EXT_disjoint_timer_query WebGL working group (public_webgl 'at' khronos.org) Contributors to ARB_occlusion_query Contributors to EXT_timer_query Contributors to ARB_timer_query Ben Vanik, Google Inc. Daniel Koch, TransGaming Inc. Florian Boesch (pyalot 'at' gmail.com) Members of the WebGL working group 26 Specifies that queries' results only become available at certain well-defined times. This extension provides a query mechanism that can be used to determine the amount of time it takes to fully complete a set of GL commands, and without stalling the rendering pipeline. It uses the query object mechanisms first introduced in the occlusion query extension, which allow time intervals to be polled asynchronously by the application. This version of the disjoint_timer_query extension is exposed only on on WebGL 1.0 contexts. See the _webgl2 version of the extension for how it is exposed on WebGL 2.0 contexts. typedef unsigned long long GLuint64EXT; [NoInterfaceObject] interface WebGLTimerQueryEXT : WebGLObject { }; [NoInterfaceObject] interface EXT_disjoint_timer_query { const GLenum QUERY_COUNTER_BITS_EXT = 0x8864; const GLenum CURRENT_QUERY_EXT = 0x8865; const GLenum QUERY_RESULT_EXT = 0x8866; const GLenum QUERY_RESULT_AVAILABLE_EXT = 0x8867; const GLenum TIME_ELAPSED_EXT = 0x88BF; const GLenum TIMESTAMP_EXT = 0x8E28; const GLenum GPU_DISJOINT_EXT = 0x8FBB; WebGLTimerQueryEXT? createQueryEXT(); void deleteQueryEXT(WebGLTimerQueryEXT? query); [WebGLHandlesContextLoss] boolean isQueryEXT(WebGLTimerQueryEXT? query); void beginQueryEXT(GLenum target, WebGLTimerQueryEXT query); void endQueryEXT(GLenum target); void queryCounterEXT(WebGLTimerQueryEXT query, GLenum target); any getQueryEXT(GLenum target, GLenum pname); any getQueryObjectEXT(WebGLTimerQueryEXT query, GLenum pname); }; Returns true if the passed WebGLTimerQueryEXT is valid and false otherwise. Returns false if the query's invalidated flag is set. target accepts TIME_ELAPSED_EXT. target accepts TIME_ELAPSED_EXT. target accepts TIMESTAMP_EXT. target and pname accept the following combinations of parameters. The return type of this method depends on the parameter queried.
targetpnamereturned type
TIME_ELAPSED_EXTCURRENT_QUERYWebGLQuery?
TIMESTAMP_EXTCURRENT_QUERYnull
TIME_ELAPSED_EXTQUERY_COUNTER_BITS_EXTGLint
TIMESTAMP_EXTQUERY_COUNTER_BITS_EXTGLint
pname accepts QUERY_RESULT_EXT or QUERY_RESULT_AVAILABLE_EXT.
The return type of this method depends on the parameter queried:
pnamereturned type
QUERY_RESULT_EXTGLuint64EXT
QUERY_RESULT_AVAILABLE_EXTboolean

In order to ensure consistent behavior across platforms, queries' results must only be made available when the user agent's event loop is not executing a task. In other words:
  • A query's result must not be made available until control has returned to the user agent's main loop.
  • Repeatedly fetching a query's QUERY_RESULT_AVAILABLE_EXT parameter in a loop, without returning control to the user agent, must always return the same value.
A query's result may or may not be made available when control returns to the user agent's event loop. It is not guaranteed that using a single setTimeout callback with a delay of 0, or a single requestAnimationFrame callback, will allow sufficient time for the WebGL implementation to supply the query's results.
This change compared to the original extension specification is enforced in order to prevent applications from relying on being able to issue a query and fetch its result in the same frame. In order to ensure best portability among devices and best performance among implementations, applications must expect that queries' results will become available asynchronously.
pname accepts TIMESTAMP_EXT or GPU_DISJOINT_EXT.
The return type depends on the parameter queried:
pnamereturned type
TIMESTAMP_EXTGLuint64EXT
GPU_DISJOINT_EXTboolean
  1. Can getQueryObjectEXT be exposed in its current form according to ECMAScript semantics? ECMAScript's de-facto concurrency model is "shared nothing" communicating event loops. Is it acceptable for sequential calls to getQueryObjectEXT to return different answers? Note that Date.now() advances during script execution, so this may be fine; but if concerns are raised, then the API may need to be redesigned to use callbacks.

