Graphics Reference
In-Depth Information
6
High Dynamic Range
Imaging
6.1 Response Curves and HDR Imaging
The main purpose of image-based rendering is to reproduce the real world faith-
fully and with photorealism. However, film and digital cameras are intrinsically
limited in their ability to capture real world radiance values. In imagery, the range
of reproducible light intensity is limited by the characteristics of the film or sen-
sor array: if there is not enough light, nothing gets recorded; if there is too much,
the film or sensor saturates. The minimum and maximum values that can be re-
coded by a device is known as the
dynamic range
of the device. A photograph
of a scene that has a greater range of radiance values than the dynamic range of
the camera—as real scenes often do—cannot faithfully capture the lighting in the
scene. When using these images in rendering in techniques such as IBR, it can
substantially influence the appearance of the resulting images. This is why it be-
came necessary to capture a greater range of radiance values than is possible with
an ordinary camera. This process is called
high dynamic range
(HDR) imaging.
6.1.1 Response Curves
As described in Chapter 5, a pixel in the sensor of a digital camera records the
incident irradiance integrated over the time it is exposed. This value is called
the
exposure
. If the scene is static, the irradiance is constant and the exposure is
simply the product of the irradiance and the amount of time the shutter is open:
X
=
E
Δ
t
.
(6.1)
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