1

Computer GraphicsComputer Graphics

Chapter 1: Introduction to Graphics

2

Computer graphics History

• Computer graphics generally means creation, storage and manipulation of models and images

• Such models come from diverse and expanding set of fields including physical, mathematical, artistic, biological, and even conceptual (abstract) structures

3

Frame from animation by William Latham, shown at SIGGRAPH 1992. Latham uses rules that govern patterns of natural forms to create his artwork.

Graphics Library (GL)

• Examples: OpenGL™, DirectX™, Windows Presentation Foundation™ (WPF), RenderMan™

• Primitives (chars, lines, polygons, meshes,…)• Attributes

• color• line style• material properties for 3D

• Lights• Transformations• Immediate mode vs. retained mode

• immediate mode: no stored representation, package holds only attribute state, and application must completely draw each frame

• retained mode: library compiles and displays from scene graph that it maintains, a complex DAG. It is a display-centered extract of the Application Model

4

What is computer graphics

5

Some research fields of computer graphics

6

Basic notions

• Computer graphics – process of producing a picture or image using the computer

• Computer interfaces popular on personal computers• Desktop publishing • Realistic images generated using mathematical and physical methods

7

Steps for creating a picture

• Creating a model • Perform necessary transformation• Lighting and rendering the object• The goal is the creation of an image by writing a program instead of

taking a picture with a camera• There exists an analogy between writing graphical programs and

taking pictures by a camera

8

Pixels

• Pixel or “Picture element” is the simplest element in computer graphics

• Single location on the computer screen or printout• Value of each pixel is the range from white to black or range of

intensities of red, green, blue (RGB) colors.

9

Frame buffers

• The pixels of an image are organized into two dimensional grid – frame buffer

• Multiple frame buffers can be stored in computer memory• Double buffering – first image is drawn into frame buffer and sent to

display. While the user is looking on the display, the next picture is drawing to the second buffer.

10

Windows

• Image don’t fill the entire screen but is drawn into a window• Pixels are adressed within the window based on their location• Relative position of each pixel enables moving the window and

change its size

11

Realistic images

• Calculating pixel values to create impression of a realistic picture• Simulation of objects from the real word• Approximation of physical properties of objects• Limitations given by computing time and memory space

12

The Graphics Pipeline

• Modeling – creates an internal representation of the objects in the scene

• Rendering – converts the screen description into image• Display – shows the image on the output device

13

The Graphics Pipeline

14

Object Specification

Model

TransformationScene Description

Scene Description Model

TransformationView and Light Specification

Clipping and Hidden Surface Removal

Shading Image

The Graphics Pipeline

15

Display TransformationImage Output

Applications of Computer Graphics

1. Display of information2. Design3. Simulation and animation4. User interfaces

16

• Display of information• Maps • GIS (geographic information system)• CT (computer tomography)• MRI (magnetic resonance imaging) • PET (positron-emission tomography)• Fluid flow, molecular biology, mathematics…

17

Display of Information

18

Applications of Computer Graphics.

• Design• CAD (computer-aided design):

VLSI (very-large-scale integrated) circuits• Together with other tools:

architecture or interior design

19

Interior Design

20

Applications of Computer Graphics

• Simulation and animation• Flight simulation – pilot training• Games and educational software• Benefits:

• Less cost• Less danger, e.g. combination with the VR (virtual reality) techniques can help surgical

interns and astronauts

21

Applications of Computer Graphics

• User interfaces• Friendly working environment:

windows, icons, menus, pointing devices

22

Interface for a painting program

Application Programmer’s Interface

23Specifications of the functions in the graphics library

•The OpenGL graphics system is an Application Programming Interface (API) to graphics hardware.

