Choose maximum for frame drop limit depending on the number of

allocated frames.
The current code uses a hard coded frame drop limit of 3 and
doesn't adhere to it's documentation when testing whether frames
shall be dropped. As a result frame drop limit is actually 4,
which means that the decoder is asked to drop some frames when
the number of frames waiting for displaying is less then 4.
Consider a video out device like xxmc which only supplies 8
frames. For MPEG2 decoding, two frames will be used by the
decoder (for the current frame and the forward reference frame)
and two further frames will be used in the video out loop (the
current and the previous frame) so that at any given time (under
perfect conditions) there will be 4 frames waiting to be displayed.
But when there are delays in scheduling, it might happen that
there are only 3 frames ready for displaying and thus will result
in asking the decoder to drop frames.
The changes therefore determine the maximum frame drop limit in
dependence of the number of allocated frames and make the
detection work like documented. In the above scenario, the maximum
number actually used for frame drop limit will then be 2 which
allows to compensate some scheduling delays without causing the
decoder to drop frames.

1 files changed, 25 insertions, 6 deletions

diff --git a/src/xine-engine/video_out.c b/src/xine-engine/video_out.c
index 852e13524..c75b7058a 100644
--- a/src/xine-engine/video_out.c
+++ b/src/xine-engine/video_out.c
@@ -129,6 +129,7 @@ typedef struct {
 
   int                       current_width, current_height;
   int64_t                   current_duration;
+  int                       frame_drop_limit_max;
   int                       frame_drop_limit;
   int                       frame_drop_cpt;
   int                       crop_left, crop_right, crop_top, crop_bottom;
@@ -473,25 +474,28 @@ static int vo_frame_draw (vo_frame_t *img, xine_stream_t *stream) {
       duration = img->duration;
     
     /* Frame dropping slow start:
-     *   The engine starts to drop frames if there is less than frame_drop_limit
+     *   The engine starts to drop frames if there are less than frame_drop_limit
      *   frames in advance. There might be a problem just after a seek because
      *   there is no frame in advance yet.
      *   The following code increases progressively the frame_drop_limit (-2 -> 3)
      *   after a seek to give a chance to the engine to display the first frames
-     *   smootly before starting to drop frames if the decoder is really too
+     *   smoothly before starting to drop frames if the decoder is really too
      *   slow.
+     *   The above numbers are the result of frame_drop_limit_max beeing 3. They
+     *   will be (-4 -> 1) when frame_drop_limit_max is only 1. This maximum value
+     *   depends on the number of video buffers which the output device provides.
      */
     if (stream && stream->first_frame_flag == 2)
       this->frame_drop_cpt = 10;
 
     if (this->frame_drop_cpt) {
-      this->frame_drop_limit = 3 - (this->frame_drop_cpt / 2);
+      this->frame_drop_limit = this->frame_drop_limit_max - (this->frame_drop_cpt / 2);
       this->frame_drop_cpt--;
     }
     frames_to_skip = ((-1 * diff) / duration + this->frame_drop_limit) * 2;
 
     /* do not skip decoding until output fifo frames are consumed */
-    if (this->display_img_buf_queue->num_buffers > this->frame_drop_limit ||
+    if (this->display_img_buf_queue->num_buffers >= this->frame_drop_limit ||
         frames_to_skip < 0)
       frames_to_skip = 0;
       
@@ -1832,8 +1836,6 @@ xine_video_port_t *_x_vo_new_port (xine_t *xine, vo_driver_t *driver, int grabon
   this->overlay_source->init (this->overlay_source);
   this->overlay_enabled       = 1;
 
-  this->frame_drop_limit      = 3;
-  this->frame_drop_cpt        = 0;
 
   /* default number of video frames from config */
   num_frame_buffers = xine->config->register_num (xine->config,
@@ -1854,6 +1856,23 @@ xine_video_port_t *_x_vo_new_port (xine_t *xine, vo_driver_t *driver, int grabon
   if (num_frame_buffers<5) 
     num_frame_buffers = 5;
 
+  /* Choose a frame_drop_limit which matches num_frame_buffers.
+   * xxmc for example supplies only 8 buffers. 2 are occupied by
+   * MPEG2 decoding, further 2 for displaying and the remaining 4 can
+   * hardly be filled all the time.
+   * The below constants reserve buffers for decoding, displaying and
+   * buffer fluctuation.
+   * A frame_drop_limit_max below 1 will disable frame drops at all.
+   */
+  this->frame_drop_limit_max  = num_frame_buffers - 2 - 2 - 1;
+  if (this->frame_drop_limit_max < 1)
+    this->frame_drop_limit_max = 1;
+  else if (this->frame_drop_limit_max > 3)
+    this->frame_drop_limit_max = 3;
+
+  this->frame_drop_limit      = this->frame_drop_limit_max;
+  this->frame_drop_cpt        = 0;
+
   this->extra_info_base = calloc (num_frame_buffers,
 					  sizeof(extra_info_t));