nvenc: Adjust DTS when bframe is enabled

NVDEC driver always uses input timestamp without adjustment
even if bframe encoding was enabled.
So DTS can be larger than PTS when bframe was enabled.
To ensure PTS >= DTS, we should adjust the timestamp manually
based on the PTS difference between the first
encoded frame and the second one. That's also the maximum PTS/DTS
difference.

sys/nvcodec/gstnvbaseenc.c

@@ -585,6 +585,9 @@ gst_nv_base_enc_start (GstVideoEncoder * enc)
   }
 #endif
 
+  /* DTS can be negative if bframe was enabled */
+  gst_video_encoder_set_min_pts (enc, GST_SECOND * 60 * 60 * 1000);
+
   return TRUE;
 }
 
@@ -1066,6 +1069,57 @@ gst_nv_base_enc_bitstream_thread (gpointer user_data)
 
     g_async_queue_push (nvenc->available_queue, state_in_queue);
 
+    /* Ugly but no other way to get DTS offset since nvenc dose not adjust
+     * dts/pts even if bframe was enabled. So the output PTS can be smaller
+     * than DTS. The maximum difference between DTS and PTS can be calculated
+     * using the PTS difference between the first frame and the second frame.
+     */
+    if (nvenc->bframes > 0) {
+      if (nvenc->dts_offset == 0) {
+        if (!nvenc->first_frame) {
+          /* store the first frame to get dts offset */
+          nvenc->first_frame = frame;
+          continue;
+        } else {
+          if (nvenc->first_frame->pts >= frame->pts) {
+            GstClockTime duration = 0;
+
+            GST_WARNING_OBJECT (enc, "Could not calculate DTS offset");
+
+            if (nvenc->input_info.fps_n > 0 && nvenc->input_info.fps_d > 0) {
+              duration =
+                  gst_util_uint64_scale (GST_SECOND, nvenc->input_info.fps_d,
+                  nvenc->input_info.fps_n);
+            } else if (nvenc->first_frame->duration > 0 &&
+                GST_CLOCK_TIME_IS_VALID (nvenc->first_frame->duration)) {
+              duration = nvenc->first_frame->duration;
+            } else {
+              GST_WARNING_OBJECT (enc,
+                  "No way to get frame duration, assuming 30fps");
+              duration = gst_util_uint64_scale (GST_SECOND, 1, 30);
+            }
+
+            nvenc->dts_offset = duration * nvenc->bframes;
+          } else {
+            nvenc->dts_offset = frame->pts - nvenc->first_frame->pts;
+          }
+
+          /* + 1 to dts_offset to adjust fraction */
+          nvenc->dts_offset++;
+
+          GST_DEBUG_OBJECT (enc,
+              "Calculated DTS offset %" GST_TIME_FORMAT,
+              GST_TIME_ARGS (nvenc->dts_offset));
+        }
+
+        nvenc->first_frame->dts -= nvenc->dts_offset;
+        gst_video_encoder_finish_frame (enc, nvenc->first_frame);
+        nvenc->first_frame = NULL;
+      }
+
+      frame->dts -= nvenc->dts_offset;
+    }
+
     flow = gst_video_encoder_finish_frame (enc, frame);
 
     if (flow != GST_FLOW_OK) {
@@ -1079,6 +1133,11 @@ gst_nv_base_enc_bitstream_thread (gpointer user_data)
 
 error_shutdown:
   {
+    if (nvenc->first_frame) {
+      gst_clear_buffer (&nvenc->first_frame->output_buffer);
+      gst_video_encoder_finish_frame (enc, nvenc->first_frame);
+      nvenc->first_frame = NULL;
+    }
     g_atomic_int_set (&nvenc->last_flow, GST_FLOW_ERROR);
     g_async_queue_push (nvenc->available_queue, SHUTDOWN_COOKIE);
 
@@ -1087,6 +1146,11 @@ error_shutdown:
 
 exit_thread:
   {
+    if (nvenc->first_frame) {
+      gst_video_encoder_finish_frame (enc, nvenc->first_frame);
+      nvenc->first_frame = NULL;
+    }
+
     GST_INFO_OBJECT (nvenc, "exiting thread");
 
     return NULL;

sys/nvcodec/gstnvbaseenc.h

@@ -143,6 +143,10 @@ typedef struct {
 
   GstFlowReturn   last_flow;          /* ATOMIC */
 
+  /* the first frame when bframe was enabled */
+  GstVideoCodecFrame *first_frame;
+  GstClockTime dts_offset;
+
   /*< protected >*/
   /* device capability dependent properties, set by subclass */
   gboolean        weighted_pred;