[osg-users] How I build OpenSceneGraph manually for desktop, mobile, and web

[michael kapelko]

Hi.

Recently I've started building OpenSceneGraph without using official
CMakeLists.txt file. I've decided to describe why and how of this
process.

1. WHY.

I use OpenSceneGraph to develop cross-platform applications. While
desktop builds work mostly fine, anything else (Android, iOS, Web) is
easy to break and hard to fix.

Here's my list of top annoying things I had to deal with frequently:

1.1. Inability to build Lua plugin out of the box on macOS

My project's CMakeLists.txt uses `ADD_DEFINITIONS("-std=c++11")` to
force C++11. This breaks Lua plugin building on macOS because Clang
says C language does not have `C++11`, which makes sense. However,
Linux's GCC simply warns about it.

1.2. Hacking PNG plugin safety checks for Android

To build OpenSceneGraph's PNG plugin for Android, I need to hack
CMake's PNG plugin detection because OpenSceneGraph's CMakeLists.txt
uses standard `FIND_PNG`. While this works fine on desktops, Android
sucks at supporting STL, exceptions, and other standard stuff in old
APIs. I personally target API 14.

1.3. Maintaining two different builds for iOS - simulator and device

You have to configure and work with two different versions of the same
application for both simulator and device. Which means you can't
switch between device and simulator in the single project. Not to
mention how many hacks went into OpenSceneGraph's CMakeLists.txt to
support iOS.

1.4. Building OpenSceneGraph takes A LOT of time

Even firing 10 parallel build processes with 'make -j10' is damn slow
because OpenSceneGraph has a ton of files.

2. HOW.

2.1. Build OpenSceneGraph as part of the project into static library

Here's CMakeLists.txt I currently use for all platforms except iOS:
https://github.com/OGStudio/openscenegraph-cross-platform-examples/blob/c42fcbaaf4064b0853246e68837bc91d2b1842c9/libs/OpenSceneGraph/CMakeLists.txt

2.2. Build OpenSceneGraph libraries from custom source files

Custom source files include OpenSceneGraph source files directly.
Here's how osgGA.cpp looks like:
https://github.com/OGStudio/openscenegraph-cross-platform-examples/blob/c42fcbaaf4064b0853246e68837bc91d2b1842c9/libs/OpenSceneGraph/src/osgGA.cpp

This allows to build OpenSceneGraph multitudes time faster at the
expense of using more RAM during building.
As for plugins, I no longer need to hack my way through plugin
dependencies' safety checks, I simply include necessary source files.

2.3. Prepare OpenSceneGraph header files for each platform in advance

OpenSceneGraph and OpenThreads require different headers (osg/GL,
osg/Version, osg/Config, etc.) for different platforms and (probably)
compilers. I simply generated those headers by configuring stub
projects.

2.4. Build OpenSceneGraph as part of iOS project without CMakeLists.txt

Wrap all C++ files (.cpp extension) into Objective-C++ files (.mm
extension) and include them into Xcode project. Now you can switch
between simulator and device just like in other iOS projects.

3. EXAMPLE

OpenSceneGraph cross-platform examples now use described approach.
Check out how easy it is to build them for desktop, mobile, and web
platforms: https://github.com/OGStudio/openscenegraph-cross-platform-examples