credit FreeType for the hinting interpreter

Where the TrueType spec leaves details open, the interpreter follows FreeType's ttinterp.c (rounding, original distances, IUP, movement along near-perpendicular vectors, tolerance of invalid references). The FreeType License asks for the credit line in the documentation: it is in the source header, THIRD_PARTY_NOTICES.md and the README, and the license text ships as licenses/FTL.txt. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Ljnxm1Qjrocq1izGajLx8J
matmen · Oct 15, 2026 · Oct 15, 2026 · Oct 15, 2026 · Oct 15, 2026 · 9b07e0a103863a739c806fe57745abaaf2762c59
commit 9b07e0a103863a739c806fe57745abaaf2762c59
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -35,7 +35,6 @@
 - Text rendering: glyph outlines touching the left edge of their bitmap could come out inverted (a rasterizer
   rounding issue); hinting made it common
 
-
 ## 3.0.0
 
 ImageScript is now written in TypeScript and has no WebAssembly or native binaries anymore. Every codec and renderer

diff --git a/README.md b/README.md
@@ -92,7 +92,9 @@ The deflate/inflate implementation is derived from [fflate](https://github.com/1
 The JPEG codec is based in part on the work of the Independent JPEG Group (libjpeg) and on
 [pdf.js](https://github.com/mozilla/pdf.js), the glyph rasterizer is translated from
 [font-rs](https://github.com/raphlinus/font-rs). The WebP codecs follow [RFC 9649](https://www.rfc-editor.org/rfc/rfc9649) and [RFC 6386](https://www.rfc-editor.org/rfc/rfc6386)
-and reproduces [libwebp](https://chromium.googlesource.com/webm/libwebp)'s color conversion. See [THIRD_PARTY_NOTICES.md](THIRD_PARTY_NOTICES.md) for their licenses.
+and reproduce [libwebp](https://chromium.googlesource.com/webm/libwebp)'s color conversion, the TrueType hinting follows
+[FreeType](https://freetype.org)'s interpreter (portions of this software are copyright © 2026 The FreeType Project).
+See [THIRD_PARTY_NOTICES.md](THIRD_PARTY_NOTICES.md) for their licenses.
 
 ---
 

diff --git a/THIRD_PARTY_NOTICES.md b/THIRD_PARTY_NOTICES.md
@@ -13,6 +13,7 @@ following projects, under their own licenses. Each derived file says so in its h
 | [RFC 9649](https://www.rfc-editor.org/rfc/rfc9649) (WebP Image Format) | `src/codecs/vp8l.ts` (lossless bitstream, short distance code table), `src/codecs/webp.ts` (container) | BSD-3-Clause (Revised BSD) |
 | [RFC 6386](https://www.rfc-editor.org/rfc/rfc6386) (VP8 Data Format and Decoding Guide, with the libvpx reference decoder) | `src/codecs/vp8-tables.ts` (probability and quantizer tables), `src/codecs/vp8.ts` (decoding procedures) | BSD-3-Clause |
 | [libwebp](https://chromium.googlesource.com/webm/libwebp) | `src/codecs/vp8.ts` (YUV to RGB conversion, "fancy" chroma upsampling), `src/codecs/webp.ts` (animation frame blending) | BSD-3-Clause |
+| [FreeType](https://freetype.org) | `src/render/hinting.ts` (behavior and parts of the logic of the TrueType interpreter) | FreeType License |
 
