Freescale: Why is the Bit Rate( and file size) for H264 encoded videos on Freescale iMX6 inversely proportional to the resolution of video?

[imx_learner]

I am trying to build a live video streaming embedded server on Freescale imx6 with Gstreamer and h264 codecs. The client(Linux PC) is connected via a wireless link that streams the video transmitted by the server.

Recently I performed some test to measure the average bitrate for a h264 encoded file on a Freescale imx6 processor. I used the GStreamer element 'testvideosrc', ran it for 60 seconds and encoded it using the gstreamer-imx plugins that utilizes the hardware encoder (h264) and saved the file, and calculated the bitrate. Following is the command :

timeout --signal=SIGINT 60s \
 gst-launch-1.0 -ev videotestsrc pattern=18 ! \
 video/x-raw,width=100,height=50,framerate=10/1 ! \ 
imxvpuenc_h264 bitrate=5 ! h264parse ! \
 filesink location=60s.h264 && \
 echo $(( $(stat --format=%s 60s.h264) / 60 * 8)) 

The test was performed at 10 fps for different resolutions, and the bitrates are as follows:

Resolution                                              BitRate  
100 x 50                                               ~120.3K
320 x 240                                              ~93.7K 
640 x 480                                              ~52.7K
1280 x 960                                             ~32.3K 

The bitrate I calculated was for the file transfer rate, that I require to transfer this wirelessly over the communication link.

I am trying to understand the behaviour of the H264 encoder, as to why the bitrate is inversely proportional to the increasing resolution sizes. I had hoped that with increasing resolutions, the bitrate will also increase and hence was opting for a lower resolution image, for streaming through the wireless link. Can anyone explain, why this behaviour exists with the h264 codedc? Or am I at fault in understanding the bitrate demand.

Here is the file-size of the encoded video file.

Resolution                                              File size in bytes
  100 x 50                                                3886780
  320 x 240                                               704543
  640 x 480                                               397098
  1280 x 960                                              235108

Could anyone explain this behaviour? 

Please address this question to the H264 encoder experts, who can explain this behaviour.

[joanxie]

the bitrate you set is too small, do you know what the real frame rate is? pls check frame rate when you set the different resolution.

[imx_learner]

Hello,

Any updates for this question? 

Regards.

[imx_learner]

Hello Joan,

Here are more numbers at different framerates:

at 30 fps

Resolution                                              File size in bytes                            Bitrate
  100 x 50                                                3474013                                       455.16K
  320 x 240                                               692716                                        92.36K
  640 x 480                                               397563                                        53.01K
  1280 x 960                                              234252                                       31.23K

at 60 fps

Resolution                                              File size in bytes                            Bitrate
  100 x 50                                                3470160                                       425.43K
  320 x 240                                              703225                                          93.76K
  640 x 480                                              399125                                          53.216K 
  1280 x 960                                              233787                                        31.168K

I do not understand "the real frame rate". How do I check this?

Regards.