glBeginQuery

glBeginQuery: man2/glBeginQuery.xml

glBeginQuery and glEndQuery delimit the boundaries of a query object. If a query object with name id does not yet exist it is created. When glBeginQuery is executed, the query object's samples-passed counter is reset to 0. Subsequent rendering will increment the counter once for every sample that passes the depth test. When glEndQuery is executed, the samples-passed counter is assigned to the query object's result value. This value can be queried by calling glGetQueryObject with pname GL_QUERY_RESULT. Querying the GL_QUERY_RESULT implicitly flushes the GL pipeline until the rendering delimited by the query object has completed and the result is available. GL_QUERY_RESULT_AVAILABLE can be queried to determine if the result is immediately available or if the rendering is not yet complete.

If the samples-passed count exceeds the maximum value representable in the number of available bits, as reported by glGetQueryiv with pname GL_QUERY_COUNTER_BITS, the count becomes undefined. An implementation may support 0 bits in its samples-passed counter, in which case query results are always undefined and essentially useless. When GL_SAMPLE_BUFFERS is 0, the samples-passed counter will increment once for each fragment that passes the depth test. When GL_SAMPLE_BUFFERS is 1, an implementation may either increment the samples-passed counter individually for each sample of a fragment that passes the depth test, or it may choose to increment the counter for all samples of a fragment if any one of them passes the depth test. glBeginQuery and glEndQuery are available only if the GL version is 1.5 or greater.