Axis WDR vs WDR?By: John Honovich, Published on Jun 14, 2013
Axis has two types of WDR, and if you do not know the difference between the two, join the club. One is called WDR with Dynamic Contrast and the other is called WDR with Dynamic Capture. They even sound almost identical. Recently, we had debates in the IPVMU class about this with attendees conflating both and us arguing that performance differences between the two were dramatic. In this note, we explain the differences between the two and how to best use (and not use) these features.
Background: Readers not familiar with WDR should first read our WDR tutorial.
Contrast vs Capture
Axis' 'Capture' mode is what IPVM refers to as 'true' WDR using multiple exposures while Axis' 'Contrast' mode is what we consider a 'fake' digital WDR.
Axis explained the technical differences as follows. With dynamic contrast:
"The camera sensor captures an image with higher bit depth than what the camera can send out. The camera then performs an advanced tone mapping technique, where some brightness levels are dropped to decrease the bit depth to a format that the computer screen can handle. In this tone mapping, both the darkest and the lightest parts are taken into account, resulting in more details on both ends – i.e. why the term “contrast” is used. A camera with a more powerful sensor/processing power can perform more advanced tone mapping."
And here's Axis' explaining dynamic capture:
"The camera ‘captures’ several images in fast sequence with different exposure levels between them. These images are then put together to a composite, where both the lightest and the darkest parts are kept. This image has a much higher bit depth, higher than a computer screen can handle. Therefore, a tone mapping is made – to downsize the image."
While many, if not most of, Axis professional cameras support Dynamic Contast, only two lines support the more powerful Dynamic Capture, the Q1604 box and P3384 domes.
In our WDR scene tests, cameras with Dynamic Contrast (like the P1344) delivered no noticeable improvements with WDR enabled while the Q1604 with Dynamic Capture, delivered top tier quality (see the Q1604 / P1344 WDR test results).
Beware of Dynamic Contrast
If you really need WDR from Axis, you should use Dynamic Capture cameras (Q1604/P1384) and stay away from the Dynamic Contrast ones. Moreover, if you are specifying WDR, be careful not to accept WDR with Dynamic Contrast cameras. To avoid this, you can add require WDR implemented with multiple exposures as this is the standard technique for true WDR MP cameras.
Axis is not the first company with WDR terms that confuse though given their market power and the nearly identical sounding terms, their approach is likely to cause problems. Currently, Axis has a WDR white paper [link no longer available] but that only mentions Dynamic Capture. Axis says they are working on new material to better explain the differences. Perhaps they can follow the lead of others and rename 'Dynamic Contrast' as simply DWDR, the more common acronym identifying 'fake' WDR.