public void SetCharacterSpacing( int characterIndex, int characterCount, float leadingSpacing, float trailingSpacing, float minimumAdvance )
Leading spacing is the spacing before each character, in reading order.
Trailing spacing is the spacing after each character, in reading order.
Minimum advance width is the minimum amount to move forward from one character to the next, to prevent characters from becoming too thin or zero-width. This must be zero or greater.