Background

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Background
Perception
Display Considerations
Film and Video, Analog and Digital Technology
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Perception
Persistence of Vision
Perception of Motion
Flicker, critical flicker rate
Refresh rate v. update rate
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Display Issues
Double buffering
Shadows, drop shadows
Motion Blur
Compositing Alpha channel
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Double Buffering
A
Write into buffer
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Double Buffering
A
B
Write into buffer
Display buffer
A
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Double Buffering
B
C
Clear and write into buffer
Display buffer
B
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Double Buffering

• Requires additional memory
• Allows for instantaneous update of screen
• Writing to buffer may not be real-time
• May be supported in hardware
• Can use more than two buffers

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Shadows
Without shadows hard to tell relative distances,
sizes, and height
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Shadows
But calculating shadows is expensive - basically
a second visibility calculation from the point of
view of the light source
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Shadows
Drop shadow Even an approximation to the real
shadow helps
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Shadows
For better approximations, copy the data and
smash it down to the ground
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Motion Blur

• Sample the pixel over single frame time
• Move objects during frame time
• Blend colors
• Usually jitter the samples in time

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Motion Blur
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Billboarding
For complex objects (e.g. trees) Use 2D elements
that always face the camera
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Compositing

• Z buffer keep z values with color buffer
• Compare z values at corresponding pixels
• Keep all or nothing

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Compositing

• Z buffer keep z values with color buffer
• Compare z values at corresponding pixels
• 2. Compute partial coverage
• Interpolate corner z values
• Compare corner values for pixel and blend

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Compositing
Alpha Channel Value between 0 and 1 Combined
partial coverage and transparency
Computed during rendering in front of a null
background
2 1/2 D blend based on alpha of image in front
alpha
RGB
32 bit pixel values
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Compositing - example
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Analog Image Technology
Film Various formats (e.g. mm widths of 8, 16,
35, 65, 70) Vary by placement of sound track,
perforations, frame Speeds 18 fps or, more
usually, 24fps Usually doubly or triply projected
Video (raster scan) Interlace v. progressive
scan Number of scanlines (e.g. 525, 625) Aspect
ratio (e.g. 43, 169) Field rate (e.g., 59.94
Hz, 50 Hz)
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Video - Raster Pattern
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Interlaced Raster Pattern
Frame v. Field
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Video
Video Information RGB YUV (Betamax) Y-C
(S-VHS) NTSC
NTSC 29.97Hz, interlaced 43 aspect ratio 480
scanlines 640 (square) pixels
HDTV 60Hz 1080i or 720p 169 aspect ratio
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Video
Analog
Digital
Binary
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Codecs
Tradeoffs Symmetry v. asymmetry Lossy v.
lossless Speed Space (compression ratio) Video v.
Television
Compression techniques Run-length
encoding LZW Discrete cosine transform Wavelet
compression Fractal compression Vector
quantization
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Digital Formats
Codecs GIF Motion JPEG MPEG Cinepak Sorenson Indeo
3.2 RLE Video I
File formats MPEG MJPEG Gif89a
Movie formats Quicktime Video for Windows
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Digital Video Formats
D1 D2 D3 D5 D6 DVCam DVCPRO Digital8 Ampex
DCT Digital Betacam