Encoding 4k H.264 Videos

Previzion is capable of playing back 4k H.264 videos, however these are quite demanding on the system and will severely restrict the complexity of the rest of the scene.  Currently we have been successful in playing back two 4k videos encoded at 35mbit at 23.98PsF, and 25Psf.  29.97Psf and 60i can only play back a single 4k video without frame drops.

HandBreak Workflow

Our current encoding workflow is not optimized but outputs 4K video files which go into Previzion.

  • Using RedCineX output a 4k ProRes video file of the clip or select of the clip
  • Convert to h.264 using HandBrake
  • Set Video Codec to “H.264 (x264) and “Video Quality” to “Constant Quality” with “RF:” around 18.25 to get a 35Mbit video.

Adobe AfterEffects Workflow

You may also output 4K videos directly from Adobe AfterEffects as H.264.
  • For your Composition Settings, your comp dimensions should be:
    • Width: 4096px
    • Height: 2160px
    • Square Pixels
      CompSettings
  • When your are ready to render, set Output Module Settings to:
    • Format: H.264
      4kAESettings
    • Click on “Format Options…” and set your H.264 Options’ Bitrate Settings to:
      • Bitrate Encoding: CBR (Continuous Bit Rate)
      • Bitrate [Mbps]: 35 (or lower)
      H264CBR
  • All other default settings should be left unchanged:
    • Multiplexing: MP4
    • Stream Compatibility: Standard
    • Video Settings Profile: High and Level 5.1
    • Key Frame Distance: disabled

H264 4K Playback Application and Limitations

When using video backgrounds, it becomes incredibly useful to have high resolution background videos.  Where stitched video plates and HD video plates start pixelating or showing resolution limits, 4k background plates shine.  Previzion can decode up to two simultaneous 4k video depending on their encoding settings.

As impressive as it is to decode multiple 4k video streams, there are major limiting factors to this feature.  The primary ones are heat, bit rate and bit rate type (CBR over VBR).

Heat can become a problem when using multiple video streams in a poorly ventilated environment.  If running Previzion System in a hot, poorly ventilated room, 4k videos can cause a thermal overload and trigger an internal beep that signals this overload.  At the point that this beep is triggered, with our motherboards, no harm comes to your hardware.  Only if the beep is ignore do temperatures climb to harmful ranges.

Bit rate is important because it determines the rate in which each “real” frame is encoded to.  The recommended bit rate is about 32 MBps to 35 MBps.  Lower bit rates buy you more CPU cycles.

Bit rate type is directly related to the above described because it determines how often and to what extent video decoding oscillates in CPU cycles demand.  The recommended setting is to use Constant Bit Rate, or CBR, because it will cap the amount of information a give frame has.  This setting prevents Previzion from having spikes in its CPU demand, which can cause errors and dropped frames.

The number of 4k stream Previzion currently supports can be seen as follows:

 
Frame Rate (in fps) Max Number of Simultaneous 4K Streams
23.98 (24) 2 to 4
25 2 to 4
29.97 1 to 3
60i 1 to 2
 

Previzion has different hardware configurations depending on the generation of Previzion system.  The higher estimates in the previous table portray the newest Previzion configuration.

Because of the nature of H264, 4k sizes is not officially supported.  It’s important to know that, if using more than one 4k H264 video stream, the video streams may not stay in sync overtime.  With that in mind, we don’t recommend using a 4K H264 video stream with a corresponding 4k H264 alpha video stream as it will end up getting out of sync.

Finally, we strongly recommend against mixing H264 and MPEG2 video streams as they both take up CPU cycles, but different resources that end up taking over your Previzion system’s resources.

Compatibility Notes:

-Currently QuickTime based transcoders and players are unable to create or play 4k h.264 files.  This includes QuicktTme 7, QuickTime X, Compressor, and Final Cut 7.  For quality control playback we have been successful with using VLC for playback.

-Frequently the limiting factor in playing back these 4k files will be hard disk speed.  I have only been able to play them back smoothly off of SSD drives not spinning drives.  Another possible alternative for a system without a SSD would be to use a RAMdisk utility to turn a block of ram into temporary fast storage for playback.

-In some system configurations, leaving two or more 4K videos playing for too long may cause thermal error.  This is usually indicated by an error beeping.

Last Modified: May 28, 2013