Computer Graphics Hardware

Computer Graphics Hardware
Computer Graphics
Graphics Hardware
Display Devices
– Vector Scan
• Image stored as line segments (vectors) that can be
drawn anywhere on display device
– Raster Scan
• Image stored as a 2D array of color values in a memory
area called the frame buffer
• Each value stored determines the color/intensity of an
accessible point on display device
Both based historically on CRT (TV)
– Electron beam accelerated toward screen
• focused
• deflected
• strikes phosphorescent material on screen
-->pixel that glows
A Pixel
? Visible
point where electron beam hits
? Screen phosphors glow & fade
? Have a finite size
– Not a mathematical point
Maximum number of pixels that can be plotted
without overlap
? Expressed as: # horizontal X # vertical pixels
? Depends on:
phosphor used
focusing system (how small a point)
Speed/precision of deflection system
video memory size (raster scan)--as we'll see
Aspect Ratio
Ratio of # of pixel columns to # of pixel rows
? Examples:
– SVGA VESA mode 100h: 640 X 400, A.R. = 1.6
– Standard Windows: 1280X1024. A.R. = 1.25
Pixel Ratio (often called Aspect Ratio)
– Ratio of pixel height to pixel width
– Ratio of # of horizontal pixels to vertical pixels
needed to produce equal length lines
– For a square screen, A.R. = P.R.
– If Pixel Ratio != 1, figures are distorted
Dot Pitch
Minimum distance between centers of
adjacent pixels of same color
? Should be less than 0.28 mm for sharp
? For fixed sized screen
– Decreasing distance between pixels ==>
Increase Resolution
– So dot pitch determines max resolution
After beam leaves a phosphor, it fades
? Definition of persistence:
– Time to reduce initial intensity to 10% of original value
– Value depends on type of phosphor (10 - 100 msec.)
Finite persistence==>screen must be redrawn
– Refresh rate determined by persistence
Example: If persistence = 20 msec
– 1st pixel on screen invisible after that time ==>
• screen must be refreshed once every 20 msec
• so refresh rate must be > 50 Hz.
? If
refresh is too slow: flicker
? If refresh is too fast: shadowing
Graphics Hardware Systems
CPU--Runs program in main memory
– specifies what is to be drawn
CRT--does the actual display
Display Controller--Provides analog voltages needed to
move beam and vary its intensity
DPU—generates digital signals that drive display controller
– (offloads task of video control to separate processor)
VRAM--Stores data needed to draw the picture
– Dual-ported (written to by CPU, read from by DPU)
– Fast (e.g., 1000X1000, 50 Hz ==> 20 nsec access time!)
– Also called Refresh Buffer or Frame Buffer
I/O devices--interface CPU with user
Flat-Panel Displays
? Technologies
to replace CRT monitors
? Reduced volume, weight, power needs
– Thinner: can hang on a wall
– Higher resolution (High Definition)
? Two
– Emissive and non-emissive
Flat Panel Displays: Emissive
– Convert electrical energy to light
– Plasma panels (gas-discharge displays)
• Voltages fired to intersecting vertical/horizontal
conductors cause gas to glow at that pixel
• Resolution determined by density of conductors
• Pixel selected by x-y coordinates of conductors
• These are “raster” devices
– Other technologies
• All require storage of x-y coordinates of pixels
• Examples:
– Thin-film electroluminescent dipslays
– LEDs
– Flat CRTs
Flat Panel Displays:
Non-emissive Devices
– Use optical effects to convert ambient light to
pixel patterns
– Example: LCDs
• Pass polarized light from surroundings through liquid
crystal material that can be aligned to block or transmit
the light
• Voltage applied to 2 intersecting conductors
determines whether the liquid crystal blocks or
transmits the light
– Like emissive devices, require storage of x-y
coordinates of pixel to be illuminated
Vector Scan Systems
Also called random, stroke, calligraphic displays
? Images drawn as line segments (vectors)
? Beam can be moved to any position on screen
? Refresh Buffer stores plotting commands
– So Frame Buffer often called "Display File”
– provides DPU with needed endpoint
– Pixel size independent of frame buffer
• ==> very high resolution
Advantages of Vector Scan
? High
resolution (good for detailed line
? Crisp lines (no "jaggies")
? High contrast (beam can dwell on a single
point for some time ==> very bright)
? Selective erase (remove commands from
display file)
? Animation (change line endpoints slightly
after each refresh)
Disadvantages of Vector Scan
Complex drawings can have flicker
– Many lines
• so if time to draw > refresh time ==> flicker
– High cost--very fast deflection system needed
– Hard to get colors
– No area fill
• so it’s difficult to use for realistic (shaded) images
– 1960s Technology, only used for special purpose stuff
Raster Scan Systems
(TV Technology)
Beam continually traces a raster pattern
? Intensity adjusted as raster scan takes place
In synchronization with beam
Beam focuses on each pixel
Each pixel’s intensity is stored in frame buffer
So resolution determined by size of frame buffer
Each pixel on screen visited during each scan
– Scan rate must be >= 30 Hz to avoid flicker
Simplest system: one bit per pixel
– frame buffer called a bitmap
Gray Scale: N bits/pixel
– 2^N intensities possible
– memory intensive
• Example:1000 X 1000 X 256 shades of gray
==> 8 Mbits
Scan Conversion
? Process
of determining which pixels need
to be turned on in the frame buffer to draw
a given graphics primitive
? Need
algorithms to efficiently scan convert
graphics primitives like lines, circles, etc.
