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Lecture Notes CMSC 427/828M

CMSC 427/828M: Computer Graphics1

Spring 1997 Dave Mount

Lecture 1: Course Introduction (Thursday, Jan 30, 1997) Review of syllabus. No lecture today.

Lecture 2: Graphics Systems and Models (Tuesday, Feb 4, 1997) Read: Chapter 1 in Angel.

Computer Graphics: Computer graphics is concerned with producing images using a computer.

The task of producing photo-realistic images is an extremely complex one, but this is a field that is in great demand because of the nearly limitless variety of applications.

Our approach to teaching computer graphics will be based on a top-down approach. In a traditional bottom-up approach we would begin by discussing primitive 2-dimensional graphics from the level of illuminating individual pixels on a raster graphics devices, and work up successively to complex 3-dimensional graphics. Instead a top-down approach begins by asking the question of how to model 3-dimensional graphics at a very high level, and then later considering what sorts of low-level tools will be needed to implement this model.

In this first lecture, we begin with the basics of 3-dimensional graphics: graphics display systems and basic models for 3-d graphics.

Graphics Devices: The standard interactive graphics device today is called a raster display. As

with a television, the display consists of a two-dimensional array of dots, called pixels. The screen has a phosphor coating, that allows each pixel to be illuminated momentarily when struck by an electron beam. A pixel is either illuminated (white) or not (black). The level of intensity can be varied to achieve arbitrary gray values.

In color raster devices each pixel is a composite of (typically) three components: red, green, and blue, that are combined in varying proportions to achieve different colors (and there are three electron beams). The number of bits used for each pixel is called the depth of the display. Simple color graphics systems, like the ones we will be using will have typically 8-bit depth. Higher-end graphics systems will have at least 24-bit depth, and often much more. 8-bit depth is quite limiting, since it implies there are only 256 different colors that can be displayed at any one time. We will discuss how it is that these systems still manage to produce realistic looking images.

The image is stored as a two-dimensional array of pixel values (called a frame buffer), and the display hardware produces the image line-by-line (called raster lines). For each pixel on each line the intensity of the pixel is varied by varying the intensity of the electron beam as it scans across the line. Since the phosphor only holds its color for a short period of time, the display must be continuously updated (from 30 to 60 times a second) to avoid a "flickering" affect.

1Copyright, David M. Mount, 1997, Dept. of Computer Science, University of Maryland, College Park, MD, 20742. These lecture notes were prepared by David Mount for the course CMSC 427/828M, Computer Graphics, at the University of Maryland, College Park. Permission to use, copy, modify, and distribute these notes for educational purposes and without fee is hereby granted, provided that this copyright notice appear in all copies.

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Lecture Notes CMSC 427/828M

A hardware device called a video controller reads the frame buffer and produces the image on the display. An example of this type of configuration is shown below.

CPU

Display Processor