保证视频质量

在接下来的两年里, as we approach a world where processing power is sufficient to produce decent or even high-质量 real-time encodes, the differentiator in the professional encoding and transcoding markets will soon shift from one of performance to one of 质量. 

Or at least that’s one theory. 毕竟, 理由是, if everyone’s encoder is capable of generating content in real time, or even faster than real time, what else but 质量 will differentiate solutions from one another?

While we’re not yet at a time when all systems perform equally, we are at a transition point at which it makes sense to begin to address 质量 as a defining factor. 

Defining Quality Approaches

在质量争论中, there seem to be two basic approaches: a pragmatic approach and a technical one. 

The pragmatic approach often comes down to a decision by content owners and their “golden eye” compressionists, balancing “good enough” against too-lengthy encoding times. 仍然, the balance doesn’t limit premium content owners from wanting both speed and 质量: The Holy Grail is a fast, robust system to encode content in real time (or faster than real time for file-based transcoding) at the highest possible 质量.

换句话说, a suspension of the laws of physics, as one participant in Transitions’ 2010 inaugural “Best Workflows” study succinctly put it after viewing preliminary results from this set of comparative tests in mid- to late 2010, where we attempted to balance performance, 质量, and workflow differentiators in a few of the world’s best encoding and transcoding systems.

技术方法, 另一方面, seeks to break down 质量 into numerical values, using a variety of tests that have varying degrees of correlation to human perception. While this quantitative approach does not yet address some of the newer delivery solutions, such as adaptive bitrate (ABR) and peer-to-peer (P2P) delivery, the technical approach is a starting point to baseline content 质量.

What exactly are some of the technical elements of this 质量 debate, and how do we approach 质量 measurements beyond just what looks good to an individual?

While it would be nice to just list them off in four or five bullet points, it turns out that even the elements themselves are up for debate, depending on whether the discussion is around the 质量 of the video luminance, 图像, 或者配送网络.

“For the analysis of decoded video,” wrote Stefan Winkler and Praveen Mohandas in a paper submitted to the IEEE Transmission Broadcasting Journal, “we can distinguish data metrics, which measure the fidelity of the signal without considering its content, 图像度量, which treat the video data as the visual 信息 that it contains. For compressed video delivery over packet networks, there are also packet- or bitstream-based metrics, which look at the packet header 信息 and the encoded bitstream directly without fully decoding the video. 此外, metrics can be classified into full-reference, 并没有提到, and reduced-reference metrics based on the amount of reference 信息 they require.”

明白了? 换句话说, depending on which portion of the network you are measuring, there are a number of 质量 measurements at hand.