Advantages of Raster Scan
Low cost (TV technology)
? Area fill (entire screen painted on each scan)
? Colors
? Selective erase (just change contents of
frame buffer)
? Bright display, good contrast
– but not as good as vector scan can be:
– can’t make beam dwell on a pixel
? Large
memory requirement for high
– (but cost of VRAM has decreased a lot!)
? Aliasing
(due to finite size of frame buffer)
– Finite pixel size
– Jagged lines (staircase effect)
– Moire patterns, scintillation, "creep" in
? Raster
scan is the principal “now”
technology for graphics displays!
Tektronix Direct View
Storage Tube
? 1st
"inexpensive" graphics display
? Extension of vector scan technique
? Two electron guns
– writing gun
– flood gun
? Writing
gun beam knocks electrons out
leaves + charges behind (constitute image)
? Flood
gun supplies continuous source of
unfocused electrons
– migrate toward the + charges on grid
– pass through grid and strike screen phosphors
--> lighted dots
– electrons continue to hit + charges
– continuous light (Up to an hour)
Erasure of DVST image
1. Plus charge applied to entire grid
– Attracts electrons to entire grid
– Entire screen flashes (Image gone)
2. Minus charge applied to entire grid
– Provides electrons that can be knocked out
by writing gun
– Ready to draw next image with writing gun
Advantages to DVST
? No
refresh needed
– unlimited image complexity possible
? High
? Crisp lines
? Low cost
– no fast refresh circuitry needed
Disadvantages to DVST
? No
selective erase
– whole image or nothing
? No
? Low light output
– poor contrast
– must use in subdued light
? No
? No area fill
Interlaced Displays
All even then all odd screen lines scanned
? Typically 1/60 second each
– Same image presented twice in 1/30 second
– Image changed at 1/2 non-interlaced frequency
less demands on image generation system
can be less expensive
30 Hz is borderline for flicker
lower quality image (seeing half the image at a time)
Color Display Hardware (raster)
? Each
pixel composed of 3 phosphors
– glow red, green, and blue
electron guns shoot their beams through
a shadow mask
– so beams hit the sensitive phosphors
? Intensity
of 3 beams determines how bright
each phosphor glows
? Human eye detects an additive color mix
– e.g., max red, green, & blue perceived as white
Direct color systems
Frame buffer divided into bit planes
? A bit plane contributes one bit to the color of
each pixel on the screen
? If resolution of the screen is W x H pixels:
– a bit plane is a W x H x 1 bit memory
Bit planes can be organized into 3 sets
– Each called a color channel: (R, G, B)
– Bit planes of a color channel provide the intensity
values fed to that channel’s electron gun
A system with N bit planes per color channel:
– 2^N red, 2^N green, & 2^N blue shades
– 2^3N different colors displayable simultaneously
True Color & High Color
? True
color: direct color system with:
so 2^24 = 16,777,216 different colors
possible for each pixel on screen
More colors than discernable by human eye
? High
color: direct color system with:
Nr=5, Ng=6, Nb=5
2^16 = 65, 536 different colors possible
Indirect Color Systems
? Values
stored in bit planes are indices
into one or more color lookup tables
– CLUT stores R, G, B intensity values
– # of bit planes determines # of colors
displayable simultaneously on screen
– width of CLUTs determines # of possible
Indirect Color Systems, continued
If system has N bit planes per color channel
? And each set of bit planes indexes a CLUT of
width w,
– Then number of entries in each CLUT = 2 N
– We say there are 2 3N colors displayable chosen from a
total of 2 3w possible colors
– Each set of 2 3N colors often called a palette
– CLUTs often called palette registers
Advantages to Indirect Color
? Wide
CLUTs (large w) ==>huge number of
possible colors
? Modest # of bitplanes (small N) ==>
VRAM not excessive in size
? Also, number CLUT entries is modest
– So we get lots of possible colors with
relatively little memory expense
? Fast
animation for certain effects
– just change contents of CLUTS
Down Side to Indirect Color
? Ultimately
number of colors on screen at
any one time is limited by number of bit
planes (N)
– Even if there’s a large number of possible
colors (large CLUT w), only a small fraction
of them are usable at one time
– So graphics applications must set up CLUTs
with values corresponding to most frequently
occurring colors in scene
• Different scenes might require different
combinations of colors in the CLUTs
? Can
be slower: 2nd memory access
Color Graphics on a PC
