Image Processing Reference
In-Depth Information
Fig. 2.11
Three different camera settings resulting in three different depth-of-fields
To sum up, the following interconnected issues must be considered: distance to
object, motion of object, zoom, focus, depth-of-field, focal length, shutter, aperture,
and sensor. In Figs.
2.11
and
2.12
some of these issues are illustrated. With this
knowledge you might be able to appreciate why a professional photographer can
capture better images than you can!
2.3
The Image Sensor
The light reflected from the object of interest is focused by some optics and now
needs to be recorded by the camera. For this purpose an image sensor is used. An
image sensor consists of a 2D array of cells as seen in Fig.
2.13
. Each of these
cells is denoted a
pixel
and is capable of measuring the amount of incident light and
convert that into a voltage, which in turn is converted into a digital number.
The more incident light the higher the voltage and the higher the digital number.
Before a camera can capture an image, all cells are emptied, meaning that no charge
is present. When the camera is to capture an image, light is allowed to enter and
charges start accumulating in each cell. After a certain amount of time, known as the
exposure time
, and controlled by the
shutter
, the incident light is shut out again. If
the exposure time is too low or too high the result is an underexposed or overexposed
image, respectively, see Fig.
2.14
.
Many cameras have a built-in intelligent system that tries to ensure the image
is not over- or underexposed. This is done by measuring the amount of incoming
light and if too low/high correct the image accordingly, either by changing the ex-
posure time or more often by an
automatic gain control
. While the former improves
the image by changing the camera settings, the latter is rather a post-processing step.
Both can provide more pleasing video for the human eye to watch, but for automatic
video analysis you are very often better off disabling such features. This might sound
counter intuitive, but since automatic video/image processing is all about manipu-
lating the incoming light, we need to understand and be able to foresee incoming
light in different situations and this can be hard if the camera interferes beyond our
control and understanding. This might be easier understood after reading the next
chapter. The point is that when choosing a camera you need to remember to check
if the automatic gain control is mandatory or if it can be disabled. Go for a cam-
era where it can be disabled. It should of course be added that if you capture video
