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Background

This article explores the background and considerations of film and video display, including perception, persistence of vision, motion perception, double buffering, shadows, motion blur, and compositing. It also discusses analog and digital technology, as well as different video formats and codecs.

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Background

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Presentation Transcript

  1. Background Perception Display Considerations Film and Video, Analog and Digital Technology

  2. Perception Persistence of Vision Perception of Motion Flicker, critical flicker rate Refresh rate v. update rate

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

  4. Double Buffering A Write into buffer

  5. Double Buffering A B Write into buffer Display buffer A

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

  7. 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

  8. Shadows Without shadows hard to tell relative distances, sizes, and height

  9. Shadows But calculating shadows is expensive - basically a second visibility calculation from the point of view of the light source

  10. Shadows Drop shadow: Even an approximation to the real shadow helps

  11. Shadows For better approximations, copy the data and smash it down to the ground

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

  13. Motion Blur

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

  15. 10 6 9 9 10 6 9 9 Compositing Z buffer: keep z values with color buffer Compare z values at corresponding pixels Keep all or nothing

  16. 9 8 9 10 9 6 10 6 9 9 9 6 10 9 8 9 10 6 9 9 9 10 9 6 9 8 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

  17. 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

  18. Compositing - example

  19. 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. 4:3, 16:9) • Field rate (e.g., 59.94 Hz, 50 Hz)

  20. Video - Raster Pattern

  21. Interlaced Raster Pattern Frame v. Field

  22. 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 • 1080i or 720p • 16:9 aspect ratio

  23. Video Analog Digital Binary

  24. 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

  25. 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

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

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