        // Example (1) -- uses beginQueryEXT/endQueryEXT.
        var ext = gl.getExtension('EXT_disjoint_timer_query');
        var query = ext.createQueryEXT();
        ext.beginQueryEXT(ext.TIME_ELAPSED_EXT, query);

        // Draw object
        gl.drawElements(...);

        ext.endQueryEXT(ext.TIME_ELAPSED_EXT);

        // ...at some point in the future, after returning control to the browser and being called again:
        var available = ext.getQueryObjectEXT(query, ext.QUERY_RESULT_AVAILABLE_EXT);
        var disjoint = gl.getParameter(ext.GPU_DISJOINT_EXT);

        if (available && !disjoint) {
          // See how much time the rendering of the object took in nanoseconds.
          var timeElapsed = ext.getQueryObjectEXT(query, ext.QUERY_RESULT_EXT);

          // Do something useful with the time.  Note that care should be
          // taken to use all significant bits of the result, not just the
          // least significant 32 bits.
          adjustObjectLODBasedOnDrawTime(timeElapsed);
        }

        //----------------------------------------------------------------------

        // Example (2) -- same as the example above, but uses queryCounterEXT instead.
        var ext = gl.getExtension('EXT_disjoint_timer_query');
        var startQuery = ext.createQueryEXT();
        var endQuery = ext.createQueryEXT();
        ext.queryCounterEXT(startQuery, ext.TIMESTAMP_EXT);

        // Draw object
        gl.drawElements(...);

        ext.queryCounterEXT(endQuery, ext.TIMESTAMP_EXT);

        // ...at some point in the future, after returning control to the browser and being called again:
        var available = ext.getQueryObjectEXT(endQuery, ext.QUERY_RESULT_AVAILABLE_EXT);
        var disjoint = gl.getParameter(ext.GPU_DISJOINT_EXT);

        if (available && !disjoint) {
          // See how much time the rendering of the object took in nanoseconds.
          var timeStart = ext.getQueryObjectEXT(startQuery, ext.QUERY_RESULT_EXT);
          var timeEnd = ext.getQueryObjectEXT(endQuery, ext.QUERY_RESULT_EXT);

          // Do something useful with the time.  Note that care should be
          // taken to use all significant bits of the result, not just the
          // least significant 32 bits.
          adjustObjectLODBasedOnDrawTime(timeEnd - timeStart);
        }

        //----------------------------------------------------------------------

        // Example (3) -- check the number of timestamp bits to determine how to best
        // measure elapsed time.
        var ext = gl.getExtension('EXT_disjoint_timer_query');
        var timeElapsedQuery;
        var startQuery;
        var endQuery;

        var useTimestamps = false;

        if (ext.getQueryEXT(ext.TIMESTAMP_EXT, ext.QUERY_COUNTER_BITS_EXT) > 0) {
          useTimestamps = true;
        }

        // Clear the disjoint state before starting to work with queries to increase
        // the chances that the results will be valid.
        gl.getParameter(ext.GPU_DISJOINT_EXT);

        if (useTimestamps) {
          startQuery = ext.createQueryEXT();
          endQuery = ext.createQueryEXT();
          ext.queryCounterEXT(startQuery, ext.TIMESTAMP_EXT);
        } else {
          timeElapsedQuery = ext.createQueryEXT();
          ext.beginQueryEXT(ext.TIME_ELAPSED_EXT, timeElapsedQuery);
        }

        // Draw object
        gl.drawElements(...);

        if (useTimestamps) {
          ext.queryCounterEXT(endQuery, ext.TIMESTAMP_EXT);
        } else {
          ext.endQueryEXT(ext.TIME_ELAPSED_EXT);
        }

        // ...at some point in the future, after returning control to the browser and being called again:
        var disjoint = gl.getParameter(ext.GPU_DISJOINT_EXT);
        if (disjoint) {
          // Have to redo all of the measurements.
        } else {
          var available;
          if (useTimestamps) {
            available = ext.getQueryObjectEXT(endQuery, ext.QUERY_RESULT_AVAILABLE_EXT);
          } else {
            available = ext.getQueryObjectEXT(timeElapsedQuery, ext.QUERY_RESULT_AVAILABLE_EXT);
          }

          if (available) {
            var timeElapsed;
            if (useTimestamps) {
              // See how much time the rendering of the object took in nanoseconds.
              var timeStart = ext.getQueryObjectEXT(startQuery, ext.QUERY_RESULT_EXT);
              var timeEnd = ext.getQueryObjectEXT(endQuery, ext.QUERY_RESULT_EXT);
              timeElapsed = timeEnd - timeStart;
            } else {
              timeElapsed = ext.getQueryObjectEXT(query, ext.QUERY_RESULT_EXT);
            }

            // Do something useful with the time.  Note that care should be
            // taken to use all significant bits of the result, not just the
            // least significant 32 bits.
            adjustObjectLODBasedOnDrawTime(timeElapsed);
          }
        }
    
Initial revision. Based on public_webgl discussion, specified that queries' results only become available at well-defined times. Recast as EXT_disjoint_timer_query and harmonized with mirrored extension. Added NoInterfaceObject extended attribute. Revised language regarding queries' availability based on feedback from Jeff Gilbert. Promoted to community approved after discussion on public_webgl. Added example choosing measurement technique based on number of timestamp bits. Added clarifying note that this document only applies to WebGL 1.0. Minor reformatting and typo fixes.