In order to use particular OpenGL extensions in your OpenGL application, Chromium must also implement those extensions. This page describes how OpenGL extensions are implemented in Chromium and how to add new ones.
The include/cr_versions.h file defines a preprocessor token for each extension Chromium supports. For example:
#define CR_ARB_multitexture 1
#define CR_ARB_texture_cube_map 1
#define CR_EXT_blend_color 1
#define CR_EXT_blend_minmax 1
#define CR_EXT_blend_subtract 1
Most Chromium client programs are ordinary OpenGL applications and should
GL_-prefixed tokens to test for extensions. However,
Chromium-specific applications may use these tokens if needed.
Internally, Chromium uses these tokens to determine which OpenGL
extensions should be enabled. If one wants to disable a particular
extension in Chromium you can simply comment-out the appropriate
When adding a new extension to chromium you should surround all the new
code for that extension with
#ifdef CR_(extension name) and
At runtime, Chromium clients should use
glGetString(GL_EXTENSIONS) to determine if extensions are
Internally, Chromium will compute the extension string by examining the extensions supported by each back-end OpenGL implementation, finding the intersection, and propogating the list up through the SPU DAG to be sure the extensions are supported by all the Chromium SPUs.
Extension strings and OpenGL limits like maximum texture size and number of light sources for each SPU are stored in the mothership. When each SPU is initialized it reports its OpenGL extensions and limits to the mothership. Some SPUs, like the tilesort SPU, will query the mothership for its children/server SPU limits in order to determine its own limits.
Here are the steps for adding a new extension to Chromium:
CR_* token to include/cr_versions.h.
CRExtensionStatestructure in the include/state/cr_limits.h file.
crStateExtensionsInitin spu/state_tracker/state_limits.c to initialize the new boolean. Update the extension strings in state_tracker/state_limits.c as well.
#endifso that it can be disabled if desired. See the
__enableSet()in spu/state_tracker/state_enable.c for an example.
A short-cut to adding new extensions is to implment them for the application-side only. This means that the extension can only be used by SPUs running on the application node (such as the render SPU or readback SPU when using a sort-last configuration). An examples of an application-only extension is GL_NV_fence.
Basically, only the OpenGL API dispatcher needs to be updated. The
extension should be added to the
string in state_tracker/state_limits.c.