Some applications provide features that reflect (or mirror) objects drawn in the client area. Applications that contain reflection capabilities use the SetWorldTransform function to set the appropriate values in the world-space to page-space transformation. This function receives a pointer to an XFORM structure containing the appropriate values. The eM11 and eM22 members of XFORM specify the horizontal and vertical reflection components, respectively.
The reflection transformation creates a "mirror" image of an object with respect to either the x- or y-axis. In short, reflection is just negative scaling. To produce a horizontal reflection, x-coordinates are multiplied by –1. To produce a vertical reflection, y-coordinates are multiplied by – 1.
Horizontal reflection can be represented by the following algorithm:
x' =–x
where x is the x-coordinate and x' is the result of the reflection.
The 2-by-2 matrix that produced horizontal reflection contains the following values:
|-1 0|
|0 1| Vertical reflection can be represented by the following algorithm:
y' = -y
where y is the y-coordinate and y' is the result of the reflection.
The 2-by-2 matrix that produced vertical reflection contains the following values:
|1 0|
|0 -1|
The horizontal-reflection and vertical-reflection operations can be combined into a single operation by using the following 2-by-2 matrix:
|-1 0|
|0 -1|