Cesar A. Gonzales

Two-pass MPEG-2 variable-bit-rate encoding

Many MPEG-2 encoding applications are real-time; this implies that the video signal must be encoded with no significant lookahead. However, there exist non-real-time applications that do enable us to first analyze a video sequence entirely, and, using the analysis results, to optimize a second encoding pass of the same data. One example of such an application is the digital video disk (DVD), which is designed to facilitate a variable-bit-rate (VBR) output stream. In that case, it is possible to let the MPEG-2 encoder produce a video sequence with a constant visual quality over time. This is in contrast to constant-bit-rate (CBR) systems, where the rate is constant but the visual quality varies with the coding difficulty. This paper describes a two-pass encoding system that has as its objective to produce an optimized VBR data stream in a second pass. In a first pass, the video sequence is encoded with CBR, while statistics concerning coding complexity are gathered. Next, the first-pass data is processed to prepare the control parameters for the second pass, which performs the actual VBR compression. In this off-line processing stage, we determine the target number of bits for each picture in the sequence, such that we realize the VBR objective. This means that the available bits are appropriately distributed over the different video segments such that constant visual quality is obtained. To be able to quantify the constant visual quality, perceptual experiments are described and a practical model is fitted to them. Exceptional cases such as scene changes and fades are detected and dealt with appropriately. We also ensure that the second-pass compression process does not violate the decoder buffer boundaries. Finally, the encoding is performed again, but now under control of the processed first-pass data. During the running of this second pass, a run-time bit-production control mechanism monitors the accuracy and validity of the first-pass data, correcting errors in prediction and observing the buffer boundaries. Results are compared to CBR operation.

Single-pass constant- and variable-bit-rate MPEG-2 video compression

Most real-time MPEG-2 encoders are designed to perform in a constant-bit-rate (CBR) mode, in which buffer constraints are imposed to circumvent large deviations from a desired rate at any instant in time. Although such streams are generally good-quality sequences, certain types of operations or environments call for a more efficient real-time CBR encoder. The first part of the paper describes how a better-quality CBR video stream can be produced by estimating the relative complexity of a picture in comparison with the average complexity of the partially encoded stream and using it to adjust the compression parameters in a single-pass mode of operation. Our CBR encoder is particularly attractive for digital broadcast and editing environments, in which representations of higher-fidelity video objects in both display and freeze modes are constantly pursued. The second part of the paper describes the real-time generation of video streams with a variable-bit-rate (VBR) encoder. This mode of operation is highly desirable for home entertainment and recreational events. We propose a robust single-pass VBR video encoder algorithm which is capable of learning and adapting itself to the complexity of image segments and thereafter creating streams which have constant visual picture quality. The new VBR scheme displays a better performance than the CBR encoder, particularly when special effects such as scene transitions, fades, or luminance changes are to be compressed. Both CBR and VBR encoders are fully compliant with the MPEG-2 standard and are easily implementable with IBM encoder architecture. Compression results for the new single-pass encoding algorithms and comparisons with previous CBR schemes are provided. The result suggests the suitability of our VBR approach for record/playback in storage media such as digital video disc (DVD) players, disk-based camcorders, and digital videocassette recorders (DVCRs). It further reflects the importance of our single-pass CBR scheme for providers of broadcast services, for which it allows more video programs to be allocated to a selected communication link, and for in-studio applications, for which it greatly facilitates visual analysis of captured streams.

Video compression algorithm with adaptive bit allocation and quantization

The emerging ISO MPEG video compression standard is a hybrid algorithm which employs motion compensation, spatial discrete cosine transforms, quantization, and Huffman coding. The MPEG standard specifies the syntax of the compressed data stream and the method of decoding, but leaves considerable latitude in the design of the encoder. Although the algorithm is geared toward fixed-bit-rate storage media, the rules for bit rate control allow a good deal of variation in the number of bits allocated to each picture. In addition, the allocation of bits within a picture is subject to no rules whatsoever. One would like to design an encoder that optimizes visual quality of the decoded video sequence subject to these bit rate restrictions. However, this is difficult due to the elusive nature of a quantitative distortion measure for images and motion sequences that correlates well with human perception. This paper describes an MPEG encoder designed to produce good quality coded sequences for a wide range of video source characteristics and over a range of bit rates. The novel parts of the algorithm include a temporal bit allocation strategy, spatially adaptive quantization, and a bit rate control scheme.

