H.264 is the new standard for high definition digital video, and for good reason. A codec based on the H.264 standard compresses a digital video file so that it only uses half the space of MPEG-2 (the DVD standard), to deliver the same quality video. This means you can enjoy stunning HD video the way it was meant to be seen without sacrificing speed or performance.
What is H.264?
Video for DivX Plus® is based on the H.264 standard, a state-of-the-art digital format for efficiently encoding high definition video. Why is H.264 so great? To make a long story short, H.264 encompasses a collection of powerful features that enable the delivery of high-quality video at very low data rates. Using DivX Plus Software you can create, enjoy and share stunning HD video in the same ways that you can with standard definition DivX® video. This includes creating personal libraries on your hard drive, burning files to disc or transferring them via USB drives to your DivX Plus devices, or even serving them to visitors on your web page using the DivX Plus Web Player.
The H.264 standard is already set to supersede some of the formats that are commonplace today such as DV, the standard used by many consumer video cameras, and MPEG-2, the standard for DVD video and some types of cable TV and digital broadcast.
Why DivX Plus?
The H.264 standard supports the notion that different categories of decoder device may have individual requirements and capabilities. For example, compared to a desktop computer, mobile devices may have lower display resolution, less internal memory and processing power, and a desire to maintain battery life. By selecting the H.264 bitstream features used during content creation, we can significantly influence the playback requirements. DivX Plus HD represents a carefully selected balance of features developed in collaboration with our manufacturing partners that enable both strong compression and interoperability across a huge range of device categories including DVD players, Blu-ray players, digital TVs, set-top boxes and more.
Brace yourself -- we've got some serious details
A note of caution before proceeding: If you're only interested in a high-level overview of what H.264 is and why we think it's so great, you have hopefully been enlightened by the above paragraphs. If, on the other hand, you're hungering for a more technical discussion of the intricacies of video compression and the advancements that H.264 brings to the dark art of video encoding, the next 900 words or so are for you.
Digital video compression: The basics
Digital video is compressed to economize on space, whether it's bandwidth or media, and a codec (compressor/decompressor) does the job of encoding and decoding. In our case that means taking a sequence of raw video and transforming it into a series of bits we call the bitstream, and the process of transforming that bitstream back into raw video. By improving the techniques upon which the codec is based, we're able to transmit higher quality video using the same bandwidth as before. Typically both the encoder and decoder will become more complex, but this is okay because computers are always getting faster.
An H.264 encoder reduces the amount of information required to reproduce the input video by exploiting redundancy in the pictures it's encoding, both spatially (within the same picture) and temporally (between pictures). Temporally, an encoder processes each frame, subdividing the picture into a grid of blocks and searching previous or future frames for each block for matching texture, a technique known as motion estimation. Once a suitable match is found, a decoder can later reproduce the texture of that block using only a vector pointing to the matching reference texture along with a little information to correct any small texture differences. Spatially, where motion estimation fails to find suitable matches, an encoder can use the texture of nearby blocks within the same frame to predict the block texture and store only the difference between the prediction and the actual block texture. This is more efficient than storing the complete texture directly but still more costly than motion estimation. H.264 encoders act as “lossy” compressors; their goal is not to reproduce the original picture exactly but instead to choose the optimal means to reduce the data rate while preserving visual quality as best as possible. With suitable settings differences can be unperceivable even when compression over raw input approaches 100:1.
The H.264 standard offers substantial performance improvements over its predecessors. For example, a DVD can hold one two-hour movie compressed using MPEG-2 encoding (typical for DVD video) but four hours of video using an H.264 codec. H.264 encoding used by DivX Plus is even more efficient than the popular DivX 6 codec, which is based on the MPEG-4 ASP standard, H.264's predecessor.
What's new in H.264?
H.264, as used by the DivX Plus format, has a variety of new features that improve picture quality and compression over the DivX 6 (ASP) codec:
In-loop deblocking:
Deblocking is a CPU-intensive technique that attempts to remove blocking artifacts in the decoded picture. The DivX ASP decoder uses deblocking as an optional post-processing technique to improve quality during playback, typically applied depending on the availability of free CPU time. This allows fast computers to see the best possible picture and slower computer to dial back post-processing to achieve smoother playback.H.264 instead enforces deblocking on every frame during both encoding and decoding. The result is that encoding becomes more efficient because there is less noise present in reference pictures but consequently there is no option to disable deblocking to boost playback performance on slower systems.
More reference pictures:
The ASP and H.264 standards both use motion estimation to efficiently code each frame, reconstructing new frames by moving texture from already decoded pictures around. Under the ASP standard the only inter-frame references permitted are to the next or previous frames, and frames that make both references cannot serve as references themselves. These restrictions are important for older devices because they limit the memory required to decode any stream as well as reducing the number of reference frames an encoder might spend time searching during motion estimation. The H.264 standard vastly expands the system of reference pictures by allowing a single frame to reference many past and future frames and all frame types to serve as reference pictures.
Quarter-pixel motion estimation
When the DivX ASP encoder searches for blocks in past and future frames it evaluates motion vectors down to a half-pixel precision. Half-pixel searches can be performed quickly during encoding and reconstructing the texture from motion vectors with half-pixel precision during playback is also very fast. H.264 uses quarter-pixel precision for motion search and this leads to longer search times during encoding as well as more complex texture reconstruction during playback. Although the processing requirements are higher, more accurate prediction leads to sharper pictures and improved encoding efficiency.
Smaller block sizes, improved prediction
Both ASP and H.264 use 16x16 pixel blocks as their fundamental block sizes. Under ASP blocks can be subdivided into four 8x8 partitions, but H.264 allows for block subdivisions down to 4x4 pixels. Smaller block sizes are beneficial in areas of low spatial resolution and therefore particularly useful for standard definition and mobile content. H.264 also offers much greater flexibility in intra-frame prediction so when motion estimation fails or keyframes are coded block texture can be stored with greater efficiency than the ASP standard allows.
What's special about the DivX Plus implementation of H.264?
It is a common misconception that H.264 is a format with only a single form. Different profiles of H.264 present different feature sets that are not strict subsets of one another. This leads to problems of interoperability. We want to unify the H.264 format into one form so that users can create and play their digital media on their video cameras, portable media players and TVs—seamlessly. The H.264 format's powerful compression and scalability allows us to deliver on the promise of the true cinematic viewing on your computer, in the living room and on the go. Still reading? Check out DivX Labs to keep up with the DivX community.
