1. Rotating 180°, 90° clockwise, 90° counterclockwise,
flipping horizontally or vertically is easy: select Image - Rotate,
and then the appropriate direction. Rotating just a few degrees
is a bit trickier.

In both images a measuring line
has been placed between two points. The "Info" Palette
indicates that the rotation of Image 2 is .5 degrees off compared
to Image 1. (The negative sign indicates that each image would
need to be rotated counterclockwise in order to be perfectly
horizontal.)

2. In both images, choose the same two points to check for
angle. Using the measuring tool (see "Sizing") draw
a measuring line from one point to the other.

3. Check the angle of the lines under "Info."

4. Calculate the difference between the angle in Image 1 and
Image 2, and determine whether Image 2 needs to be rotated clockwise
or counterclockwise to match Image 1.

Rotating the "canvas" means that the entire image
is rotated, including all layers that might be added to a file.
The computer interpolates pixels along the edges to fill in the
space that would be left blank by the rotation process.