A locally quadratic model of the motion estimation error criterion function and its application to subpixel interpolations

Accurate motion estimation is essential to effective motion compensated video signal processing, and subpixel resolutions are required for high quality applications. It is observed that around the optimum point of the motion estimation process the error criterion function is well modeled as a quadratic function with respect to the motion vector offsets. This locally quadratic functional model decomposes the motion estimation optimization at subpixel resolutions into a two-stage pipelinable processes: full-search at full-pixel resolution and interpolation at any subpixel resolution. Practical approximation formulas lead to the explicit computations of both motion vectors and error criterion functional values at subpixel resolutions.

DCT coding for motion video storage using adaptive arithmetic coding

Abstract We describe a motion compensated hybrid DCT/DPCM video compression scheme, which has been proposed for the storage of compressed digital video. The scheme, an extension of a current CCITT/ISO proposal for compressing still-frame images, differs from other hybrid DCT proposals in several ways. (1) Reduction of DCT block-artifacts by implementing inverse AC-quantizers which are of finer granularity than the corresponding encoder quantizers. This process, which we label ‘AC-Correction’, is applied to intraframe coded frames. (2) Entropy coding by means of an adaptive binary arithmetic coder. Arithmetic coding results in better compression efficiency than Huffman coding, thus improving image quality when a fixed bandwidth channel is available. (3) Non-linear loop filtering that preserves edge definition. (4) Tight tolerances in the rate control of groups of frames.

Flexible scalable digital video coding

Abstract In this paper, we describe a video coding algorithm which combines the high visual quality of hybrid motion-compensated transform-based video coding techniques with the functional advantages of scalable, multi-resolution video. The technique produces a hierarchical video data representation by incorporating a simple frequency domain pyramid in a hybrid motion-compensated prediction/discrete cosine transform video coding algorithm. Compared to a single-layer hybrid scheme, this method has a very low penalty in coding efficiency and code complexity.

Motion video adaptive quantization in the transform domain

The authors developed a minimax adaptive quantization algorithm that operates in the discrete cosine transform domain conforming to the Moving Picture Experts Group (MPEG) standard. The algorithm is designed to optimize image quality by adapting a quantizer scaling factor to the local characteristics of the video pictures while preserving a constraint on the average output bit rate. The algorithm is well suited for real-time encoder implementations of current video compression standards, such as MPEG and H.261. >

Requirements for motion-estimation search range in MPEG-2 coded video

The motion-estimation search range required for interframe encoding with the MPEG-2 video compression standard depends on a number of factors, including video content, video resolution, elapsed time between reference and predicted pictures, and, just as significantly, pragmatic considerations in implementing a cost-effective solution. In this paper we present a set of experimental results that provide a probabilistic characterization of the size of motion vectors for different types of video, from well-known standard test sequences to fast-paced sports sequences to action movie clips. We study the impact of search range on compression efficiency and video quality. Finally, and on the basis of these results, we conclude with recommendations for target search ranges suitable for high-quality compression of standard and high-definition video.

Efficient and low-cost video transcoding

We present a low cost high performance scheme for video transcoding. Our scheme includes the information look-ahead structure and the transcoding algorithm. It is this look-ahead mechanism that greatly facilitates the use of information embedded in the bitstream for informative and intelligent transcoding. Furthermore, our transcoding algorithm has four new features: 1) picture complexity look-ahead for efficient rate allocation and control; 2) dynamic bit allocation weighting on picture types based on the scene complexity and bit rate; 3) adaptive nonlinear rate control on macroblocks based on complexity distribution in the picture; 4) Intra frame requantization adjustment to reduce the transcoding distortion. The software simulation results have proven that the performance of our transcoding scheme is significantly better than the reencoding scheme and comparable to the direct encoding scheme.

Low-cost and efficient logo insertion scheme in MPEG video transcoding

In this paper, we work on the problem of inserting logo into part of a previously compressed video sequence while transcoding it to a video stream with lower rate. In most of the transcoding architecture, it has been widely accepted to reuse the motion information extracted from the original incoming bit stream. For our transcoding and logo insertion process, we preserve the same motion information as the incoming video stream for the parts that were unaffected by logo, and propose an efficient algorithm to code logo areas. We first find the proper range of pictures that might be affected by logo insertions, analyze logo-affected parts of those pictures, and perform corresponding algorithms depending on the picture types. In addition, we develop a method to achieve better bit allocation during logo insertion. The efficiency of our algorithm is demonstrated by simulation.