The human visual system is organized in a modular manner. This makes it 
possible to analyze the different modules relatively independently from 
others. The human stereopsis, for example, could be studied and 
formulated in relative isolation from other modules, like distinguishing 
surface orientation from texture, or shape from shading. The modules are 
normally grouped into the three processes or levels of early, intermediate 
and late vision. Early vision involves digital image processing tasks which 
are performed relatively independently and concurrently on different 
regions of the image. The result is a primal sketch which abstracts away 
from the raw image consisting primarily of the geometrical distribution 
and orientation of abrupt changes in the intensity function. Stereopsis is 
one of several modules of intermediate vision. In order to find 
corresponding points, information from late vision such as the structure 
of objects, is not needed. Matching is not performed on the level of the raw 
image, rather on the level of the primal sketch. Primal sketches abstract 
away from the raw images and are already closer to meaning something. 
The original image has a lot of redundant information. Physical and 
geometrical properties of neighboring patches on a surface in the ob ject 
Space normally do not vary a great deal, except along discontinuities. Thus, 
much of the information of the pixels which Correspond to these patches is 
redundant. The primal sketches can be thought of as a representation of 
the raw image consisting of relevant information only, primarily the 
geometrical distribution and orientation of intensity changes. Abrupt 
intensity changes are found by convolving the raw image with the 
Laplacian of a Gaussian operator, V G(x,y). The operator combines in one 
Step the smoothing of the Image by taking the second derivative of the 
intensity function. Where the second derivative crosses zero, the first 
derivation attains its highest absolute values, and this is where the gray 
levels change fastest. In the human visual system the zero crossings are 
used for the matching process. Since the ZEro crossings often are identical 
with the physical boundaries in the ob ject, the method of matching zero 
Crossings is often referred to as feature matching. 
Interestingly enough the human visual System convolves the raw image as 
perceived on the retina in the same manner. Ganglion cells in the visual 
cortex integrate light Intensity from photoreceptors on different 
locations of the retina. Two types of ganglion cells make it possible to 
transmit positive as well as negative values. In order to detect coarse and 
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