clGetGLContextInfoKHR

Parameters

Description

Get OpenGL context information.

Notes

The OpenCL specification in section 9.7 defines how to share data with texture and buffer objects in a parallel OpenGL implementation, but does not define how the association between an OpenCL context and an OpenGL context or share group is established. This extension defines optional attributes to OpenCL context creation routines which associate a GL context or share group object with a newly created OpenCL context. If this extension is supported by an implementation, the string "cl_khr_gl_sharing" will be present in the CL_DEVICE_EXTENSIONS string described in the table of allowed values for param_name for clGetDeviceInfo or in the CL_PLATFORM_EXTENSIONS string described in the table of allowed values for param_name for clGetPlatformInfo.

This section discusses OpenCL functions that allow applications to use OpenGL buffer, texture, and renderbuffer objects as OpenCL memory objects. This allows efficient sharing of data between OpenCL and OpenGL. The OpenCL API may be used to execute kernels that read and/or write memory objects that are also OpenGL objects.

An OpenCL image object may be created from an OpenGL texture or renderbuffer object. An OpenCL buffer object may be created from an OpenGL buffer object.

OpenCL memory objects may be created from OpenGL objects if and only if the OpenCL context has been created from an OpenGL share group object or context. OpenGL share groups and contexts are created using platform specific APIs such as EGL, CGL, WGL, and GLX. On MacOS X, an OpenCL context may be created from an OpenGL share group object using the OpenCL platform extension cl_apple_gl_sharing. On other platforms including Microsoft Windows, Linux/Unix and others, an OpenCL context may be created from an OpenGL context using the Khronos platform extension cl_khr_gl_sharing. Refer to the platform documentation for your OpenCL implementation, or visit the Khronos Registry at http://www.khronos.org/registry/cl/ for more information.

Any supported OpenGL object defined within the GL share group object, or the share group associated with the GL context from which the CL context is created, may be shared, with the exception of the default OpenGL objects (i.e. objects named zero), which may not be shared.

OpenGL and Corresponding OpenCL Image Formats

The table below (Table 9.4) describes the list of GL texture internal formats and the corresponding CL image formats. If a GL texture object with an internal format from the table below is successfully created by OpenGL, then there is guaranteed to be a mapping to one of the corresponding CL image format(s) in that table. Texture objects created with other OpenGL internal formats may (but are not guaranteed to) have a mapping to a CL image format; if such mappings exist, they are guaranteed to preserve all color components, data types, and at least the number of bits/component actually allocated by OpenGL for that format.

If the cl_khr_gl_depth_images extension is enabled, the following new image formats are added to table 9.4 in section 9.6.3.1 of the OpenCL 2.0 extension specification. If a GL texture object with an internal format from table 9.4 is successfully created by OpenGL, then there is guaranteed to be a mapping to one of the corresponding CL image format(s) in that table.

Lifetime of [GL] Shared Objects

An OpenCL memory object created from an OpenGL object (hereinafter refered to as a "shared CL/GL object") remains valid as long as the corresponding GL object has not been deleted. If the GL object is deleted through the GL API (e.g. glDeleteBuffers, glDeleteTextures, or glDeleteRenderbuffers), subsequent use of the CL buffer or image object will result in undefined behavior, including but not limited to possible CL errors and data corruption, but may not result in program termination.

The CL context and corresponding command-queues are dependent on the existence of the GL share group object, or the share group associated with the GL context from which the CL context is created. If the GL share group object or all GL contexts in the share group are destroyed, any use of the CL context or command-queue(s) will result in undefined behavior, which may include program termination. Applications should destroy the CL command-queue(s) and CL context before destroying the corresponding GL share group or contexts.

Synchronizing OpenCL and OpenGL Access

In order to ensure data integrity, the application is responsible for synchronizing access to shared CL/GL objects by their respective APIs. Failure to provide such synchronization may result in race conditions and other undefined behavior including non-portability between implementations.

Prior to calling clEnqueueAcquireGLObjects, the application must ensure that any pending GL operations which access the objects specified in mem_objects have completed. This may be accomplished portably by issuing and waiting for completion of a glFinish command on all GL contexts with pending references to these objects. Implementations may offer more efficient synchronization methods; for example on some platforms calling glFlush may be sufficient, or synchronization may be implicit within a thread, or there may be vendor-specific extensions that enable placing a fence in the GL command stream and waiting for completion of that fence in the CL command queue. Note that no synchronization methods other than glFinish are portable between OpenGL implementations at this time.

When the extension cl_khr_egl_event is supported: Prior to calling clEnqueueAcquireGLObjects, the application must ensure that any pending EGL or EGL client API operations which access the objects specified in mem_objects have completed. If the cl_khr_egl_event extension is supported and the EGL context in question supports fence sync objects, explicit synchronisation can be achieved as set out in section 5.7.1. If the cl_khr_egl_event extension is not supported, completion of EGL client API commands may be determined by issuing and waiting for completion of commands such as glFinish or vgFinish on all client API contexts with pending references to these objects. Some implementations may offer other efficient synchronization methods. If such methods exist they will be described in platform-specific documentation. Note that no synchronization methods other than glFinish and vgFinish are portable between all EGL client API implementations and all OpenCL implementations. While this is the only way to ensure completion that is portable to all platforms, these are expensive operation and their use should be avoided if the cl_khr_egl_event extension is supported on a platform.

Similarly, after calling clEnqueueReleaseGLObjects, the application is responsible for ensuring that any pending OpenCL operations which access the objects specified in mem_objects have completed prior to executing subsequent GL commands which reference these objects. This may be accomplished portably by calling clWaitForEvents with the event object returned by clEnqueueReleaseGLObjects, or by calling clFinish. As above, some implementations may offer more efficient methods.

The application is responsible for maintaining the proper order of operations if the CL and GL contexts are in separate threads.

If a GL context is bound to a thread other than the one in which clEnqueueReleaseGLObjects is called, changes to any of the objects in mem_objects may not be visible to that context without additional steps being taken by the application. For an OpenGL 3.1 (or later) context, the requirements are described in Appendix D ("Shared Objects and Multiple Contexts") of the OpenGL 3.1 Specification. For prior versions of OpenGL, the requirements are implementation-dependent.

Attempting to access the data store of an OpenGL object after it has been acquired by OpenCL and before it has been released will result in undefined behavior. Similarly, attempting to access a shared CL/GL object from OpenCL before it has been acquired by the OpenCL command queue, or after it has been released, will result in undefined behavior.

If the cl_khr_gl_event extension is supported, then the OpenCL implementation will ensure that any such pending OpenGL operations are complete for an OpenGL context bound to the same thread as the OpenCL context. This is referred to as implicit synchronization.

Errors

Returns CL_SUCCESS if the function is executed successfully. If no device(s) exist corresponding to param_name, the call will not fail, but the value of param_value_size_ret will be zero. Otherwise returns one of the following:

Specification

OpenCL Specification

Also see

clCreateContext, cl_khr_gl_sharing, cl_khr_gl_event

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