In High Sierra and iOS 11, Apple will be implementing a new video encoding process called High Efficiency Video Coding, or HEVC. It promises to shrink video size by a huge margin, supporting new high-resolution content while saving disk space. But what makes HEVC better than H.264, the video codec it plans to replace?
HEVC, also known as H.265, is a video compression standard designed for the newest generations of high-resolution video. It’s a successor to the widely-used H.264 codec (also called AVC or MPEG-4 Part 10) and offers some major improvements over that now-aging compression scheme. Eventually, HEVC (or H.265) may replace H.264 completely, but that might take some time to truly take hold.
HEVC was developed by the Joint Collaborative Team on Video Coding (JCT-VC), a group of video coding experts that started working on the compression standard back in 2010. Apple announced support for it at WWDC, and will be rolling it out to all iPhone users with devices newer than the iPhone 6 with the release of iOS 11.
Why Is HEVC Better than H.264?
The HEVC codec offers some major improvements over the H.264 codec, which was first developed in the hazy days of 2003. There are far more improvements that we can cover here, but these are the highlights for consumers.
HEVC provides major improvements in compression when compared to the H.264 codec it’s replacing. In fact, the newer codec can compress video nearly twice as efficiently as its predecessor. With HEVC, a video of the same apparent visual quality would take up only half as much space. Alternatively, a video of the same file size and bit rate could be significantly better looking.
Part of this improvement comes from an increased macroblock size. Macroblocks define the image area used for compression calculations, and larger macroblocks are required to efficiently compress high resolution video. H.264 allows for only 16 x 16 pixel macroblocks, which are too small to be efficient with video above 1080p. HEVC provides for 64 x 64 pixel macroblocks (now called coding tree units or CTUs), allowing for greater encoding efficiency at higher resolutions.
Improved intraframe motion prediction
A major element in video compression is predicting motion (or the lack thereof) between frames. When a pixel stays static (a solid background image, or for example) a smart video codec can save space by referencing it, rather than reproducing it. With improved motion prediction, HEVC can provide smaller file sizes and increased compression quality.
Improved intraframe prediction
Video compression also benefits from analyzing “movement” within individual frames, allowing single frames of video to be compressed more efficiently. This can be achieved by describing pixels layouts with a mathematical function rather than actual pixel values. The function takes up less space than pixel data, shrinking file size. However, the codec must support a sufficiently advanced mathematical function for this technique to be truly useful. HEVC’s intraframe prediction function is far more detailed than H.264’s, allowing for 33 directions of motion over H.264’s nine directions.
HEVC uses tiles and slices which can be decoded independently from the rest of a frame. This means that the decoding process can be split up across multiple parallel process threads, taking advantage of more efficient decoding opportunities on now-standard multi-core processors. With video resolutions getting higher, this kind of improved efficiency is required to decode video at a watchable pace on lower-end hardware.
Higher maximum frame size
The world is getting higher-res, and HEVC supports that. With HEVC, video can be encoded at up to 8K UHD or 8192 pixels × 4320 pixels. Currently, only a handful of cameras can even produce 8K video, and very few monitors can display that kind of resolution. But just as HD is today’s standard, we can expect 4K and eventually 8K to rise to similar prominence eventually.
The HEVC codec is specifically supported by the current generation of Intel processors. The Kaby Lake line of processors contains special instruction sets for encoding and decoding HEVC video, as should future generations. This gives the codec a major speed and consistency advantage when compared to other high-res video codecs. Considering the popularity and technical superiority of the H.264 codec, it’s not surprising that Intel would choose to throw their hardware might behind its successor.
Of course, this doesn’t limit HEVC use to Kaby Lake processors, but it does mean that computers using Kaby Lake chips will play HEVC video more fluidly. And considering the computational overhead required to encode and decode high-resolution HEVC video is significant, this could mean a major difference between hardware- and software-supported implementations of HEVC.
Conclusion: Where Is HEVC Found?
Support for HEVC isn’t limited to macOS High Sierra and iOS 11. The video format will also be used in Apple’s tvOS and Safari web browser for streaming video, and other companies are broadening their support. It’s still not as popular as the erstwhile H.264, but it is gaining ground.
Just last month Microsoft released a free extension for Windows 10 that adds support for HEVC video decoding. Netflix’s 4K content is streamed with the HEVC codec on supported hardware. YouTube, on the other hand, does not use HEVC, instead opting for their competing VP9 compression scheme.
But with HEVC greater efficiency, we’re likely to see that codec dominate the marketplace in the years to come.
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