The following predicates are provided in order to be able to determine whether or not a transformation has a particular characteristic.

transformation-equal[Generic Function]

Arguments: transformation1 transformation2

Summary: Returns t if the two transformations have equivalent effects (that is, are mathematically equal); otherwise, it returns nil .

identity-transformation-p[Generic Function]

Summary: Returns t if transformation is equal (in the sense of transformation-equal ) to the identity transformation; otherwise, it returns nil .

translation-transformation-p[Generic Function]

Summary: Returns
t
if *
transformation*
is a pure translation, that is, a transformation that moves every point by the same distance in *
x*
and the same distance in *
y*
. Otherwise, it returns
nil
.

invertible-transformation-p[Generic Function]

Summary: Returns t if transformation has an inverse; otherwise, it returns nil .

reflection-transformation-p[Generic Function]

Summary: Returns t if transformation inverts the "handedness" of the coordinate system; otherwise, it returns nil . Note that this is a very inclusive category--transformations are considered reflections even if they distort, scale, or skew the coordinate system, as long as they invert the handedness.

rigid-transformation-p[Generic Function]

Summary: Returns t if transformation transforms the coordinate system as a rigid object, that is, as a combination of translations, rotations, and pure reflections. Otherwise, it returns nil .

Rigid transformations are the most general category of transformations that preserve magnitudes of all lengths and angles.

even-scaling-transformation-p[Generic Function]

Summary: Returns
t
if transformation multiplies all *
x*
-lengths and *
y*
-lengths by the same magnitude; otherwise, it returns
nil
. This includes pure reflections through vertical and horizontal lines.

scaling-transformation-p[Generic Function]

Summary: Returns
t
if transformation multiplies all *
x*
-lengths by one magnitude and all *
y*
-lengths by another magnitude; otherwise, it returns
nil
. This category includes even scalings as a subset.

rectilinear-transformation-p[Generic Function]

Summary: Returns t if transformation will always transform any axis-aligned rectangle into another axis-aligned rectangle; otherwise, it returns nil . This category includes scalings as a subset, and also includes 90 degree rotations.

Rectilinear transformations are the most general category of transformations for which the bounding rectangle of a transformed object can be found by transforming the bounding rectangle of the original object.

CommonLisp Interface Manager 2.0 User's Guide - 30 Jul 2004