Video optimization for web
Web users love videos (high-quality videos). In fact, 4x as many customers would rather watch a video about a product than read about it. But adding videos to your website can cause a lot of performance problems, and it needs to be optimized to handle them.
Video optimization refers to a set of technologies used to improve consumer viewing experience improving video quality and reducing video start times or re-buffering events, and the amount of network bandwidth consumed. Taking special consideration to play videos on the wide variety of mobile devices and networks.
Video optimization starts with the video format. In fact, MP4 and WebM have become the standard formats for platforms like YouTube and Vimeo. This formats pack high-quality videos into small file sizes. Moreover, HTML5 can now deliver video content without the aid of a plugin, using this two formats (MP4 and WebM).
It is important to know that MP4 and WebM files are containers. This containers may contain one or more streams and use different codecs. This distintion between a container and a codec is important as files with the same container can have their contents encoded with different codecs.
Nowadays, there are three main options for video encoding:
H264 (also known as AVC): Packaged in a MP4 file is the universally supported standard for web and mobile devices.
VP9: Supported by Chrome, Firefox and Android is the current open source codec used in WebM files. As an advantage, WebM VP9 encoded videos tend to be lighter than H264 encoded files.
H265 (also known as HEVC): Packaged as a MP4 file it is about 30 percent more efficient than the H264 (lighter files with the same visual quality). Currently H265 works only on IE, Edge, Safari, and iOS 11 and up.
|Extension||Video Codec||Audio Codec||Support|
|webm||VP9||vorbis/opus||Chrome, Firefox, and Android|
|mp4||H265||aac||Safari, Edge, and iOS|
Modern codecs, such as HEVC (H265) and VP9, render sharing pixels across frames more efficient, encoding the key frames while maintaining the visual fidelity of videos. In fact, they are about 30-50 percent more efficient than the H264 standard. However, HEVC and VP9 do not enjoy universal browser or device support, so these options must be provided with a fall back H264 version.
There are several types of compression artifacts, but they can be categorized by whether they are temporal (time/sequence-based) or spatial (location-based). If you can see the artifact when the video is paused, then it is probably a spatial artifact. If it is much more visible while the video plays, then it is likely temporal.
To convert your videos to one of this formats, one of the best tools available is HandBrake. This open source application provides good presets to reduce video files without degrading image quality.
For instance, we can optimize the Sintel trailer just using the default options to produce a high quality MP4 file ready for the web, reducing the original size from 28.2MB to 19.2MB. (Note that the file is saved by HandBrake with the m4v extension, but this is an standard mp4 file renamed)
This generated file can be directly delivered from your server, as a progressive video using the HTML5 video tag standard for playback in the user's browser.
<video width="100%" controls> <source src="Sintel_Trailer.1080p.DivX_Plus_HD.m4v" type="video/mp4"> </video>
The HTML5 video tag also supports multiple sources to allow the browser to choose from. The browser then starts at the top of the list (H265) and goes down (VP9) until it finds a version it can play. This way, you can get optimal cross-browser support for video compression.
Check for H265 support first and then VP9 support. If neither of them works, fall back to the H264 manifest.
<video width="100%" controls> <source src="Sintel_Trailer.1080p.DivX_Plus_HD.mp4" type="video/mp4; codecs=hevc"> <source src="Sintel_Trailer.1080p.DivX_Plus_HD.webm" type="video/webm; codecs=vp9"> <source src="Sintel_Trailer.1080p.DivX_Plus_HD.m4v" type="video/mp4"> </video>
This approach can be great for short-form videos (under 20 seconds) and for videos that are displayed at a low resolution (such as ads and previews). But since your videos are delivered as a single file, this video optimization is not enough to provide an optimal experience to all users, especially in the case of mobile users.
For these users, videos must follow the principles of responsive web design to make your website adaptable to different screens sizes and display resolutions. Hence, sending down a video too large for the screen is not only a waste of resources and increased bandwidth costs on your part, but also a waste of your users' bandwidth.
In that case, the solution appears as Adaptive Bitrate Streaming to deliver a lightweight and optimal experience to all users.
Adaptive bitrate streaming
Adaptive Bit-Rate (ABR) technology solves some of the challenges with video streaming. These techniques use multiple versions of the video available at different resolutions and bitrates to provide the quality / bitrate that works best for every user, considering their network bandwidth (like 3G or LTE 4G) and visualization resolution.
ABR takes into account the network conditions and the available resolution of the viewer to choose the best video version to be displayed every time. It enables videos to start quicker, with fewer buffering interruptions, and at the best possible quality for the current device and network connection. For instance, it dynamically switches to a lower-bitrate (lower-quality) version of the video when encountering slow network conditions.
Although ABR are excellent methods for packaging and delivering content streams that will perform well on a variety of networks and devices, they can be expensive to implement due to the equipment, storage capacity and expertise needed. In particular, ABR streaming requires that video streams be encoded at multiple bit rates, screen sizes and quality levels, a process that complicates content preparation and management workflows.
HLS and MPEG-DASH
Two main protocols have becoming standard for video web streaming:
HTTP Live Streaming (HLS) is an HTTP-based media streaming communications protocol implemented by Apple Inc. It is natively supported by Safari, OS X, and iOS software.
Dynamic Adaptive Streaming over HTTP (MPEG-DASH) is an HTTP-based media streaming protocol either. But is more recent than HLS, it is codec agnostic, and supports some additional characteristics.
Both protocols work by breaking the overall stream into a sequence of small HTTP-based file downloads (chunks). As the stream is played, the client may select from a number of different alternate streams containing the same material encoded at a variety of data rates, allowing the streaming session to adapt to the available data rate.
At the start of the streaming session, the client software downloads a master playlist file containing the metadata for the various sub-streams that are available. The client software then decides what to download from the available media files, based on predefined factors such as device type, resolution, current bandwidth, size, etc.
As a result, to deliver videos using adaptive streaming, you need multiple copies of your video prepared at different resolutions, qualities, and data rates. These are called video representations or variants.
Bitrate and resolution are the properties that correlate to the amount of data in a media file. Selected a codec, the number of pixels (video resolution) and bitrate set the video quality.
Bitrate = total pixels/second * bytes/pixels
Thus, a 10-second video with a 10MB file size has a bitrate of 8 mbps (1 megabyte per second * 8 megabits per megabyte).
- Bitrate is the measure of how many bits are used to encode one second of a stream. The more bits used to encode a second of stream, the higher the quality. For instance, the table bellow compares web bitrates with common home video formats. Unsurprisingly, bitrates for the web are low. Values are given in megabits per second (Mbs).
|Desktop web video||2 Mbs|
|4G mobile video||0.7 Mbs|
|3G mobile video||0.35 Mbs|
- Resolution is the amount of information in a single frame of video, given as the number of logical pixels in each dimension. For instance, the following list is common for 16:9 video resolutions.
There is no universal best parameters to adjust video bitrates. They should be selected for every video content, testing the result and tweaking the bitrates to your requirements. Keep in mind that a given bitrate at a constant resolution would result in a video of higher visual quality in H265 and VP9 than in H264.
Most of transcoding services provide fixed configurations for ABR variants. This means poor results with high dynamic videos and a huge bandwidth waste with low dynamic videos.
We perform a per-title encoding, analyzing each video to adapt the variants bitrate, balancing compression and contents to provide the best visualization experience at the minimum file size.