Configure GOPs with VEGA acceleration
Open GOPs have been implemented in this version. You can select either open GOP or closed GOP.
An advanced parameter is available to allow configuration of sub-GOPs with more than three B-frames in HEVC. Auto/Off/2/3 options are available via the GUI.
The theoretical GOP length of the output streams (in frames) is calculated as follows:
expectedGOPSize = (keyFramePeriodms) * framerate + 500 / 1000
The Key Frame Period can be directly modified in the UI. The Key Frame Period corresponds to the distance between two IRAP frames (also called GOP size).
The produced GOP length is constrained by the underlying HW encoder implementation.
The nearest possible value to the expected GOP Size is chosen among the supported ones and depends on the codec.
- Supported values for H.264 are 12, 15, 24, 30, 60, 240.
- Supported value for HEVC are 12, 24, 25, 30, 32, 60, 64, 100, 128, 240, 248.
A GOP size of 12 or 15 is possible with H.264 and GOP structure of IBP, if the Live Encoding configuration is updated to set the minimum size Key Frame Period (KFP) to 400ms (for 12) or 500ms (for 15).
B-frames option is not valid for all codecs with VEGA acceleration:
- “Auto”, “Off” and “2” B-frames are available for H.264.
- “Auto”, “Off” and “3” B-frames are available for HEVC.
Other GOP structures are available via advanced parameters:
vega.forcedHevcGopIB=true: only applicable for HEVC, forces the GOP structure to IB.vega.forcedBframes=XXX: overwrite B-frames selection, applicable values depend on codec.
Examples of extended configurations:
- H265 IBBBBBBBP:
vega.forcedBframes=7 - H265 IBBBB:
vega.forcedHevcGopIB=true - H265 IBB:
vega.forcedHevcGopIB=true,vega.forcedBframes=2 - H264/H265 IBP:
vega.forcedBframes=1 - H.264/H.265 I-only: B-frames configuration from the GUI set to off. (Encoding > Video Encoding > Codec settings page) and
vega.IFramesOnly = true.
The GOP size must be divisible by reference frame number, otherwise it causes video image to be broken near GOP boundary. That’s why in some specific cases, the actual GOP length varies from the configured Key Frame Period. One example with Key Frame Period = 500ms with HEVC 720p60 streams, the actual GOP size will be 28 frames.