[gimp] devel-docs: add a README.md.

[Jehan <jehanp src gnome org>]

commit 1aeee787a84c94bc8fd92ab34489c5d433df047b
Author: Jehan <jehan girinstud io>
Date:   Thu Jan 20 22:11:27 2022 +0100

    devel-docs: add a README.md.
    
    This will be the root page for the developer documentation. Note that
    there are other files in this directory (old `README` included) which
    will need to be deleted but I don't do it just yet on purpose until I
    checked them and integrate anything which could be of interest back into
    the new documentation.

 devel-docs/README.md | 149 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 149 insertions(+)
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diff --git a/devel-docs/README.md b/devel-docs/README.md
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+# Developers documentation
+
+This manual holds information that you will find useful if you
+develop a GIMP plug-in or want to contribute to the GIMP core.
+
+People only interested into plug-ins can probably read just the
+[Plug-in development](#plug-in-development) section. If you wish to
+contribute to all parts of GIMP, the whole documentation is of interest.
+
+[[TOC]]
+
+## Plug-in development
+### Concepts
+#### Basics
+
+Plug-ins in GIMP are executables which GIMP can call upon certain
+conditions. Since they are separate executables, it means that they are
+run as their own process, making the plug-in infrastructure very robust.
+No plug-in should ever crash GIMP, even with the worst bugs. If such
+thing happens, you can consider this a core bug.
+
+On the other hand, a plug-in can mess your opened files, so a badly
+developed plug-in could still leave your opened images in an undesirable
+state. If this happens, you'd be advised to close and reopen the file
+(provided you saved recently).
+
+Another downside of plug-ins is that GIMP currently doesn't have any
+sandboxing ability. Since we explained that plug-ins are run by GIMP as
+independant processes, it also means they have the same rights as your
+GIMP process. Therefore be careful that you trust the source of your
+plug-ins. You should never run shady plug-ins from untrusted sources.
+
+GIMP comes itself with a lot of plug-ins. Actually nearly all file
+format support is implemented as a plug-in (XCF support being the
+exception: the only format implemented as core code). This makes it a
+very good base to study plug-in development.
+
+#### Procedural DataBase (PDB)
+
+Obviously since plug-ins are separate processes, they need a way to
+communicate with GIMP. This is the Procedural Database role, also known
+as **PDB**.
+
+The PDB is our protocol allowing plug-ins to request or send information
+from or to the main GIMP process.
+
+Not only this, but every plug-in has the ability to register one or
+several procedures itself, which means that any plug-in can call
+features brought by other plug-ins through the PDB.
+
+#### libgimp and libgimpui
+
+The GIMP project provides plug-in developers with the `libgimp` library.
+This is the main library which any plug-in needs. All the core PDB
+procedures have a wrapper in `libgimp` so you actually nearly never need
+to call PDB procedures explicitly (exception being when you call
+procedures registered by other plug-ins; these won't have a wrapper).
+
+The `libgimpui` library is an optional one which provides various
+graphical interface utility functions, based on the GIMP toolkit
+(`GTK`). Of course, it means that linking to this library is not
+mandatory (unlike `libgimp`). Some cases where you would not do this
+are: because you don't need any graphical interface (e.g. a plug-in
+doing something directly without dialog, or even a plug-in meant to be
+run on non-GUI servers); because you want to use pure GTK directly
+without going through `libgimpui` facility; because you want to make
+your GUI with another toolkit…
+
+The whole C reference documentation for both these libraries can be
+generated in the main GIMP build with the `--enable-gi-docgen` autotools
+option or the `-Dgi-docgen=enabled` meson option (you need to have the
+`gi-docgen` tools installed).
+
+TODO: add online links when it is up for the new APIs.
+
+### Programming Languages
+
+While C is our main language, and the one `libgimp` and `libgimpui` are
+provided in, these 2 libraries are also introspected thanks to the
+[GObject-Introspection](https://gi.readthedocs.io/en/latest/) (**GI**)
+project. It means you can in fact create plug-ins with absolutely any
+[language with a GI binding](https://wiki.gnome.org/Projects/GObjectIntrospection/Users)
+though of course it may not always be as easy as the theory goes.
+
+The GIMP project explicitly tests the following languages and even
+provides a test plug-in as a case study:
+
+* [C](https://gitlab.gnome.org/GNOME/gimp/-/blob/master/extensions/goat-exercises/goat-exercise-c.c) (not a 
binding)
+* [Python 
3](https://gitlab.gnome.org/GNOME/gimp/-/blob/master/extensions/goat-exercises/goat-exercise-py3.py)
+  (binding)
+* [Lua](https://gitlab.gnome.org/GNOME/gimp/-/blob/master/extensions/goat-exercises/goat-exercise-lua.lua)
+  (binding)
+* [Vala](https://gitlab.gnome.org/GNOME/gimp/-/blob/master/extensions/goat-exercises/goat-exercise-vala.vala)
+  (binding)
+* 
[Javascript](https://gitlab.gnome.org/GNOME/gimp/-/blob/master/extensions/goat-exercises/goat-exercise-gjs.js)
+  (binding, not supported on Windows for the time being)
+
+One of the big advantage of these automatic bindings is that they are
+full-featured since they don't require manual tweaking. Therefore any
+function in the C library should have an equivalent in any of the
+bindings.
+
+TODO: binding reference documentation.
+
+**Note**: several GObject-Introspection's Scheme bindings exist though
+we haven't tested them. Nevertheless, GIMP also provides historically
+the "script-fu" interface, based on an integrated Scheme implementation.
+It is different from the other bindings (even from any GI Scheme
+binding) and doesn't use `libgimp`. Please see the [Script-fu
+development](#script-fu-development) section.
+
+### Tutorials
+
+TODO: at least in C and in one of the officially supported binding
+(ideally even in all of them).
+
+### Porting from GIMP 2 plug-ins
+
+## Script-fu development
+
+`Script-fu` is its own thing as it is a way to run Scheme script with
+GIMP. It is itself implemented as an always-running plug-in with its own
+Scheme mini-interpreter and therefore `Script-fu` scripts do not use
+`libgimp` or `libgimpui`. They interface with the PDB through the
+`Script-fu` plug-in.
+
+### Tutorials
+
+### Porting from GIMP 2 scripts
+
+## GEGL operation development
+
+## Custom data
+### Brushes
+### Dynamics
+### Patterns
+### Themes
+### Icon themes
+
+## GIMP extensions (*.gex*)
+
+## Core development
+### Newcomers
+
+### Core Contributors
+
+### Directory structure of the GIMP source tree
+
+### Advanced concepts