White Paper
causing excessive noise in the affected areas. The effect of
as signal-to-noise ratio discontinuity, and appears as areas of excessive noise around the middle of the dynamic range.
is present, it could produce motion-like artifacts resulting in colorful noise patterns. Measures to suppress motion artifacts in WDR are known as motion compensation.
Motion compensation may produce visual artifacts in a special case of capturing WDR scene containing very bright
captured primarily with the short shutter. If the duration of the short shutter is shorter than the duration of the half-cycle of
Noise
variation s betwee n th e shor t an d th e lon g shutte r images . These variations may be mistaken by the on-camera motion compensation as legitimate motion, causing visual artifacts. In this scenario, disabling motion compensation may be a reasonable solution.
LIMITATIONS OF WDR IMAGING It should be noted that in some of the mass produced WDR technologies, the choice of available shutter ratios is very limited due to certain technological and cost constraints. As a result, problems caused by selecting the inadequate shutter
artifacts highly dependent on the scene itself.
A further limitation of WDR imaging is related to motion artifacts. Due to different shutter speeds, parts of the scene that were moving at the time of capture will appear at different locations in the two (or more) individual snapshots that make up the composed WDR image. For example, consider a dark object that moves across a bright background scene. If the object was stationary, the dark pixels in the composed WDR image would come from the long shutter image, while the surrounding outline would come from the short shutter image. However, because the object is moving, its outline in the short shutter image does not match its outline in the long shutter image. A decision has to be made as to how to
motion is accompanied by motion blur, which produces brightness values in-between the brightness of the object and the background. In the case of color WDR imaging, motion artifacts may produce bright color contours around the moving objects unless special computational measures are taken in image processing. Moreover, when excessive temporal noise
Motion artifacts from combining different shutter images
DISPLAYING WDR VIDEO ON A TV MONITOR Are there any special considerations in displaying the wide dynamic range video on a television monitor? The answer is yes: monitors are generally incapable of displaying wide dynamic range imagery, they are limited to a dynamic range which is 200-300 times narrower than that of a typical WDR camera. Fortunately, there is a solution: the problem is solved by taking advantage of one of the many idiosyncrasies of human visual perception which places much more importance on local contrast variations, and much less on the global variations between the large areas making up the overall scene. To take advantage of this, the WDR image is put through a nonlinear image processing process known as tone mapping, which reassigns pixel brightness values to achieve the reduction of global contrast while preserving the local one. Consider a typical
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