Background Perception Display Considerations Video Technology

Perception Persistence of Vision Positive Afterimage Perception of Motion Flicker, flicker rate Refresh rate v. update rate

Display Issues Double buffering Shadows, drop shadows Motion Blur Compositing Alpha channel

Double Buffering A Write into buffer

Double Buffering A B Write into buffer Display buffer A

Double Buffering C B Clear and write into buffer Display buffer B

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

Shadows

Shadows

Shadows

Shadows

Shadows

Shadows

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

Compositing Z buffer: keep z values with color buffer Compare z values at corresponding pixels Keep all or nothing OR compute partial coverage Interpolate corner z values Compare corner values for pixel and blend Use transparancy values (alpha) in addition to partial coverage

Analog Image Technology Film Various formats (e.g. mm widths of 8, 16, 35, 65, 70) Vary by placement of sound track, performations, frame Speeds 18 fps or, more usually, 24fps Usually doubly projected Video (raster scan) Interlace v. progressive scan Number of scanlines (e.g. 525, 625) Aspect ratio (e.g. 4:3, 16:9) Field rate (e.g., 59.94 Hz, 50 Hz)

Video - Raster Pattern

Interlaced Raster Pattern Frame v. Field

Video Video Information RGB YUV (Betamax) Y-C (S-VHS) NTSC NTSC: 29.97Hz, interlaced 4:3 aspect ratio ~480 scanlines ~640 (square) pixels HDTV: 60Hz progressive scan 16:9 aspect ratio

Video Analog Digital Binary

Codecs Tradeoffs Compression techniques 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

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

Digital Video Formats D1 D2 D3 D5 D6 DVCam DVCPRO Digital8 Ampex DCT Digital Betacam