 ## fflate, Fast CRC32 in JavaScript
 
@@ -142,3 +143,14 @@ SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSE
 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 ```
+
+## FreeType
+
+Portions of this software are copyright © 2026 The FreeType Project (https://freetype.org). All rights reserved.
+
+The TrueType hinting interpreter in `src/render/hinting.ts` is written after the TrueType instruction specification.
+Where the specification leaves details open, its behavior and parts of its logic follow FreeType's interpreter
+(`src/truetype/ttinterp.c`): the fixed point rounding, measuring original distances on the unscaled outline, the
+interpolation of untouched points (IUP), the handling of near-perpendicular freedom and projection vectors, and the
+tolerance of invalid references. FreeType is used under the FreeType License, included as
+[licenses/FTL.txt](licenses/FTL.txt).
diff --git a/licenses/FTL.txt b/licenses/FTL.txt
@@ -0,0 +1,169 @@
+                    The FreeType Project LICENSE
+                    ----------------------------
+
+                            2006-Jan-27
+
+                    Copyright 1996-2002, 2006 by
+          David Turner, Robert Wilhelm, and Werner Lemberg
+
+
+
+Introduction
+============
+
+  The FreeType  Project is distributed in  several archive packages;
+  some of them may contain, in addition to the FreeType font engine,
+  various tools and  contributions which rely on, or  relate to, the
+  FreeType Project.
+
+  This  license applies  to all  files found  in such  packages, and
+  which do not  fall under their own explicit  license.  The license
+  affects  thus  the  FreeType   font  engine,  the  test  programs,
+  documentation and makefiles, at the very least.
+
+  This  license   was  inspired  by  the  BSD,   Artistic,  and  IJG
+  (Independent JPEG  Group) licenses, which  all encourage inclusion
+  and  use of  free  software in  commercial  and freeware  products
+  alike.  As a consequence, its main points are that:
+
+    o We don't promise that this software works. However, we will be
+      interested in any kind of bug reports. (`as is' distribution)
+
+    o You can  use this software for whatever you  want, in parts or
+      full form, without having to pay us. (`royalty-free' usage)
+
+    o You may not pretend that  you wrote this software.  If you use
+      it, or  only parts of it,  in a program,  you must acknowledge
+      somewhere  in  your  documentation  that  you  have  used  the
+      FreeType code. (`credits')
+
+  We  specifically  permit  and  encourage  the  inclusion  of  this
+  software, with  or without modifications,  in commercial products.
+  We  disclaim  all warranties  covering  The  FreeType Project  and
+  assume no liability related to The FreeType Project.
+
+
+  Finally,  many  people  asked  us  for  a  preferred  form  for  a
+  credit/disclaimer to use in compliance with this license.  We thus
+  encourage you to use the following text:
+
+   """
+    Portions of this software are copyright © <year> The FreeType
+    Project (https://freetype.org).  All rights reserved.
+   """
+
+  Please replace <year> with the value from the FreeType version you
+  actually use.
+
+
+Legal Terms
+===========
+
+0. Definitions
+--------------
+
+  Throughout this license,  the terms `package', `FreeType Project',
+  and  `FreeType  archive' refer  to  the  set  of files  originally
+  distributed  by the  authors  (David Turner,  Robert Wilhelm,  and
+  Werner Lemberg) as the `FreeType Project', be they named as alpha,
+  beta or final release.
+
+  `You' refers to  the licensee, or person using  the project, where
+  `using' is a generic term including compiling the project's source
+  code as  well as linking it  to form a  `program' or `executable'.
+  This  program is  referred to  as  `a program  using the  FreeType
+  engine'.
+
+  This  license applies  to all  files distributed  in  the original
+  FreeType  Project,   including  all  source   code,  binaries  and
+  documentation,  unless  otherwise  stated   in  the  file  in  its
+  original, unmodified form as  distributed in the original archive.
+  If you are  unsure whether or not a particular  file is covered by
+  this license, you must contact us to verify this.
+
+  The FreeType  Project is copyright (C) 1996-2000  by David Turner,
+  Robert Wilhelm, and Werner Lemberg.  All rights reserved except as
+  specified below.
+
+1. No Warranty
+--------------
+
+  THE FREETYPE PROJECT  IS PROVIDED `AS IS' WITHOUT  WARRANTY OF ANY
+  KIND, EITHER  EXPRESS OR IMPLIED,  INCLUDING, BUT NOT  LIMITED TO,
+  WARRANTIES  OF  MERCHANTABILITY   AND  FITNESS  FOR  A  PARTICULAR
+  PURPOSE.  IN NO EVENT WILL ANY OF THE AUTHORS OR COPYRIGHT HOLDERS
