[gegl] bin: rework audio timing for video playback

[Øyvind "pippin" Kolås <ok src gnome org>]

commit 6e1fd149ccd7e982aace8fc984ef2654d93fc0cf
Author: Øyvind Kolås <pippin gimp org>
Date:   Fri Mar 22 02:35:44 2019 +0100

    bin: rework audio timing for video playback
    
    Relying on newly added mrg API mrg_pcm_audio_get_queued_length, after
    all UI and cairo drawing is complete for a frame, queue the related PCM
    data if this was a video-frame, then wait until 0.25 of the previous
    frames PCM data or less is queued before proceeding with flipping/displaying
    the current frame.
    
    Video playback works better with mipmap rendering even when no mipmap rendering
    is happening due to fewer buffer copies.

 bin/ui-core.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)
---
diff --git a/bin/ui-core.c b/bin/ui-core.c
index f9ce4d91a..991111ad0 100644
--- a/bin/ui-core.c
+++ b/bin/ui-core.c
@@ -3996,9 +3996,14 @@ static void iterate_frame (GeState *o)
            temp_buf[i*2+1] = audio->data[1][i] * 32767.0 * 0.46;
          }
 
-         mrg_pcm_queue (mrg, (void*)&temp_buf[0], sample_count);
-         while (mrg_pcm_get_queued (mrg) > sample_count / 2)
-            g_usleep (50);
+           mrg_pcm_queue (mrg, (void*)&temp_buf[0], sample_count);
+
+           /* after queing our currently decoded audio frame, we
+              wait until the pcm buffer is nearly ready to play
+              back our content
+            */
+           while (mrg_pcm_get_queued_length (mrg) > (1.0/fps) * 1.25  )
+              g_usleep (100);
          }
 
 
@@ -4798,10 +4803,6 @@ static void gegl_ui (Mrg *mrg, void *data)
        break;
   }
 
-  if (o->playing)
-  {
-    iterate_frame (o);
-  }
 
   if (o->show_controls && 0)
   {
@@ -5009,6 +5010,15 @@ static void gegl_ui (Mrg *mrg, void *data)
      ui_show_bindings (mrg, o);
   }
 
+  /* iterate frame, and queue pcm last - since this might cause
+     waiting with presenting the video frame until the pcm data
+     of the frame is about to be played
+   */
+  if (o->playing)
+  {
+    iterate_frame (o);
+  }
+
 }
 
 /***********************************************/