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.h and include/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.