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Unformatted text preview: Purdue University: ECE438 - Digital Signal Processing with Applications 1 ECE438 - Laboratory 10: Image Processing (Week 2) October 6, 2010 1 Introduction This is the second part of a two week experiment in image processing. In the first week , we covered the fundamentals of digital monochrome images, intensity histograms, pointwise transformations, gamma correction, and image enhancement based on filtering. During this week, we will cover some fundamental concepts of color images. This will include a brief description on how humans perceive color, followed by descriptions of two standard color spaces . We will also discuss an application known as halftoning , which is the process of converting a gray scale image into a binary image. 2 Color Images 2.1 Background on Color Color is a perceptual phenomenon related to the human response to different wavelengths of light, mainly in the region of 400 to 700 nanometers (nm). The perception of color arises from the sensitivities of three types of neurochemical sensors in the retina, known as the long (L), medium (M), and short (S) cones . The response of these sensors to photons is shown in Figure 1. Note that each sensor responds to a range of wavelengths. Due to this property of the human visual system, all colors can be modeled as combi- nations of the three primary color components: red (R), green (G), and blue (B). For the purpose of standardization, the CIE (Commission International de l’Eclairage — the Inter- national Commission on Illumination) designated the following wavelength values for the three primary colors: blue = 435 . 8 nm , green = 546 . 1 nm , and red = 700 nm . The relative amounts of the three primary colors of light required to produce a color of a given wavelength are called tristimulus values . Figure 2 shows the plot of tristimulus values using the CIE primary colors. Notice that some of the tristimulus values are negative , which indicates that colors at those wavelengths cannot be reproduced by the CIE primary colors. Questions or comments concerning this laboratory should be directed to Prof. Charles A. Bouman, School of Electrical and Computer Engineering, Purdue University, West Lafayette IN 47907; (765) 494- 0340; [email protected] Purdue University: ECE438 - Digital Signal Processing with Applications 2 400 450 500 550 600 650 700 0.2 0.4 0.6 0.8 1 1.2 wavelength (nm) relative sensitivity L M S Figure 1: Relative photon sensitivity of long (L), medium (M), and short (S) cones. 400 450 500 550 600 650 700-1-0.5 0.5 1 1.5 2 2.5 wavelength (nm) tristimulus values r( λ ) g( λ ) b( λ ) Figure 2: Plot of tristimulus values using CIE primary colors. 2.2 Color Spaces A color space allows us to represent all the colors perceived by human beings. We previously noted that weighted combinations of stimuli at three wavelengths are sufficient to describe all the colors we perceive. These wavelengths form a natural basis, or coordinate system, from which the color measurement process can be described. In this lab, we willsystem, from which the color measurement process can be described....
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