Rick Parent - CIS681 Background Perception Display Considerations Film and Video, Analog and Digital Technology

Rick Parent - CIS681 Persistence of Vision Refresh rate v. update rate Perception of Motion Flicker, critical flicker rate Perception

Rick Parent - CIS681 Double buffering Compositing Alpha channel Shadows, drop shadows Motion Blur Display Issues

Rick Parent - CIS681 Double Buffering A Write into buffer

Rick Parent - CIS681 Double Buffering A Write into buffer B Display buffer A

Rick Parent - CIS681 Double Buffering C Clear and write into buffer B Display buffer B

Rick Parent - CIS681 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

Rick Parent - CIS681 Shadows Without shadows hard to tell relative distances, sizes, and height

Rick Parent - CIS681 Shadows But calculating shadows is expensive - basically a second visibility calculation from the point of view of the light source

Rick Parent - CIS681 Shadows Drop shadow: Even an approximation to the real shadow helps

Rick Parent - CIS681 Shadows For better approximations, copy the data and smash it down to the ground

Rick Parent - CIS681 Motion Blur Sample the pixel over single frame time Move objects during frame time Blend colors Usually jitter the samples in time

Rick Parent - CIS681 Motion Blur

Rick Parent - CIS681 Billboarding For complex objects (e.g. trees) Use 2D elements that always face the camera

Rick Parent - CIS681 Compositing Z buffer: keep z values with color buffer 1.Compare z values at corresponding pixels 2.Keep all or nothing

Rick Parent - CIS681 Compositing Z buffer: keep z values with color buffer 1.Compare z values at corresponding pixels 2.Compute partial coverage: Interpolate corner z values Compare corner values for pixel and blend

Rick Parent - CIS681 Compositing Alpha Channel Value between 0 and 1 Combined partial coverage and transparency Computed during rendering in front of a null background RGB alpha 2 1/2 D blend based on alpha of image in front 32 bit pixel values

Rick Parent - CIS681 Compositing - example

Rick Parent - CIS681 Analog Image Technology 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., Hz, 50 Hz) 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

Rick Parent - CIS681 Video - Raster Pattern

Rick Parent - CIS681 Interlaced Raster Pattern Frame v. Field

Rick Parent - CIS681 Video NTSC: 29.97Hz, interlaced 4:3 aspect ratio ~480 scanlines ~640 (square) pixels HDTV: 60Hz 1080i or 720p 16:9 aspect ratio Video Information RGB YUV (Betamax) Y-C (S-VHS) NTSC

Rick Parent - CIS681 Video Analog Digital Binary

Rick Parent - CIS681 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

Rick Parent - CIS681 Digital Formats Movie formats Quicktime Video for Windows File formats MPEG MJPEG Gif89a Codecs GIF Motion JPEG MPEG Cinepak Sorenson Indeo 3.2 RLE Video I

Rick Parent - CIS681 Digital Video Formats D1 D2 D3 D5 D6 DVCam DVCPRO Digital8 Ampex DCT Digital Betacam