x264enc: Work around a rate control issue in libx264

When variable framerate is disabled in libx264 (which occurs when using
the zerolatency tuning), libx264 ignores timestamps but still uses the
timebase leading to messed up rate control with our nanosecond timebase.

We work around this issue by setting the timebase to the reciprocal of
the framerate and we validate that the framerate is suitable.

This has been fixed upstream in libx264 but there are non-fixed versions
in the wild so this workaround is still needed.

Fixes bug #632861

1 files changed, 19 insertions, 1 deletions

diff --git a/ext/x264/gstx264enc.c b/ext/x264/gstx264enc.c
index b5b4bae2..a99a8e19 100644
--- a/ext/x264/gstx264enc.c
+++ b/ext/x264/gstx264enc.c
@@ -346,7 +346,7 @@ static const GFlagsValue tune_types[] = {
   {0x0, "No tuning", "none"},
   {0x1, "Still image", "stillimage"},
   {0x2, "Fast decode", "fastdecode"},
-  {0x4, "Zero latency", "zerolatency"},
+  {0x4, "Zero latency (requires constant framerate)", "zerolatency"},
   {0, NULL, NULL},
 };
 
@@ -1159,6 +1159,24 @@ gst_x264_enc_init_encoder (GstX264Enc * encoder)
       break;
   }
 
+#if X264_BUILD >= 81 && X264_BUILD < 106
+  /* When vfr is disabled, libx264 ignores buffer timestamps. This causes
+   * issues with rate control in libx264 with our nanosecond timebase. This
+   * has been fixed upstream in libx264 but this workaround is required for
+   * pre-fix versions. */
+  if (!encoder->x264param.b_vfr_input) {
+    if (encoder->x264param.i_fps_num == 0) {
+      GST_ELEMENT_ERROR (encoder, STREAM, ENCODE,
+          ("Constant framerate is required."),
+          ("The framerate caps (%d/%d) indicate VFR but VFR is disabled in libx264. (Is the zerolatency tuning in use?)",
+              encoder->x264param.i_fps_num, encoder->x264param.i_fps_den));
+      return FALSE;
+    }
+    encoder->x264param.i_timebase_num = encoder->x264param.i_fps_den;
+    encoder->x264param.i_timebase_den = encoder->x264param.i_fps_num;
+  }
+#endif
+
 #ifdef X264_PRESETS
   if (encoder->profile
       && x264_param_apply_profile (&encoder->x264param,