+void
+metrics_end_frame_pre_swap (metrics_t *metrics)
+{
+ GLint64 swap_end_timestamp, frame_time_ns, latency_ns;
+
+ /* Don't leave any counters running over the end_frame work we
+ * do here. The counters will be started again at the end of
+ * metrics_end_frame_post_swap. */
+
+ metrics_counter_stop (metrics);
+
+ /* Now that an entire frame's worth of content has gone by, we
+ * can be sure that the timer quiery for the previous frame's
+ * swap is available.
+ *
+ * Note: The only case in which this query isn't immediately
+ * available would be if there was effectively nothing in this
+ * frame. In that case, triggering a block on this query
+ * result is not a concern (since an empty frame is not
+ * interesting, and all of our counters are stopped anyway).
+ *
+ * Accepting this one block is much better than adding a
+ * linked list for these results that would reaslisticly never
+ * have more than one entry anyway.
+ */
+
+ glGetQueryObjecti64v (metrics->swap_end_timestamp_id,
+ GL_QUERY_RESULT, &swap_end_timestamp);
+
+ if (frames > 0) {
+ /* Subtract previous frame's timestamp to get frame time. */
+ frame_time_ns = swap_end_timestamp - metrics->previous_swap_end_timestamp;
+
+ latency_ns = swap_end_timestamp - metrics->swap_begin_timestamp;
+
+ /* We've waited one frame to ensure we have a timestamp
+ * result. So the time we've actually measured here is
+ * for the previous frame. */
+ printf ("Frame %d: %.2f ms (latency: %.2f ms)\n",
+ frames - 1, frame_time_ns / 1e6, latency_ns / 1e6);
+ }
+
+ metrics->previous_swap_end_timestamp = swap_end_timestamp;
+
+ /* Before the actual swap call, we get the current timestamp
+ * value. This is a synchronous get so we can use this as the
+ * baseline for a frame latency measurment. */
+
+ glGetInteger64v (GL_TIMESTAMP, &metrics->swap_begin_timestamp);
+}