H.265 Codec: Rate control settings

VBV Buffer Size

Specify the size of the VBV buffer (kbits). Enables VBV in ABR mode.
In CRF mode, VBV Maximum Bitrate must also be specified.
Default 0 (vbv disabled)

VBV Maximum Bitrate

Maximum local bitrate (kbits/sec). Will be used only if vbv-bufsize is also non-zero.
Both vbv-bufsize and vbv-maxrate are required to enable VBV in CRF mode.
Default 0 (disabled)

VBV Initial Buffer

Initial buffer occupancy. The portion of the decode buffer which must be full before the decoder will begin decoding. Determines absolute maximum frame size. May be specified as a fractional value between 0 and 1, or in kbits.
Default 0.9

Min CRF

Specify an lower limit to the rate factor which may be assigned to any given frame (ensuring a min compression factor).

Max CRF

Specify an upper limit to the rate factor which may be assigned to any given frame (ensuring a max QP).
This is dangerous when CRF is used in combination with VBV as it may result in buffer underruns.

IP Ratio

QP ratio factor between I and P slices.
This ratio is used in all of the rate control modes.
Default 1.4

PB Ratio

QP ratio factor between P and B slices.
This ratio is used in all of the rate control modes.
Default 1.3

QP Minimum

sets a hard lower limit on QP allowed to ratecontrol.
Default 0

QP Maximum

sets a hard upper limit on QP allowed to ratecontrol.
Default 69

QP Step

The maximum single adjustment in QP allowed to rate control.
Default 4

qComp

qComp sets the quantizer curve compression factor.
It weights the frame quantizer based on the complexity of residual (measured by lookahead).
It’s value must be between 0.5 and 1.0. Default value is 0.6. Increasing it to 1.0 will effectively generate CQP.

Chroma Cb QP Offset

Offset of Cb chroma QP from the luma QP selected by rate control. This is a general way to spend more or less bits on the chroma channel.
Default 0

Chroma Cr QP Offset

Offset of Cr chroma QP from the luma QP selected by rate control. This is a general way to spend more or less bits on the chroma channel.
Default 0

AQ Mode

Adaptive Quantization operating mode. Raise or lower per-block quantization based on complexity analysis of the source image. The more complex the block, the more quantization is used. This offsets the tendency of the encoder to spend too many bits on complex areas and not enough in flat areas.

• disabled
• AQ enabled
• AQ enabled with auto-variance (default)
• AQ enabled with auto-variance and bias to dark scenes.
  This is recommended for 8-bit encodes or low-bitrate 10-bit encodes, to prevent color banding/blocking.
• AQ enabled with auto-variance and edge information.

AQ strength

Adjust the strength of the adaptive quantization offsets. Setting AQ strength to 0 disables AQ. At aq-modes 2 and 3, high aq-strengths will lead to high QP offsets resulting in a large difference in achieved bitrates.
Default 1.0.

QP size

Enable adaptive quantization for sub-CTUs. This parameter specifies the minimum CU size at which QP can be adjusted, ie. Quantization Group size. Allowed range of values are 64, 32, 16, 8 provided this falls within the inclusive range [maxCUSize, minCUSize]. Default: same as maxCUSize

QP Adaptation Range

Delta-QP range by QP adaptation based on a psycho-visual model.

CUTree

Enable the use of lookahead’s lowres motion vector fields to determine the amount of reuse of each block to tune adaptive quantization factors. CU blocks which are heavily reused as motion reference for later frames are given a lower QP (more bits) while CU blocks which are quickly changed and are not referenced are given less bits. This tends to improve detail in the backgrounds of video with less detail in areas of high motion. Default enabled

Lossless

Enables true lossless coding by bypassing scaling, transform, quantization and in-loop filter processes. This is used for ultra-high bitrates with zero loss of quality. Reconstructed output pictures are bit-exact to the input pictures. Lossless encodes implicitly have no rate control, all rate control options are ignored. Slower presets will generally achieve better compression efficiency (and generate smaller bitstreams). Default disabled.

AQ motion

Adjust the AQ offsets based on the relative motion of each block with respect to the motion of the frame. The more the relative motion of the block, the more quantization is used.
Default disabled. Experimental Feature

Strict CBR

Enables stricter conditions to control bitrate deviance from the target bitrate in ABR mode. Bit rate adherence is prioritised over quality. Rate tolerance is reduced to 50%.
Default disabled.

Specialised ratecontrol algorithm for film grain content

This parameter strictly minimises QP fluctuations within and across frames and removes pulsing of grain.
Default disabled.