0 Color and tone correction block details

Color and tone correction block details

   Many different approaches have been developed to acquire trichromatic color images.
Some cameras scan a scene with a trilinear sensor incorporating color filters to capture
three channels of color information for every pixel in the scene [51], [52].  These cameras
can achieve very high spatial resolution and excellent color quality.  Because the exposure
of each scan line of the scene is sequential, the exposure time for each line must be a small
fraction of the capture time for the whole scene. These cameras often take seconds or even
minutes to acquire a full scene,  so they work poorly for recording scenes that have any
motion or variation in lighting.  Some cameras use a single panchromatic area sensor and
filters in the lens system to sequentially capture three color channels for every pixel in the
scene. These are faster than the linear scanning cameras, since all pixels in a color channel
are  exposed  simultaneously,  although  the  channels  are  exposed sequentially.            

These  are commonly used and particularly effective in astronomy and other scientific applications
where motion is not a factor.         Some cameras use dichroic beam splitters and three area
sensors to simultaneously capture three channels of color for each pixel in the scene [53],
[54].  This is particularly common and successful in high-quality video cameras, where a
high pixel rate makes the processing for demosaicking difficult to implement well.  These
are the fastest cameras, since all color channels are exposed simultaneously. Each of these
approaches can perform well, but they increase the cost and complexity of the camera and
restrict the range of camera operation.


Cost and complexity issues drive consumer digital color cameras in the direction of a
single sensor that captures all color information simultaneously.                Two approaches based
on single array sensors are presently used.           One approach fabricates a sensor with three
layers of photodiodes and uses the wavelength-dependent depth of photon absorption to
provide spectral sensitivity [55], [56]. This system allows sampling three channels of color
information at every pixel,  although the spectral sensitivity poses several challenges for
image processing. The approach more commonly used is the fusion of multiple color chan-
nels from a sensor with a color filter array into a full-color image.  This fusion approach,
specifically with a four-channel color filter array, is the focus of this chapter.

  A camera embodying this approach includes a single lens, a single area array sensor with
a color filter array, and a processing path to convert pixel values read from the sensor to
a suitable image for color reproduction.          The image processing chain used is shown as a
block diagram in Figure 1.4.  The block labeled as color and tone processing is examined
in more detail here, as shown in Figure 1.5.  This figure breaks the overall color and tone
processing down into three distinct steps.  The first, white balance, applies gain factors to
each color channel of camera pixel values to provide an image with equal mean code values
in each color channel for neutral scene content. The second, color correction, converts the
white balanced image to a known set of color primaries,  such as the primaries used for
sRGB. The final step applies a tone correction to convert the image to a rendered image.