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Beyond the Datasheet: Understanding Axis’ WDR options

December 7th, 2017 by Bethanne Pugh

Learn about the different Axis WDR solutions

Optimum light levels are key for the delivery of clear, detailed CCTV footage. If light conditions become challenging, the usability of images will often suffer – resulting in poor quality surveillance video. To ensure optimal image capture, most IP camera manufacturers will now include a handful of enhancement technologies to help improve imaging in difficult lighting environments; one of the most common being Wide Dynamic Range.

But what is WDR? …

Put simply, ‘Dynamic Range’ refers to the difference between the darkest and brightest light levels within an image. When there is a large difference between these areas, this is better known as ‘Wide’ Dynamic Range (or High Dynamic Range). Examples of this are scenes which simultaneously contain both very bright and very dark areas – such as entranceways (1), outdoor locations with direct sunlight, or in backlight scenarios where a person is in front of a bright window. WDR technology helps to cope with this wide range of light levels by applying various techniques which extend an IP camera’s dynamic range – enabling details in both bright and dark areas to be visible (2).

Scene featuring wide dynamic range

(1) Axis image showing a scene with wide dynamic range

Clear imaging through the use of WDR technology

(2) Axis image taken using Axis WDR technology

Keen to ensure useable CCTV footage is delivered at all times, Axis currently offer a range of different WDR solutions throughout their product range – each optimsed for specific applications. But with four choices available, it’s no wonder that things can get confusing when it comes to understanding Axis’ WDR options. To offer a helping hand, we decided to summarise each WDR solution – so you can be aware of what you are getting in terms of WDR from your Axis IP camera.

WDR – Dynamic Contrast

An Axis IP camera using WDR – Dynamic Contrast will perform advanced tone mapping, which takes the dynamic range of an image and drops certain brightness levels, in order to decrease the bit depth to a format that a computer screen can handle (security cameras will typically have a bit depth of 10 bits, whilst a standard monitor has a bit depth of only 8 bits). Both the darkest and brightest levels are retained with this tone mapping technology – resulting in more details at both ends of the scale. While it does not actually extend the captured dynamic range, this digital contrast enhancement method delivers images with very few artefacts – helping to improve identification. In addition, this simple method of combatting WDR only uses one exposure and is useful in scenes with limited dynamic range and lots of motion.

WDR – Dynamic Capture

A common method for extending dynamic range, Axis’ WDR – Dynamic Capture works by taking several images in a rapid sequence – each with different exposure levels. These are then combined into a composite, where both the brightest and darkest parts are retained to result in the delivery of clear and detailed images. Due to the sequential capture, this dual-exposure method does have its limitations – such as the introduction of artefacts if there is motion in the scene. This relates to anything from fast movement to a flickering light source and can result in a final composite image that features motion blur, ghosting or increased levels of noise.

Dynamic Contrast and Dynamic Capture originally sat side by side as Axis’ two WDR solutions; one combatting WDR with contrast enhancement and the other using dual-exposure. Developments in wide dynamic range technology have resulted in the release of alternative high-performance solutions – which have already been rolled-out across the Axis product range.

WDR – Forensic Capture

Back in 2014, after heavily investing in research and development, Axis released WDR – Forensic Capture; an advanced WDR solution which combines dual-exposure and local contrast enhancement methods. Designed for maximum forensic usability, the technology uses a set of advanced algorithms to help lower noise levels and increase the image signal. This results in a video that is highly optimised for forensic purposes, with every detail captured in both dark and bright areas at the same time – even in scenes with very difficult lighting conditions. However, due to the large number of reproduced tones and enhanced details, these optimised images can often have a distinct cartoonish style (not an issue if it’s detail that you are after, but insufficient if you require more ‘natural looking’ video). This high-performance WDR, which was once only supported in Axis’ advanced IP cameras, is now being introduced into their more affordable ranges such as the Axis M20 and M31.

Forensic WDR

Designed to be Axis’ highest performance WDR solution to date, Forensic WDR uses a combination of dual exposure and local contract enhancement methods – much alike its similarly named sibling ‘WDR – Forensic Capture’. The advanced technology was introduced earlier this year, and utilises the latest generation of image processing algorithms in order to effectively reduce visible noise and artefacts. This ensures forensic levels of detail can be seen at all times – even in scenes with bright highlights and dark shadows. In addition, this WDR solution is one of the first on the market to add sufficient forensic value to ultra high-resolution cameras or in scenes that feature high levels of activity.

Handy table showing a ‘quick glance’ comparison of the four Axis WDR solutions

In summary, any form of wide dynamic range technology is going to be helpful in scenes with challenging lighting conditions. However, when you take all of the technical information into account, it’s clear that Axis’ latest WDR solutions (WDR – Forensic Capture and Forensic WDR) will offer the base on which the technology will develop further.

Kevin Bowyer – Technical Director at NW Systems Group

Kevin Bowyer

WDR-based image processing is a fairly recent development which has made a real difference to system design and camera placement. Previous technologies such as backlight compensation were simply not good enough where scenes have high dynamic range so the introduction of WDR technologies by Axis, Sony and other manufacturers, allows cameras to be placed more conveniently for the end use.

Wide Dynamic Capture in particular is interesting – frame combination and pre-processing offers exciting possibilities for the future of image processing.

WDR has a processing/CPU implication and it should be noted that other technologies such as Lightfinder (Axis) may not be available if WDR is switched on. Not all features may work simultaneously. This ensures that the camera performance is never compromised.

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