the most precise mapping between the digitized points on the tablet and the corresponding points on the screen.

Affine Preserves parallel lines, but not necessarily the angles between intersecting lines. If you specified three points, CADdirect can no longer represent this map­ping as an exact orthogonal transformation. Therefore, you have a choice of an exact affine transformation or a “best fit” orthogonal transformation.

Projective Does not preserve parallel lines or angles. If you specify exactly four points, you have a choice of an exact projective transformation or a “best fit” orthogonal or affine transformation.

 Recommended and “best fit” transformation types
 Number of points specified Transformation type recommended “Best fit” (approximate) 2 Orthogonal None 3 Affine Orthogonal 4 Projective Orthogonal, Affine 5-10 None Orthogonal, Affine

Choose the recommended type unless you know it will not be appropriate for what you are digitizing. The most appropriate type is not always the one with the least error; for example, you might digitize three points and select the orthogonal transfor­mation, even though the affine transformation would yield a closer representation of your calibration entries.

 Digitizing by selecting points (1, 2, 3, and 4) on the tablet. Corresponding screen coordinates: 1      Coordinate specification X0, Y0, Z0 2      Coordinate specification X0, Y5, Z0 3      Coordinate specification X5, Y5, Z0 4      Coordinate specification X5, Y0, Z0

 Screen result of orthogonal transformation after tracing polyline on tablet. Screen result of affine transformation after tracing polyline on tablet. Screen result of projective transformation after tracing polyline on tablet.

To calibrate your tablet for digitizing points

1   Do one of the following:

Choose Tools > Tablet > Calibrate.

Type tablet, choose Calibrate, and then press Enter.

2   Click a point on the tablet to define as the first calibration point.

3   Specify a point within the CADdirect drawing window to correspond to the point you digitized on the tablet, or enter coordinate values in the command bar.

4   Click a point on the tablet to define as the second calibration point.

Specify a point within the CADdirect drawing window to correspond to the point you digitized on the tablet, or enter coordinate values in the command bar. To specify more than the two points required, click a point on the tablet to define as the third calibration point. You can enter up to 10 points.

5   Choose the transformation type, and then press Enter.