? Graphics
capabilities depend on display
adapter (video card) in the system
? Historical development:
– CGA (Color Graphics Adapter)
– EGA (Enhanced Graphics Adapter)
– VGA (Video Graphics Array)
– Many different types of SVGA cards
– Each display adapter can function in many
different text and graphics modes
– Backwards compatibility
SVGA Adapters
? Many
? Each designed differently
– Each programmed differently at the pixel
– No compatibility
– Most compliant with VESA standards
• so VESA SVGA modes can be programmed
with relative ease
• often at the expense of performance
Setting the PC Graphics
Mode of Operation
Easiest way: use the BIOS VGA Services
via video interrupt 0x10
set AH register to 0 (set mode)
set AL to desired mode
make call to INT 0x10
INT 0x10 can be used for many other
graphics/video functions
– usually very slow
VGA Graphics Modes
? Support
all CGA and EGA modes
? 640 X 480 X 16 colors
? 320 X 200 X 256 colors (mode 13h)
? Also other modes
VGA Mode 13h--A Simple
Example of Indirect Color
? One
byte of VRAM controls one pixel
– Row major ordered
starts at address 0xa000:0000
? To set pixel at (x,y) to a given color:
– Set a segment register (ES) to start of video
– Compute pixel offset = 320 * y + x
– Load offset into a pointer register
– Set pixel by loading location with a color
(byte), e.g., MOV ES:[SI], color
VGA Mode 13h, continued
? Indirect
color control thru 256 X 18 CLUT
? Color written to VRAM is a byte-size index
into this CLUT
? Table entries: 6 bits red, 6 green, 6 blue
(0=no intensity, 63=maximum intensity)
? To
change an entry in the VGA CLUT
– use the video interrupt (10h):
• AH=10h, AL=10h, BX=CLUT position (0-255)
• DH, CH, CL = R, G, B intensity: (0-63 each)
Extension (VBE)
? Using
high resolution, high color SVGA
display modes in a standard way
? Documentation available at:
Graphics under Microsoft
? Windows
GDI does not permit direct
access to Display Adapter
? Must use GDI calls to do graphics
? Or
Special Libraries giving some access
to frame buffer
– OpenGL
– DirectX
Color Under Windows
? Direct
or Indirect
? Direct Modes:
– 16 bit “high color”
– 24 bit “true color”
• R, G, B: 8 bits each
• 224 different colors
– Use RGB() macro to get a COLORREF
• If used in low color modes, result is color
Windows Indirect Color Modes
256 entry CLUT (8 bits)
? 16 entry CLUT (4 bits)
? CLUTS called palettes
? Controlled by Windows “Palette Manager”
– A part of the GDI
Using a color in the CLUT:
– PALETTEINDEX(i) instead of RGB()
We’ll look at the 256-color palette
The System Palette
Maintained by Palette Manager
? Sort of like the physical CLUT
? Entries contain 8 bits per color channel
? 20 “static” colors initially defined
? Contents determine colors displayed
? Used by all applications
– Shared between all windows
Arbitrary changing of it could mess up color of
other windows
Changing the Palette
? Create
a “logical palette”
– Use CPalette::CreatePalette()
– Set up with desired colors
? Select
into a Device Context
? “Realize” it
– i.e., map it to the system palette
– done by calling CDC::RealizePalette()
Color Mapping with
Causes Palette Manager to compare colors in
logical palette with system palette
– exact match==>
• log. palette entry mapped to phys. palette entry
– no exact match==>
• if available free entry, copy and map
• if not, map to closest existing entry
Active foreground application mapped first
? So background window colors can change
Details in Changing System
1. Set up a logical palette structure:
Windows LOGPALETTE structure:
WORD palVersion;
// 0x300
WORD palNumEntries; // # colors to change
PALETTEENTRY palPalEntry[1] //new colors
- you may want to define & use your own
logical palette struct for more colors
// new color’s red intensity
// green intensity
// blue intensity
// usually 0
2. Create the palette:
CPalette::CreatePalette(LPLOGPALETTE pLP);
– Member function of CPalette
– Takes ptr to desired logical palette structure
– Should be typecast
– Returns nonzero if successful
3. Select it into the DC:
– pLP is a pointer to the logical palette structure created
4. Map current logical palette to system
Indirect Color in OpenGL
First tell system you’re using indexed color
field to PFD_TYPE_COLORINDEX instead of
• May not work on some systems
Set entries in window’s CLUT
– wgl: Use logical palette structure as described above
– GLUT: use glutSetColor(int color_index, r, g, b) to set
an entry in the CLUT
Selecting a color
– Glndexi(index_value); instead of glColor3f(r,g,b);
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