Bài giảng Computer graphics and virtual reality - Lesson 1: Kỹ thuật đồ họa và Hiện thực ảo

erty
G U I
MODELING RENDERING DISPLAYING
what is 
a table, a car, •
• •
( to describe) 
to the computer
Geometric Engine
(to capture)
the description
create 2D image 
from 2D / 3D 
models
Rendering Engine
generate 
image on 
screen
(to show)
the image
Raster & Display Engine
concerned with :
- viewing & projection
- drawing & clipping 
primitives
- local illumination & 
shading
- texture mapping
Thành phần trong chức năng của 
kỹ thuật đồ hoạ
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3D Graphics Over World Wide Web
SRGP
library
Pascal / C
program
X Window 
System Graphics hardware
Image
image formats, compression, transfer
graphics algorithms
colour
Drawing
packages
transformation
of objects
3D Graphics
projections
lighting,shading
lines,areas,...positions
Video
WWW
Animation
WWW
VRML
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Các chuẩn giao diện của hệ đồ hoạ
z GKS (Graphics Kernel System): chuẩn xác định các hàm đồ hoạ chuẩn, được
thiết kế như một tập hợp các công cụ đồ hoạ hai chiều và ba chiều.
– GKS Functional Description, ANSI X3.124 - 1985.
– GKS - 3D Functional Description, ISO Doc #8805:1988.
z CGI (Computer Graphics Interface System): hệ chuẩn cho các phương pháp
giao tiếp với các thiết bị ngoại vi.
z CGM (Computer Graphics Metafile): xác định các chuẩn cho việc lưu trữ và
chuyển đổi hình ảnh.
z VRML (Virtual Reality Modeling Language): ngôn ngữ thực tại ảo, một hướng
phát triển trong công nghệ hiển thị được đề xuất bởi hãng Silicon Graphics, 
sau đó đã được chuẩn hóa như một chuẩn công nghiệp.
z PHIGS (Programmers Hierarchical Interactive Graphics Standard): xác định 
các phương pháp chuẩn cho các mô hình thời gian thực và lập trình hướng 
đối tượng.
– PHIGS Functional Description, ANSI X3.144 - 1985.
– PHIGS+ Functional Description, 1988, 1992.
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Non-official industry standards
Các chuẩn của hệ đồ hoạ
z OPENGL thư viện đồ họa của hãng Silicon 
Graphics, được xây dựng theo đúng chuẩn của một 
hệ đồ họa.
– SGI’s OpenGL 1993
z DIRECTX thư viện đồ hoạ của hãng Microsoft
– Direct X/Direct3D 1997
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OpenGL
z Software interface to graphics 
hardware
z Client-server model
z 250 distinct commands
z Object specification + image 
generation
z Simple primitives: points, lines, 
polygons (pixels, images, bitmaps)
z 3D rendering
z Commands interpreted using 
client-server model
– Client (Application) issues 
commands
– Server (OpenGL) interprets 
and processes commands
– Frame buffer configuration 
done by the window system
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z Window tasks
z User input
z Complicated shapes
– OpenGL Utility Library (GLU)
– Window system support libraries
z GLX / WGL / PGL
– OpenGL Utility Toolkit (GLUT)
– OpenInventor
For portability, there are
no commands for these.
OpenGL-related Libraries
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z Direct control of graphics hardware
z Direct control of input/output devices, and sound
Application programApplication progra
Windows systemindows syste
Direct sound
Direct draw
Direct 3D
Direct input
Windows APIindows API
Direct X
..
DirectX
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OpenGL Design Goals
z SGI’s design goals for OpenGL:
– High-performance (hardware-accelerated) graphics API
– Some hardware independence 
– Natural, terse API with some built-in extensibility
z OpenGL has become a standard (competing with DirectX) because:
– It doesn’t try to do too much
z Only renders the image, doesn’t manage windows, etc.
z No high-level animation, modeling, sound (!), etc.
– It does enough
z Useful rendering effects + high performance
– Open source and promoted by SGI (& Microsoft, half-heartedly)
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49
Màn hình CRT
Raster Displays (early 70s)
like television, scan all pixels in regular pattern
use frame buffer (video RAM) to eliminate sync 
problems RAM
¼ MB (256 KB) cost $2 million in 1971
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Màn hình CRT
SONY Trinitron CRT
NEC Hybrid Mask Hitachi EDP
Standard Dot-trio
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Display Technology: 
Raster CRTs
z Raster CRT pros:
– Allows solids, not just wireframes
– Leverages low-cost CRT technology (i.e., TVs)
– Bright! Display emits light
z Cons:
– Requires screen-size memory array
– Discreet sampling (pixels)
– Practical limit on size (call it 40 inches)
– Bulky
– Finicky (convergence, warp, etc)
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Các thiết bị hiển thị dạng điểm
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CRT Displays
Advantages
z Fast repsonse (high resolution 
possible)
z Full color (large modulation 
depth of E-beam)
z Saturated and natural colors
z Inexpensive, matured 
technology
z Wide angle, high contrast and 
brightness
Disadvantages
z Large and heavy (typ. 70x70 cm, 
15 kg)
z High power consumption (typ. 
