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csc2700lect12 - A Short Introduction to Computer Graphics...

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A Short Introduction to Computer Graphics Frédo Durand MIT Laboratory for Computer Science
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Chapter I: Basics 1 Introduction Although computer graphics is a vast field that encompasses almost any graphical aspect, we are mainly interested in the generation of images of 3-dimensional scenes. Computer imagery has applications for film special effects, simulation and training, games, medical imagery, flying logos, etc. Computer graphics relies on an internal model of the scene, that is, a mathematical representation suitable for graphical computations (see Chapter II). The model describes the 3D shapes, layout and materials of the scene. This 3D representation then has to be projected to compute a 2D image from a given viewpoint, this is the rendering step (see Chapter III). Rendering involves projecting the objects (perspective), handling visibility (which parts of objects are hidden) and computing their appearance and lighting interactions. Finally, for animated sequence, the motion of objects has to be specified. We will not discuss animation in this document. 2 Pixels A computer image is usually represented as a discrete grid of picture elements a.k.a. pixels . The number of pixels determines the resolution of the image. Typical resolutions range from 320*200 to 2000*1500. For a black and white image, a number describes the intensity of each pixel. It can be expressed between 0.0 (black) and 1.0 (white). However, for internal binary representation reasons, it is usually stored as an integer between 0 (black) and 255 (white) A low-resolution digital image. Left: Black and white. Right: Color. (Image © Pixar). For a color image, each pixel is described by a triple of numbers representing the intensity of red, green and blue. For example, pure red is (255, 0, 0) and purple is (255, 0, 255). Because the image is represented by a discrete array of pixels, aliasing problems may occur. The most classical form of aliasing is the jaggy aspect of lines (see figure below). Antialiasing techniques are thus required. In the case of the line, it consists in using intermediate gray levels to “smooth” the appearance of the line. Another form of aliasing can be observed on television when people wear shirts with a fine
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