Gets the index of the character that the glyph cell bounding box contains at the specified point.
A zero-based index of the character that is located at the given point.
Note A value of -1 indicates that no such character exists. If multiple characters exist at the specified point, getCharNumAtPosition returns the index of the character that the glyphs were rendered for last.
- Scalable Vector Graphics: Text, Section 10.17.1