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Previously, we linked against libOSMesa through the linker and then used dlopen to link against libGL. This is not how libOSMesa is intended to be used. Instead, we use dlopen on libOSMesa (via bluegl), which then will map the correct GL methods for us, and for the OSMesa functions, we also map those functions from libOSMesa (instead of relying on compile-time linker).
BlueGL Mechanics
So you want to call glClear()?
Step 0: Run bluegl-gen.py
This step is only required if updating or modifying BlueGL. These artifacts should already be checked into the Filament repository.
From the libs/bluegl folder, run:
./bluegl-gen.py
The bluegl-gen.py script generates a set of files:
- assembly (proxy) files:
BlueGLCore*.S - header files:
include/BlueGLDefines.handinclude/bluegl/BlueGL.h - a private header:
include/private_BlueGL.h
Step 1: Include the BlueGL defines header:
#include <bluegl/BlueGLDefines.h>
This headers adds a bunch of defines:
...
#define glClear bluegl_glClear
...
Step 2: Include the BlueGL header after the defines header:
#include <bluegl/BlueGLDefines.h>
#include <bluegl/BlueGL.h>
This also includes the GL headers, like <GL/glcorearb.h> for you.
Step 3: Call bluegl::bind()
Internally, the BlueGL library maintains a list of function pointers:
void* __blue_glCore_glClear;
During bluegl::bind(), each function gets assigned to the appropriate symbol loaded from the OS-specific GL shared library via dlopen, dlsym, and equivalents.
Step 4: Call glClear()
Because of the prior #define, you'll actually be calling bluegl_glClear(). This is a trampoline function, defined in the BlueGLCore*.S assembly file (the exact implementation varies slightly on each platform):
.private_extern _bluegl_glClear
_bluegl_glClear:
mov ___blue_glCore_glClear@GOTPCREL(%rip), %r11
jmp *(%r11)
The invokes the __blue_glCore_glClear function, which was previously assigned to the actual GL function.