QOpenGLTimerQuery Class

Detailed Description

OpenGL timer query objects are OpenGL managed resources to measure the execution times of sequences of OpenGL commands on the GPU.

OpenGL offers various levels of support for timer queries, depending on the version of OpenGL you have and the presence of the ARB_timer_query or EXT_timer_query extensions. The support can be summarized as:

• OpenGL >=3.3 offers full support for all timer query functionality.
• OpenGL 3.2 with the ARB_timer_query extension offers full support for all timer query functionality.
• OpenGL <=3.2 with the EXT_timer_query extension offers limited support in that the timestamp of the GPU cannot be queried. Places where this impacts functions provided by Qt classes will be highlighted in the function documentation.
• OpenGL ES 2 (and OpenGL ES 3) do not provide any support for OpenGL timer queries.

OpenGL represents time with a granularity of 1 nanosecond (1e-9 seconds). As a consequence of this, 32-bit integers would only give a total possible duration of approximately 4 seconds, which would not be difficult to exceed in poorly performing or lengthy operations. OpenGL therefore uses 64 bit integer types to represent times. A GLuint64 variable has enough width to contain a duration of hundreds of years, which is plenty for real-time rendering needs.

As with the other Qt OpenGL classes, QOpenGLTimerQuery has a create() function to create the underlying OpenGL object. This is to allow the developer to ensure that there is a valid current OpenGL context at the time.

Once created, timer queries can be issued in one of several ways. The simplest method is to delimit a block of commands with calls to begin() and end(). This instructs OpenGL to measure the time taken from completing all commands issued prior to begin() until the completion of all commands issued prior to end().

At the end of a frame we can retrieve the results by calling waitForResult(). As this function's name implies, it blocks CPU execution until OpenGL notifies that the timer query result is available. To avoid blocking, you can check if the query result is available by calling isResultAvailable(). Note that modern GPUs are deeply pipelined and query results may not become available for between 1-5 frames after they were issued.

Note that OpenGL does not permit nesting or interleaving of multiple timer queries using begin() and end(). Using multiple timer queries and recordTimestamp() avoids this limitation. When using recordTimestamp() the result can be obtained at some later time using isResultAvailable() and waitForResult(). Qt provides the convenience class QOpenGLTimeMonitor that helps with using multiple query objects.