Document Types of Fonts and Subsetting

Author: aitikgupta

doc/users/fonts.rst

@@ -0,0 +1,79 @@
+All about Matplotlib and Fonts!
+===============================
+
+The story of fonts has been quite eventful throughout time. It involves
+contributions of tech giants such as the likes of Adobe, Apple and Microsoft.
+
+Types
+-----
+In practice, there are 3 types Matplotlib supports:
+
+.. list-table:: Type of Fonts
+   :header-rows: 1
+
+   * - Type 1 (PDF)
+     - Type 3 (PDF/PS)
+     - TrueType (PDF)
+   * - One of the oldest types, introduced by Adobe
+     - Similar to Type 1 in terms of introduction
+     - Newer than previous types, most commonly used today, introduced by Apple
+   * - They use simplified PostScript
+       (subset of full PostScript language)
+     - However, they use full PostScript language, which allows embedding
+       arbitrary code!
+       (in theory, even render fractals when rasterizing!)
+     - They include a virtual machine that can execute code!
+   * - These fonts support font hinting
+     - Do not support font hinting
+     - Hinting supported (virtual machine processes the "hints")
+   * - Expressed in pretty compact bytecode
+     - Expressed in simple ASCII form
+     - Expressed in binary code points
+   * - Difficult to subset!
+     - Easy to subset!
+     - Very hard to subset!
+
+NOTE: Adobe will disable support for authoring with Type 1 fonts in
+January 2023. `Read more here. <https://helpx.adobe.com/fonts/kb/postscript-type-1-fonts-end-of-support.html>`_
+
+Special Mentions
+~~~~~~~~~~~~~~~~
+- Type 42 fonts (PS):
+
+  - PostScript wrapper around TrueType fonts
+  - 42 is the `Answer to Life, the Universe, and Everything! <https://en.wikipedia.org/wiki/Answer_to_Life,_the_Universe,_and_Everything>`_
+  - Very hard to subset!
+
+- OpenType fonts:
+
+  - OpenType is a new standard for digital type fonts, developed jointly by
+    Adobe and Microsoft
+  - Generally contain a much larger character set!
+  - Limited Support with Matplotlib
+
+
+Subsetting
+----------
+Matplotlib is able to generate documents in multiple different formats. Some of
+those formats (for example, PDF, PS/EPS, SVG) allow embedding font data in such
+a way that when these documents are visually scaled, the text does not appear
+pixelated.
+
+This can be achieved by virtually embedding the *whole* font file within the
+output document. However, this can lead to **very large documents**, wherein
+most of the size bandwidth is captured by that font file data.
+
+Font Subsetting is a way to embed only the *required* glyphs within the
+documents. Fonts can be considered as a collection of glyphs, so ultimately the
+goal is to find out *which* glyphs are required for a certain piece of text,
+and embed only those within the output.
+
+Since there is almost no consistency within multiple different backends and the
+types of subsetting, this is generally difficult! Luckily, Matplotlib uses a
+fork of an external dependency called
+`ttconv <https://github.com/sandflow/ttconv>`_, which helps in embedding and
+subsetting stuff. (however, recent versions have moved away from ttconv to pure
+Python)
+
+| *Type 1 fonts are still non-subsetted* through Matplotlib.
+| **Type 3 and Type 42 fonts are subsetted**, with a fair amount of exceptions and bugs for the latter.

doc/users/index.rst

@@ -15,6 +15,7 @@ User's Guide
     installing.rst
     ../tutorials/index.rst
     interactive.rst
+    fonts.rst
     release_notes.rst
     history.rst
     license.rst