Image Formation and Digitization
Image acquisition is the first step of digital image processing and is often not properly taken into account. However, quantitative analysis of any images requires a good understanding of the image formation process. Only with a profound knowledge of all
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Image acquisition is the first step of digital image processing and is often not properly taken into account. However, quantitative analysis of any images requires a good understanding of the image formation process. Only with a profound knowledge of all the steps involved in image acquisition, is it possible to interpret the contents of an image correctly. The steps necessary for an object in the three-dimensional world to become a digital image in the memory of a computer are as follows: • Becoming visible. An object becomes visible by the interaction with light or, more generally, electromagnetic radiation. The four basic types of interaction are reflection, refraction, absorption, and scattering. These effects depend on the optical properties of the material from which the object is made and on its surface structure. The light collected by a camera system is determined by these optical properties as well as by the illumination, i.e., position and nature of the light or, more generally, radiation sources. • Projection. An optical system collects the light rays reflected from the objects and projects the three-dimensional world onto a two-dimensional image plane. • Digitization. The continuous image on the image plane must be converted into image points on a discrete grid. Furthermore, the intensity at each point must be represented by a suitable finite number of gray values (Quantization). These steps will be discussed in the following three sections. Quantization is the topic of section 4.2.2.
2.1 2.1.1
Interaction between Light and Matter Introduction
The interaction between matter and radiation is the basis for all imaging. This is more a topic of physics rather than image processing. Knowledge about this subject, however, is very useful, especially in scientific and industrial applications, where we have control on how we set up our imaging system. An approach which integrates the optical setup and the processing of the resulting images is required in order to obtain the best and most cost effective solution. In other words, if we make a serious error in the imaging
B. Jähne, Digital Image Processing © Springer-Verlag Berlin Heidelberg 1993
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2 Image Formation and Digitization a)
Light source
Camera
Camera
Figure 2.1: a) Sketch of the interaction between illumination and objects; a) objects with impermeable surfaces; b) more general arrangement showing reflection, absorption, scattering, and refraction of light from the light source to the object, the object of interest itself, and from the object back to the camera.
system, processing of the images may be costly and slow or, even worse, it might not be possible at all to correct for the resulting degradations. Applications in image processing are so widespread that a complete discussion of this topic is not possible here. We should, however, be aware of some basic facts that enable us to consider the illumination arrangement in our application properly. Interaction between illumination and the observed scene has received much attention in compu
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