The elements dx and dy specify horisontal and vertical translation. The elements m11 and m22 specify horisontal and vertical scaling. The elements m12 and m21 specify horisontal and vertical shearing. The identity matrix has m11 and m22 set to 1, all others set to 0. This matrix maps a point to itself.
Translation is the simplest transformation.
Setting dx and dy will move the coordinate system dx units along the X axis and dy units along the Y axis. Scaling can be done by setting m11 and m22 . Shearing is controlled by m12 and m21 . Setting these elements to values different from zero will twist the coordinate system. Rotation is achieved by carefully setting both the shearing factors and the scaling factors.