A snippet to determine how much horizontal shift a point should get in an italic glyph, in a UFO font.

italic-shift.py

"""
    Here’s a snippet to determine how much horizontal shift a point should get in an italic glyph.

    In practical terms, I have just used this to shift anchors in a UFO font source, in order to prepare an oblique font.

    Basically, how can we determine "x" below, given a height ("y") and an angle (italicAngle)?
  
       x
    • - - •
    |    /
    |   /
  y |  /
    | /
    |/
    •
    11.31°
    
    This is basic trigonometry, but it’s easy to forget! We need to use the "TOA" from "SOH CAH TOA."

    > tan(angle) = opposite/adjacent

    which is equivalent to...

    > opposite = tan(angle) * adjacent

    or, in Python...

    > xShift = math.tan(math.radians(slant) * yPos

    So, the below is a simplified script to do determine the x shift to add at a given y position.
  
"""

# import the Python math library
import math

# get the font object in some way
font = CurrentFont() 

# a given y postion, e.g. a mark anchor at cap height
yPos = 1000

# a clockwise or "forward" slant is negative, so we have to use the negative of the italic Angle
slant = font.info.italicAngle * -1

# Use tangent to determine the xShift. Note that we must also convert the angle to radians for this to work.
xShift = math.tan(math.radians(slant)) * yPos