+  BE LIABLE  FOR ANY DAMAGES CAUSED  BY THE USE OR  THE INABILITY TO
+  USE, OF THE FREETYPE PROJECT.
+
+2. Redistribution
+-----------------
+
+  This  license  grants  a  worldwide, royalty-free,  perpetual  and
+  irrevocable right  and license to use,  execute, perform, compile,
+  display,  copy,   create  derivative  works   of,  distribute  and
+  sublicense the  FreeType Project (in  both source and  object code
+  forms)  and  derivative works  thereof  for  any  purpose; and  to
+  authorize others  to exercise  some or all  of the  rights granted
+  herein, subject to the following conditions:
+
+    o Redistribution of  source code  must retain this  license file
+      (`FTL.TXT') unaltered; any  additions, deletions or changes to
+      the original  files must be clearly  indicated in accompanying
+      documentation.   The  copyright   notices  of  the  unaltered,
+      original  files must  be  preserved in  all  copies of  source
+      files.
+
+    o Redistribution in binary form must provide a  disclaimer  that
+      states  that  the software is based in part of the work of the
+      FreeType Team,  in  the  distribution  documentation.  We also
+      encourage you to put an URL to the FreeType web page  in  your
+      documentation, though this isn't mandatory.
+
+  These conditions  apply to any  software derived from or  based on
+  the FreeType Project,  not just the unmodified files.   If you use
+  our work, you  must acknowledge us.  However, no  fee need be paid
+  to us.
+
+3. Advertising
+--------------
+
+  Neither the  FreeType authors and  contributors nor you  shall use
+  the name of the  other for commercial, advertising, or promotional
+  purposes without specific prior written permission.
+
+  We suggest,  but do not require, that  you use one or  more of the
+  following phrases to refer  to this software in your documentation
+  or advertising  materials: `FreeType Project',  `FreeType Engine',
+  `FreeType library', or `FreeType Distribution'.
+
+  As  you have  not signed  this license,  you are  not  required to
+  accept  it.   However,  as  the FreeType  Project  is  copyrighted
+  material, only  this license, or  another one contracted  with the
+  authors, grants you  the right to use, distribute,  and modify it.
+  Therefore,  by  using,  distributing,  or modifying  the  FreeType
+  Project, you indicate that you understand and accept all the terms
+  of this license.
+
+4. Contacts
+-----------
+
+  There are two mailing lists related to FreeType:
+
+    o freetype@nongnu.org
+
+      Discusses general use and applications of FreeType, as well as
+      future and  wanted additions to the  library and distribution.
+      If  you are looking  for support,  start in  this list  if you
+      haven't found anything to help you in the documentation.
+
+    o freetype-devel@nongnu.org
+
+      Discusses bugs,  as well  as engine internals,  design issues,
+      specific licenses, porting, etc.
+
+  Our home page can be found at
+
+    https://freetype.org
+
+
+--- end of FTL.TXT ---
diff --git a/src/render/hinting.ts b/src/render/hinting.ts
@@ -1,9 +1,15 @@
+/*!
+ * TrueType hinting interpreter. Where the TrueType specification leaves details open, its behavior and parts of its
+ * logic follow FreeType's TrueType interpreter (src/truetype/ttinterp.c): fixed point rounding, original distances
+ * on the unscaled outline (IP, IUP, MDRP, MD), the IUP interpolation, the movement along near-perpendicular vectors
+ * and the tolerance of invalid references. Portions of this software are copyright © 2026 The FreeType Project
+ * (https://freetype.org). All rights reserved. FreeType License, see THIRD_PARTY_NOTICES.md and licenses/FTL.txt
+ */
 // TrueType hinting: an interpreter for the glyph programs (instructions) of TrueType fonts, following the TrueType
 // instruction set specification (Apple TrueType Reference Manual, Microsoft OpenType "TrueType instructions").
 // Hinting moves outline points so that stems and edges land on pixel boundaries at small sizes.
-// Coordinates are 26.6 fixed point numbers (64 = one pixel), kept as JS numbers. Where the specification leaves
-// details open (rounding of fixed point math, tolerance of invalid references, near-perpendicular vectors), this
-// behaves like FreeType's classic ("v35") interpreter, which the tests compare against point for point.
+// Coordinates are 26.6 fixed point numbers (64 = one pixel), kept as JS numbers. The tests compare the results with
+// FreeType's classic ("v35") interpreter point for point.
 import { Font, GlyfPoints, PathCommand, contourToPath } from './font.ts';
 
 class HintError extends Error { }