Consider AFM fonts and practicality

Author: aitikgupta

doc/users/fonts.rst

@@ -6,7 +6,8 @@ contributions of tech giants such as the likes of Adobe, Apple and Microsoft.
 
 Types
 -----
-In practice, there are 3 types Matplotlib supports:
+In practice, there are 3 types Matplotlib supports (in addition to
+'core fonts', more about which is explained later in the guide):
 
 .. list-table:: Type of Fonts
    :header-rows: 1
@@ -16,19 +17,14 @@ In practice, there are 3 types Matplotlib supports:
      - 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!
+     - Newer than previous types, used commonly today, introduced by Apple
+   * - Restricted subset of PostScript, charstrings are in bytecode
+     - Full PostScript language, allows embedding arbitrary code
        (in theory, even render fractals when rasterizing!)
-     - They include a virtual machine that can execute code!
+     - 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!
@@ -65,15 +61,52 @@ 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.
+goal is to find out *which* glyphs are required for a certain array of
+characters, 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
+.. note::
+  The role of subsetter really shines when we encounter characters like `ä`
+  (composed by calling subprograms for ``a`` and ``¨``); since the subsetter
+  has to find out *all* such subprograms being called by every glyph included
+  in the subset, and since there is almost no consistency within multiple
+  different backends and the types of subsetting, this is a generally difficult
+  problem!
+
+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)
+Python for certain types)
 
-| *Type 1 fonts are still non-subsetted* through Matplotlib.
+| *Type 1 fonts are still non-subsetted* through Matplotlib. (though one will only encounter these via `usetex`/`dviread` in PDF backend)
 | **Type 3 and Type 42 fonts are subsetted**, with a fair amount of exceptions and bugs for the latter.
+
+What to use?
+------------
+Practically, most fonts that are readily available on most operating systems or
+are readily available on the internet to download include *TrueType fonts* and
+its "extensions" such as MacOS-resource fork fonts and the newer OpenType
+fonts.
+
+PS and PDF backends provide support for yet another type of fonts, which remove
+the need of subsetting altogether! These are called **Core Fonts**, and
+Matplotlib calls them via the keyword **AFM**; all that is supplied from
+Matplotlib to such documents are font metrics (specified in AFM format), and it
+is the job of the viewer applications to supply the glyph definitions.
+
+This is especially helpful to generate *really lightweight* documents.::
+
+    # trigger core fonts for PDF backend
+    plt.rcParams["pdf.use14corefonts"] = True
+    # trigger core fonts for PS backend
+    plt.rcParams["ps.useafm"] = True
+
+    chars = "AFM ftw!"
+    fig, ax = plt.subplots()
+    ax.text(0.5, 0.5, chars)
+
+    fig.savefig("AFM_PDF.pdf", format="pdf")
+    fig.savefig("AFM_PS.ps", format="ps)
+
+.. note::
+  These core fonts are limited to PDF and PS backends only; they can not be
+  rendered in other backends.