H265 bitstream size is too large when using external rate control

[kazunori.kimura]

Hi, I’m testing NVENC external rate control feature with H265.
I found encoded bits was too large when upper part of input frame has no movement.

This case happens following platforms.

• Jetson Nano with JetPack 4.6.2
• Jetson Xavier NX with JetPack 4.6.2
• Jetson AGX Xavier with JetPack 5.1

Prepare for reprodoucing

I use this video for testing
(Creative Commons licenced video).

1. Download video using youtube-dl

$ youtube-dl -f 137 1O4qUaij0h8 -o pan.mp4

2. Create original input_src

$ ffmpeg - i pan.mp4 -pix_fmt yuv420p input_src.yuv

3. Edit video using FFmpeg. (croping and padding)

$ BAR=128 # set bar height to 128px.
$ ffmpeg -i pan.mp4 -filter:v "crop=w=in_w:h=in_h-$BAR:x=0:y=0,pad=h=1080:y=$BAR" \
  -pix_fmt yuv420p input_src2.yuv

Reproducing problem

I executed video_encode example with following parameters.

Parameter	Value	Note
nTargetFrameBits	340000	10Mbps / fps
nFrameQP	25
nFrameMinQp	1
nFrameMaxQP	51
nMaxQPDeviation	0	No effect for HEVC
nSessionMaxQP	51
Hardware Preset	fast

$ echo "166666 25 1 51 0" > hints.txt

$ video_encode input_src.yuv 1920 1080 H265 original.265 \
  --input_metadata --erh -hf hints.txt \
  -hpt 2 -fps 30 1

$ video_encode input_src2.yuv 1920 1080 H265 modified.265 \
  --input_metadata --erh -hf hints.txt \
  -hpt 2 -fps 30 1

Encoded result of modificed.265 is larger than original.265.
I checked output bitstream size with 3 kinds of bar height.

Bar height [px]	File size [byte]
0 (original)	23986425
128	26644149
256	35592979
512	89602178

I think this is strange behavior.
If bar height is large, input frame has less complexity than without bar.
So, encoded bitstream size should be smaller.
Can I improve this behavior with some other encoder settings?
And, Jetson Orin platform behaves same?

Regards.

[DaneLLL]

Hi,
Please try to set nFrameMinQp and nFrameMaxQP in a milted range and see if it helps. Also for balancing bitrate and video quality, you may try to configure CBR + virtual buffer size. Here is a topic for reference:
Random blockiness in the picture RTSP server-client -Jetson TX2 - #5 by DaneLLL

[kazunori.kimura]

Hi, DaneLLL

Thank you for your information.
I’ll try to control QP range.