EE 5359 Multimedia Processing – Summer 2008 Interim Project Report on H.264 to VP6 Transcoder Submitted by Jay R. Padia 1000 60 5145 Date: July 17, 2008Abstract VP6 is a video coding standard developed by On2 Technologies, Inc. It is the preferred codec for Macromedia Flash 8 video. VP6 assumes importance with Macromedia Flash emerging as a widely adopted video streaming technology over the internet. H.264 is currently one of the most widely accepted video coding standards in the industry. It enables high quality video at low bitrates. So there is increasing importance of techniques which can convert video from H.264 to VP6 and thereby enable high quality video transmission over the internet using Flash. The current research shows H.263 video which is a previous generation standard of H.264 can be transcoded to VP6 and complexity can be reduced upto 50%. The similarities and dissimilarities between the two encoders are used to reduce the complexity using Dynamic Search Range and Dynamic Search Window. The success in reducing complexity in the H.263 to VP6 transcoder and the available reference material related to transcoding algorithms enables us to propose a new study to find an algorithm for transcoding H.264 coding standard to VP6 coding standard. It is proposed to explore the similarities and dissimilarities between the two standards to find the right transcoding technique.Importance of the H.264 Standard H.264 [4] was proposed by the Joint Video Team (JVT) of the ITU-T Video Coding Experts Group (VCEG) and ISO/IEC Moving Pictures Experts Group (MPEG) in 2003. It is currently one of the most widely accepted industry standards. It can provide good quality video at substantially lower bitrates compared to the previous standards. It also shows more error robustness [1] [2]. H.264 has a set of innovations which can together provide a vast improvement in performance over previous generations of video codecs. MPEG-2 [21] was the most widely used video codec before the emergence of H.264. H.264 provides the same quality as MPEG-2 at a third to half the data rate. At the same data rate, H.264 can provide upto 4 times the frame size as can be seen in Table 1. H.264 provides better image quality when reaching its limits. It does not break into blocks but degrades much more smoothly, making the image softer as compression increases. H.264 is an emerging standard and over the years it can see an improvement over the current performance. It can be expected of H.264 to improve over the years, just as other standards have improved in quality and performance [3]. Table 1. H.264 data rate at various resolutions [3] Overview of H.264 Standard H.264 introduces many new features that are significantly different from the previous generation codecs. These new features make it vastly different from the existing codecs and make it much more effective. Given below is an overview of the features of H.264 video codec. Profiles and levels Like any comprehensive standard, the H.264 standard defines a set of profiles and levels to set points of conformance for various classes of applications and services. In each profile, specific encoding tools are permitted to best meet the needs of the intended scenario. H.264 includes six profiles as shown in figure 1 [4]: • Baseline. Intended for low-complexity applications such as video conferencing and mobile multimedia. • Main. Intended for the majority of general uses such as the Internet, mobile multimedia, and stored content. • Extended. Intended for streaming applications, where stream switching technologies can be beneficial. • Three High profiles (also known as Fidelity Range Extension or FRExt). Consists of three separate High profiles (High, High 10, and High 4:2:2), intended for high-end professional uses [3] [5].Fig 1. H.264 profile levels [3] 4x4 integer transform. H.264 is designed to operate on much smaller blocks of pixels than other common codecs, which mitigates blocking, smearing, and ringing artifacts. So H.264 video is crystal clear even in areas of fine detail. Because the transform is a precisely specified integer transform, it provides bit-precise reconstruction (that is, exact-match decoding) rather than statistically generated reconstruction. As a result, there can be no drift among various decoder implementations, so any compliant H.264 decoder will decode the video exactly as the content author intended it to look [3] [6]. X = input matrix; CfXCfT = core 2D transformation for X; Ef = matrix formed by scaling factors a, b, c Increased precision in motion estimation. H.264 also benefits from increased precision in motion estimation, which is the process of simplifying redundant data across a series of frames. By expressing information to 1/4-pixel resolution (fig 2) as opposed to 1/2-pixel resolution like most other codecs, H.264 represents both fast- and slow moving scenes more precisely. So objects in motion are more crisply reconstructed during decode, providing a better representation of the source material [7].Fig 2. Motion vectors in H.264 [7] Flexible block sizes in motion estimation. During motion estimation, traditional codecs commonly process frames at the macroblock level (16 pixels by 16 pixels). H.264 can process on segments within a macroblock, ranging in size from the commonly used 16x16 to as small as 4x4 as shown in fig 3, which helps to code complex motion in areas of high detail. The ability of H.264 to perform its processing on a variety of block sizes means that scenes with complicated motion are more expressively described, providing higher quality in lower data rates [7]. Fig 3(a). Macroblock partitions – 16x16, 16x8, 8x16 & 8x8 [7] Fig 3(b). Macroblock sub-partitions – 8x8, 8x4, 4x8 & 4x4 [7] Intraframe prediction. H.264 is able to gain much of its efficiency by simplifying redundant data not only across a series of frames, but also within a single frame, a technique called intraframe prediction (figure 4). The H.264 encoder uses intraframe prediction with more ways to reference neighboring pixels, so it compresses details and gradients better than previous
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