G-012 H.264 早期 经典 码率控制 提案

Joint Video Team (JVT) of ISO/IEC MPEG & ITU-T VCEG

(ISO/IEC JTC1/SC29/WG11 and ITU-T SG16 Q.6)

7

th

Meeting: Pattaya II ,Thailand, 7-14 March, 2003

Document: JVT-G012

Filename: JVT-G012.doc

Title: Adaptive Basic Unit Layer Rate Control for JVT

Status: [Input Document to JVT]

Purpose: [ Proposal]

Author(s) or

Zhengguo Li, Feng Pan, Keng Pang

ao,rsusanto}@i2r.a-

star.edu.sg

Source: [Institute for InfoComm Research]

_____________________________

(Begin text of document here: 11-point font is suggested for short documents, 10 for long ones)

1. Introduction

An encoder employs rate control as a way to regulate varying bit rate characteristics of the

coded bitstream in order to produce high quality decoded frame at a given target bit rate. Rate

control is thus a necessary part of an encoder, and has been widely studied in standards, like

MPEG 2, MPEG 4, H.263, and so on [1,2,3,4,5]. However, it has not been well studied for JVT.

RDO. Moreover, the available channel bandwidth for the coding process can be either constant

or time varying. We thus need to consider both the constant bit rate (CBR) case and the

variable bit rate (VBR) case. However, the existing schemes focus on the CBR case [1,2,3,4].

In this proposal, we present an adaptive basic unit layer rate control scheme for JVT by

introducing a concept of basic unit and a linear model. The basic unit can be a frame, a slice, or

an MB. The linear model is used to predict the MAD of current basic unit in the current frame by

that of the basic unit in the co-located position of the previous frame. The chicken and egg

dilemma is solved as follows: the target bit rate for the current frame is computed by adopting a

leaky bucket model and linear tracking theory according to the predefined frame rate, the

remaining bits are allocated to all non-coded basic units in the current frame equally because

current basic unit. We focus on the VBR case while our scheme performs equally well in the

CBR case. A virtual buffer is used in our scheme to help adjust the coding process according to

the dynamics of the channel bandwidth. The buffer is not underflowed nor overflowed. Since

the model is almost the same as the leaky bucket model, our rate control is thus conformed to

hypothetical reference decoder (HRD).

To verify our scheme, we test our scheme in both the VBR case and CBR case. The bit rate

curve for the VBR case is a predefined curve. It is shown that the number of actual generated

bits is kept close to the bit rate curve and the buffer is neither underflowed nor overflowed. For

the CBR case, we compare the coding efficiency of an encoder by using our rate control

scheme with that of an encoder using a fixed quantization parameter. The target bit rate is

should be mentioned that our scheme is one pass while F086 is partially two pass.

2. Preliminary Knowledge

In this section, we shall present the problem associated with the rate control for H.264.

2.1 The Chicken and Egg Dilemma

The coding process of a MB related to the rate control is given by

Since quantization parameters are specified in both rate control and RDO, there exists a problem

when the rate control is implemented: to perform RDO for a MB, a quantization parameter should

be first determined for the MB by using the MAD of MB. However, the MAD of current MB is only

available after performing the RDO. This is a typical chicken and egg dilemma. Because of this,

the rate control for H.264 is more difficult than those for MPEG 2, MPEG 4 and H.263. To study the

2.2 Definition of A Basic Unit

The concept of a basic unit is defined by

sequence with QCIF size, is 99. According to Definition 1,t can be 1, 3, 9, 11, 33,

or 99. The corresponding is 99, 33, 11, 9, 3, and 1, respectively.

It is noted that by employing a bigger basic unit, a higher PSNR can be achieved while the bit

fluctuation is also bigger. On the other hand, by using a small basic unit, the bit fluctuation is less

severe, but with slight loss in PSNR.

2.3 A Fluid Flow Traffic Model

We shall now present a fluid flow traffic model to compute the target bit for the current coding

frame. Let denote the total number of frames in a group of picture (GOP),

occupancy of virtual buffer after coding the jth frame. We then have

The model is similar to the leaky bucket model in [7] in the sense that In our design,

we guarantee that the bitstream is contained in the above virtual buffer. Therefore, when the

bitstream is input to an HRD with parameters and , the HRD buffer does

not overflow nor underflow. In other words, our rate control scheme is conformed to HRD.

2.4 A Linear Model for MAD Prediction

We now introduce a linear model to predict the MADs of current basic unit in the current frame

by that of the basic unit in the co-located position of the previous frame. Suppose that the

predicted MAD of current basic unit in the current frame and the actual MAD of basic unit in the co-

located position of previous frame are denoted by and , respectively. The linear

prediction model is then given by

theory [5].

2. Allocate the remaining bits to all non-coded basic units in the current frame equally.

3. Predict the MAD of current basic unit in the current frame by the linear model (3) using the

Our proposed rate control scheme is composed of two layers: GOP layer rate control and frame

layer rate control if the basic unit is selected as a frame. Otherwise, an additional basic unit layer

rate control should be added. They will be presented in detail in the following sections.

3. GOP Layer Rate Control

In this layer, we need to compute the total number of remaining bits for all non-coded frames

in each GOP and to determine the starting quantization parameter of each GOP. Same as [5], we

assume that the GOP structure is IBBPBBP... P or IPPP…P, with I being an intra-coded picture, P

being a forward predicted picture and B being a bi-directional predicted picture. The length of a

GOP is usually 15-30 [4].

3.1 Total Number of Bits

In the beginning of the ith GOP, the total number of bits allocated for the ith GOP is computed as

In the case of CBR, i.e. , Equation (5) is simplified as

(6)