[osg-users] OSG-3.6 breaking changes to GLES2 buffered objects

[michael kapelko]

Hi.
This is to continue the discussion of PR that broke web (Emscripten)
builds: https://github.com/openscenegraph/OpenSceneGraph/pull/501

The mentioned changes break rendering of the osgemscripten example
(found in OpenSceneGraph/examples/osgemscripten).
I've prepared two builds to depict the difference.
1. Build with the breaking fix:
https://kornerr.github.io/osg-pbo-gles2-fix-builds/versions/fix/osgemscripten.html
2. Build with the breaking fix reverted:
https://kornerr.github.io/osg-pbo-gles2-fix-builds/versions/reverted-fix/osgemscripten.html

The second build simply has the following GLExtension.cpp line:
  isBufferObjectSupported = isVBOSupported && isPBOSupported;
replaced with:
  isBufferObjectSupported = isVBOSupported;

So the problem is not really in PBO support, but in the fact that
possibly missing PBO support removes buffer object support. This is
wrong. The osgemscripten example uses VBO and this is what WebGL1
requires.

You can build osgemscripten example yourself (README explains how to
do it, it's easy on Linux) and see the errors.
Make sure to use the updated CMakeLists.txt I've attached to this email.
-------------- next part --------------
CMAKE_MINIMUM_REQUIRED(VERSION 2.8.0 FATAL_ERROR)

# Path to OpenSceneGraph
SET(OSG_DIR "${CMAKE_SOURCE_DIR}/../..")

# Specify critical OpenSceneGraph build variables.
SET(BUILD_OSG_APPLICATIONS NO CACHE BOOL "Do not build applications")
SET(EGL_LIBRARY "GL" CACHE STRING "Suppress linkage error")
SET(OSG_GL1_AVAILABLE OFF CACHE BOOL "Unavailable under Emscripten")
SET(OSG_GL2_AVAILABLE OFF CACHE BOOL "Unavailable under Emscripten")
SET(OSG_GLES2_AVAILABLE ON CACHE BOOL "GLES2 is what Emscripten uses")
SET(OSG_WINDOWING_SYSTEM "None" CACHE STRING "Unavailable under Emscripten")
SET(DYNAMIC_OPENTHREADS OFF CACHE BOOL "Link OpenThreads statically")
SET(DYNAMIC_OPENSCENEGRAPH OFF CACHE BOOL "Link OpenSceneGraph statically")
# Prevent CMake configuration error.
SET(_OPENTHREADS_ATOMIC_USE_GCC_BUILTINS_EXITCODE "0" CACHE STRING "Prevent cfg error")

# Set special build flags:
# * -O3: make compact binary, critical for web
# * -s USE_SDL=2: provide SDL2
SET(BUILD_FLAGS "-O3 -s USE_SDL=2")
ADD_DEFINITIONS(${BUILD_FLAGS})

# Reference OpenSceneGraph includes and libraries.
SET(OSG_SOURCE_DIR "${OSG_DIR}")
SET(OSG_BUILD_DIR "${OSG_DIR}/build/Emscripten")
INCLUDE_DIRECTORIES(${OSG_SOURCE_DIR}/include)
INCLUDE_DIRECTORIES(${OSG_BUILD_DIR}/include)
LINK_DIRECTORIES(${OSG_BUILD_DIR}/lib)

# Build OpenSceneGraph if osgViewer library is missing.
IF(EXISTS "${OSG_BUILD_DIR}/lib/libosgViewer.a")
    MESSAGE("Skip building OpenSceneGraph")
ELSE()
    MESSAGE("Build OpenSceneGraph")
    FILE(MAKE_DIRECTORY ${OSG_BUILD_DIR})
    ADD_SUBDIRECTORY(${OSG_SOURCE_DIR} ${OSG_BUILD_DIR})
ENDIF()

# Build example.
ADD_EXECUTABLE(osgemscripten osgemscripten.cpp)
# Make Emscripten generate ready-to-open HTML page.
SET(CMAKE_EXECUTABLE_SUFFIX ".html")
# Make Emscripten preload the resource.
SET_TARGET_PROPERTIES(osgemscripten PROPERTIES LINK_FLAGS "--preload-file box.osgt ${BUILD_FLAGS}")
# Copy the resource to the build directory.
CONFIGURE_FILE(box.osgt box.osgt COPYONLY)
# Libraries must be linked in the specified order.
# Otherwise you may get unsatisified linker errors.
TARGET_LINK_LIBRARIES(
    osgemscripten
    osgViewer
    osgDB
    # osgDB plugins start.
    osgdb_osg
    osgdb_serializers_osg
    # osgDB plugins end.
    osgGA
    osgText
    osgUtil
    osg
    OpenThreads
)