SSIMWAVE Archives – Page 2 of 3 – The Broadcast Knowledge

Video: Banding Impairment Detection

Posted on 16th July 2020 by Russell Trafford-Jones

It’s one of the most common visual artefacts affecting both video and images. The scourge of the beautiful sunset and the enemy of natural skin tones, banding is very noticeable as it’s not seen in nature. Banding happens when there is not enough bit depth to allow for a smooth gradient of colour or brightness which leads to strips of one shade and an abrupt change to a strip of the next, clearly different, shade.

In this Video Tech talk, SSIMWAVE’s Dr. Hojat Yeganeh explains what can be done to reduce or eliminate banding. He starts by explaining how banding is created during compression, where the quantiser has reduced the accuracy of otherwise unique pixels to very similar numbers leaving them looking the same.

Dr. Hojat explains why we see these edges so clearly. By both looking at how contrast is defined but also by referencing Dolby’s famous graph showing contrast steps against luminance where they plotted 10-bit HDR against 12-bit HDR and show that the 12-bit PQ image is always below the ‘Barten limit’ which is the threshold beyond which no contrast steps are visible. It shows that a 10-bit HDR image is always susceptible to showing quantised, i.e. banded, steps.

Why do we deliver 10-bit HDR video if it can still show banding? This is because in real footage, camera noise and film grain serve to break up the bands. Dr. Hojat explains that this random noise amounts to ‘dithering’. Well known in both audio and video, when you add random noise which changes over time, humans stop being able to see the bands. TV manufacturers also apply dithering to the picture before showing which can further break up banding, at the cost of more noise on the image.

How can you automatically detect banding? We hear that typical metrics like VMAF and SSIM aren’t usefully sensitive to banding. SSIMWAVE’s SSIMPLUS metric, on the other hand, has been created to also be able to create a banding detection map which helps with the automatic identification of banding.

The video finishes with questions including when banding is part of artistic intention, types of metrics not identifiable by typical metrics, consumer display limitations among others.

Watch now!
Speakers

Dr. Hojat Yeganeh
Senior Member Technical Staff,
SSIMWAVE Inc.

Video: No-Reference QoE Assessment: Knowledge-based vs. Learning-based

Posted on 19th June 2020 by Russell Trafford-Jones

Automatic assessment of video quality is essential for creating encoders, selecting vendors, choosing operating points and, for online streaming services, in ongoing service improvement. But getting a computer to understand what looks good and what looks bad to humans is not trivial. When the computer doesn’t have the source video to compare against, it’s even harder.

In this talk, Dr. Ahmed Badr from SSIMWAVE looks at how video quality assessment (VQA) works and goes into detail on No-Reference (NR) techniques. He starts by stating the case for VQA which is an extension, and often replacement for subjective scoring by people. Clearly this is time-consuming, can be more expensive due to involvement of people (and the time) plus requires specific viewing conditions. When done well, a whole, carefully decorated room is required. So when it comes to analysing all the video created by a TV station or automating per-title encoding optimisation, we know we have to remove the human element.

Ahmed moves on to discuss the challenges of No Reference VQA such as identifying intended blur or noise. NR VQA is a two-step process with the first being extracting features from the video. These features are then mapped to a quality model which can be done with a machine learning/AI process which is the technique which Ahmed analyses next. The first task is to come up with a dataset of videos which should be carefully chosen, then it’s important to choose a metric to use for the training, for instance, MS-SSIM or VMAF. This is needed so that the learning algorithm can get the feedback it needs to improve. The last two elements are choosing what you are optimising for, technically called a loss function, and then choosing an AI model for use.

The data set you create needs to be aimed at exploring a certain aspect or range of aspects of video. It could be that you want to optimise for sports, but if you need a broad array of genres, optimising for reducing compression or scaling artefacts may be the main theme of the video dataset. Ahmed talks about the millions of video samples that they have collated and how they’ve used that to create their metric called SSIMPLUS which can work both with a reference and without.

Watch now!
Speaker

Dr. Ahmed Badr
SSIMWAVE

Video: Subjective and Objective Quality Assessment

Posted on 13th May 2020 by Russell Trafford-Jones

Video quality is a key part of user experience, so understanding how different parts of your distribution chain can affect your video in different ways is an important factor ensuring continued quality in the service and quick fault finding where problems are reported.

Abdul Rehman from SSIMWAVE speaks at the Kitchener-Warterloo Video Technology Meetup explaining both subjective quality assessment where humans judge the quality of the video and objective quality assessments where computers analyse, often terabytes, of video to assess the quality.

Starting with a video showing examples of different problems that can occur in the chain, Abdul explains how many things can go wrong including lost or delayed data, incorrect content and service configuration checks. Display devices, nowadays, come in many shapes, sizes and resolutions which can, in turn, cause impairments with display as can the player and viewing conditions. These are only around half of the different possibilities which include the type of person – a golden eye, or a pure consumer.

In order to test your system, you may need test codecs and you will need test content. Abdul talks about subject rated databases which have images which have certain types of distortions/impairments. After seeing many examples of problem images, Abdul asks the question of who to deal with natural images which look similar or deliberate use, for creative purposes, of distorted videos.

Subjective video quality assessment is one solution to this since it uses people who are much better at detecting creative quality than computers. As such, this avoids many false positives where video may be judged as bad, but there is intent in the use. Moreover, it also represents direct feedback from your target group. Abdul talks through the different aspects of what you need to control for when using subjective video quality assessment in order to maximise its usefulness and allow results from different sessions and experiments to be directly compared.

This is to be compared against objective video quality assessment where a computer is harnessed to plough through the videos. This can be very effective for many applications meaning it can shine in terms of throughput and number of measurements. Additionally, it can make regression testing very easy. The negatives can be cost, false positives and sometimes speed – depending on the application. You then can take your pick of algorithms such as MS-SSIM, VMAF and others. Abdul finishes by explaining more about the benefits and what to look out for.

Watch now!
Speakers

Abdul Rehman
Cofounder, CEO and CTO,
SSIMWAVE

Webinar: Assessing Video Quality: Methods, Measurements, and Best Practices

Posted on 12th November 2019 by Russell Trafford-Jones

Wednesday, November 13th, 8am PST / 16:00 GMT

Bitmovin have brought together Jan Ozer from the Streaming Learning Center, their very own Sean McCarthy and Carlos Bacquet from SSIM Wave to discuss how best to assess video quality.

Fundamental to assessing video quality, of course, is what we mean by quality, which artefacts are most problematic and what drives the importance of video quality.

Quality of streaming, of course, is interdependent on the quality of the experience in general. Thinking of an online streaming system as a whole, speed of playback, smooth playback on the player itself and rebuffing are all factors of perceived quality as much as the actual codec encoding quality itself which is what is more traditionally measured.

The webinar brings together experience in measuring quality, monitoring systems and ways in which you can derive your own testing to lock on to the factors which matter to you and your business.

See the related posts below for more from Jan Ozer