Three-dimensional APIs

• Objects• Viewers• Light sources• Material properties

24

Camera(Viewer) Specifications

• Position (COP)• Orientation• Focal length• Film plane

25

Sequence of Images

• Wireframe• Flat shading

• HSR (Hidden surface removal)

• Smooth shading• Curves and surfaces

• NURBS, Bezier curves/surfaces

• Texture mapping• Bump mapping, environmental maps, antialiasing…

26

Pixels

27

A Graphics System

28

Frame Buffer

• High-end systems: VRAM or DRAM• Simpler systems: part of memory• Depth: the number of bits per pixel• True color: depth=24• Resolution: the number of pixels in the frame buffer

29

Rasterization or Scan-conversion

• Conversion of geometric entities to pixels in the frame buffer

• High-end systems• Special-purpose processors

• Simpler systems• A single and shared processor

30

Output Devices

31CRT (Cathode-ray tube)

Output Devices

• CRT• Refresh: at least 50 times per second• Interlace and non-interlace systems• Color CRTs have three colored phosphors and a shadow mask

• Other raster devices:• LCD (liquid-crystal displays)• Plasma panels and digital projection systems• Non-refreshable: printers and plotters

32

Shadow-mask CRT

33

Input Devices

• Mouse

34

Joystick Data tablet Anything else?

Hand Foot Voice Mind?

Images: Physical and Synthetic

• Image formation• Lighting• Shading• Properties of materials

35

Objects and Viewers

• Object: formed from geometric primitives• Points, lines, polygons• Vertex (pl. Vertices) is the most primitive one

• Viewer: • Locations• Viewing angles

36

Objects and Viewers

37

Objects and Viewers

38

3D world 2D image

Light and Images

39

40

Synthetic-camera Model

Film Plane

Projection Plane

Single Point Light Source

41

Ray Tracing

42

Penetrating transparent surfaces

Reflected by

Mirrors

Diffuse surfaces

Refracted

Absorbed

Human Visual System

43

Visual system does not have the same response to each color.We are most sensitive to green light

Pinhole Camera

44

Pinhole Camera

45

dz

xx

dz

yy

p

p

/

/

d

h

2tan2 1

(xp, yp, -d) is the projection of (x, y, z)

Synthetic-camera Model

46

Bellows Camera Projector

Synthetic-camera Model

47

COP(Center of Projection)

Focal Length

Synthetic-camera Model

48

Synthetic-camera Model

49

Clipping Window

Sequence of Images

• Wireframe• Flat shading

• HSR (Hidden surface removal)

• Smooth shading• Curves and surfaces

• NURBS, Bezier curves/surfaces

• Texture mapping• Bump mapping, environmental maps, antialiasing…

50

Wireframe

51

Flat Shading

52

Smooth Shading

53

Modeling With Curves/surfaces

54

Bump Mapping

55

Environmental Maps

56

Antialiasing

57

Modeling-rendering Paradigm

58

Example: Scene graph

Graphics Architecture

59

Early graphics system

Computeline segments

Drawline segments

Very high rate to avoid flickering

Graphics Architecture

60

Display-processor architecture

Graphics Architecture

61

Arithmetic pipeline: doubling the throughput!

Pipeline Architecture: Geometric pipeline

Pipelining

62

Arithmetic pipeline: doubling the throughput!

Pipeline Architecture: Geometric pipeline

Geometric Pipeline

• Transformation• Conversion between coordinate systems• Translation, rotation, scaling• Aggregate transforms by matrix multiplications

• Clipping• Could be further pipelined

63

Geometric Pipeline

• Projection• Remaining 3D objects are projected into 2D objects• Parallel or perspective projections

• Rasterization• Convert 2D objects into pixels

64

Performance Characteristics

• Latency• Throughput:

• How fast we can move geometric entities through the pipeline• How many pixels per second we can alter in the frame buffer

• Pipeline architecture is not a must• Ray tracing or radiosity for better quality

65

Summary & Notes of Lecture 1

• Application of computer graphics• A graphics system• Human visual system• Pinhole and synthetic camera models• Image formation• Geometric pipeline• Realistic images may require resolution of up to 40006000

66