140W)
z Harmful DC and AC electric and 
magnetic fields
z Flickering at 50-80 Hz (no memory 
effect)
z Geometrical errors at edges
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LCD-Liquid Crystal Display
z A transmissive technology
z Works by letting varying amounts 
of a fixed-intensity white backlight 
through an active filter
z Organnic crystals that lign
themselves together
z When external force is applied 
they realign themselves
z This is used to change 
polarisation and filter light
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Display Technology: LCDs
z Transmissive & reflective 
LCDs:
– LCDs act as light valves, not 
light emitters, and thus rely on 
an external light source.
– Laptop screen: backlit, 
transmissive display
– Palm Pilot/Game Boy: 
reflective display
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LCD Displays
Advantages
z Small footprint (approx 1/6 of CRT)
z Light weight (typ. 1/5 of CRT)
z Low power consumption (typ. 1/4 of 
CRT)
z Completely flat screen - no 
geometrical errors
z Crisp pictures - digital and uniform 
colors
z No electromagnetic emission
z Fully digital signal processing possible
z Large screens (>20 inch) on desktops
Disadvantages
z High price (presently 3x CRT)
z Poor viewing angle (typ. +/- 50 
degrees)
z Low contrast and luminance (typ. 
1:100)
z Low luminance (typ. 200 cd/m2)
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Màn hình Plasma 
z Plasma display panels
– Similar in principle to 
fluorescent light tubes
– Small gas-filled capsules 
are excited by electric field,
emits UV light
– UV excites phosphor
– Phosphor relaxes, emits 
some other color
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Display Technology
z Plasma Display Panel 
Pros
– Large viewing angle
– Good for large-format 
displays
– Fairly bright
z Cons
– Expensive
– Large pixels (~1 mm versus 
~0.2 mm)
– Phosphors gradually 
deplete
– Less bright than CRTs, 
using more power
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Display Technology: DMDs
z Digital Micromirror Devices (projectors)
– Microelectromechanical (MEM) devices, fabricated 
with VLSI techniques
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Display Technology: DMDs
z DMDs are truly digital pixels
z Vary grey levels by modulating pulse length
z Color: multiple chips, or color-wheel
z Great resolution
z Very bright
z Flicker problems
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Màn hình hữu cơ
Organic LED Arrays
z Organic Light-Emitting Diode 
(OLED) Arrays
– The display of the future? 
– OLEDs hoạt động tương tự cơ chế 
của LEDs bán dẫn
z Cấu trúc là màng chất dẻo mỏng:
– Màng film cấu tạo bởi các phần tử 
hữu cơ, các phân tử phát sáng bởi 
sự thăng hoa khí trong môi trường 
chân không
– Mầu sắc được tạo thành từ các lớp 
mầu gồm các phân tử huỳnh quang 
được kích thích.
– Mịn, không to như các hạt tinh thể 
và không phát nhiệt
– Có thể tạo màn hình rộng loại
OLEDs
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Display Technologies: 
Organic LED Arrays
z OLED pros:
– Transparent
– Flexible
– Light-emitting, and quite 
bright (daylight visible)
– Large viewing angle
– Fast (< 1 microsecond off-
on-off)
– Can be made large or small
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Display Technologies: 
Organic LED Arrays
z OLED cons:
– Not quite there yet (96x64 displays) except niche markets
z Cell phones (especially back display)
z Car stereos
– Not very robust, display lifetime a key issue
– Currently only passive matrix displays
z Passive matrix: Pixels are illuminated in scanline order (like a raster 
display), but the lack of phosphorescence causes flicker
z Active matrix: A polysilicate layer provides thin film transistors at each 
pixel, allowing direct pixel access and constant illumination
See  for more info
– Hard to compete with LCDs, a moving target
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DAC
Direct Color Framebuffer
z Store the actual intensities of R, G, and B individually in the 
framebuffer
z 24 bits per pixel = 8 bits red, 8 bits green, 8 bits blue
– 16 bits per pixel = ? bits red, ? bits green, ? bits blue
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Color Lookup Framebuffer
z Store indices (usually 8 bits) in framebuffer
z Display controller looks up the R,G,B values before triggering 
the electron guns
Frame Buffer
DAC
Pixel color = 14
Color Lookup
Table
0
1024
14
R G B
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Framebuffers: True-Color 
z A true-color ( 24-bit or 32-bit) framebuffer stores one byte 
each for red, green, and blue
z Each pixel can thus be one of 224 colors
z Pay attention to
Endian-ness
z How can 24-bit 
and 32-bit mean 
the same thing 
here?
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Framebuffers: Indexed-Color
z An indexed-color (8-bit or PseudoColor) framebuffer stores one byte per 
pixel (also: GIF image format)
z This byte indexes into a color map: 
z How many colors
can a pixel be?
z Still common on 
low-end displays 
(cell phones, PDAs,
GameBoys)
z Cute trick: 
color-map animation
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Framebuffers: Hi-Color
z Hi-Color was a popular PC SVGA standard
z Packs pixels into 16 bits:
– 5 Red, 6 Green, 5 Blue 
– Sometimes just 5,5,5
z Each pixel can be one of 216 colors
z Hi-color images can exhibit worse quantization artifacts 
than a well-mapped 8-bit image
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– X : 0 ÷ Xmax 2 màu/ 1 bit
– Y : 0 ÷ Ymax 16 màu/ 4 bit 
– 256 màu/ 8bit
– 216 màu/ 16 bit
– 224 màu/ 24 bit
– 640 × 480 × 16 → Video RAM = 2MB
– 1024 × 1024 × 24 → Video RAM = 24MB