The description and interpretations of TrueType font points and curves

A question was asked recently on another forum that made me rethink my understanding of vector formats. TrueType fonts are in a vector format and – I had previously thought – that if a vector editor was to present them for editing, it would present only the points described in the original font shape and, if there are curves, present appropriate handles at only those points.

I don’t see how one software package (Glyphs) can interpret a in Arial as 20 points and another (Illustrator) interpret it as 37 points when converted to outlines or expanded. I don’t have Glyphs to hand but can confirm that Illustrator did present 37 points when trying myself. The original poster provided illustrations of both outcomes as follows:

The a from Arial in Glyphs:

The word arial in the same font in Illustrator (focus on the points in the letter a vs the same in Glyphs):

I understand the basic concept of vector image files; that the data is a series of coordinates for points and other information mathematically detailing the amount of curve (or lack of) between points. What I don’t understand is two things:

  1. Is the data in a TrueType font not described in such a way that it
    can be very literally translated into an editable shape in
    Illustrator when outlined? Why has Illustrator added additional points? Or is the TrueType format not described using points at all (I don’t see how)?

  2. Glyphs appears to demonstrate that it’s possible to describe certain curves in a limited amount of points. Why would a highly reputed
    piece of software like Illustrator need more points to describe
    the same curve? I would see this as a poor translation and I imagine
    most users would expect this too. This overlaps with question one a
    little so perhaps a single answer covers both.


While I cannot confirm this (I checked the Glyphs Manual and Online Help/Tutorials), it seems that the Glyphs App automatically converts the TrueType outlines to PostScript Outlines (lets remember that TT Outlines are 2nd Order, PS Outlines are 3rd order. Also, OTF can contain TT or PS Outlines, while TTF can contain TT Outlines).

If you are importing the Arial typeface (which I am almost sure is a TTF file) it means you are importing TT Outlines, then Glyphs converts to PS Outlines. Hence, less points. Here is Arial.ttf converted to PS Outlines in FontLab VI (29 nodes):

Arial PS Outline

If you open the same font preserving the TT Outlines (in this example, opened in FontLab Studio 5), you get 37 nodes (remember, there is an implied node between successive control points):

Arial TT Outline

In illustrator, type is most probably handled by a type engine that preserves the outlines format, and the quadratic points are just translated on the Create Outline command (try creating the outline of the bullet, a circle: it will create 8 nodes, coherent with the quadratic subjacent format…) Hence, more points are created. Here, in illustrator cs6:

enter image description here

Same 37 nodes.

Edit: BTW, answering the question: Glyphs “simplifies” the TT curve using Bezier Degree Elevation, i.e., converting the TT outline to PS.

Source : Link , Question Author : biscuitstack , Answer Author : Pepe Ochoa

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