Published on June 26, 2007
Video: Video What is video How would you define it? Three Worlds of Video: Three Worlds of Video Computer Video Very unconstrained Broadcast Video Advanced Television (ATV/HDTV) Broadcast Video StandardsStandard Definition Television (SDTV): Broadcast Video Standards Standard Definition Television (SDTV) National Television Standards Committee NTSC 1952 US, Central America, Japan Phase Alternating Lines (PAL) Europe, Africa, Australia, and South America SECAM France (its inventor), eastern Europe, and Russia High Definition Television (HDTV): High Definition Television (HDTV) Varying resolutions 1920 x 1024: 1024i/p 1280 x 720: 720i/p Designed as a digital standard up front. Computer Video Standards: Computer Video Standards AVI Audio/Video Interleaved Fixed frame rates, simple format QuickTime (MOV) Highly flexible file format Almost anything can be put in Windows Media Format (ASF) Container format Digital Rights Management (DRM) Video Basics: Video Basics Mapping of images to time I(t) Fixed sample rates: 1 image every 0.066 seconds for example Variable sample rates: QuickTime allows arbitrary mappings Why? Video Sample Rates: Video Sample Rates fs=60 Conventional television (well, kinda…) Low quality computer monitors Progressive HDTV fs=15 or less Video phones, video conferencing, low bandwidth fs=75 to 90 or more Computer monitors What does Sample Rate determine: What does Sample Rate determine Motion continuity Do we see smooth motion or jerks Flicker Do we see the image flashing quickly Motion Continuity: Motion Continuity We are sampling underlying motion Motion continuity requirements: Motion continuity requirements We are less sensitive to continuity than to flicker Movies use 24fps We can detect less than 60fps easily 80+ fps is typically more than enough For humans, of course Flicker: Flicker Perceptible variations in brightness We can easily detect lower frequency flicker andlt;40Hz Flicker is NOT motion continuity Flicker in Motion Pictures: Flicker in Motion Pictures Motion pictures are filmed at 24fps Why do they not flicker? Raster Scanning: Raster Scanning Progressive vs. Interlaced Scan: Progressive vs. Interlaced Scan Progressive scan grabs entire frame as single image Computers use progressive scan Higher sample rate required to reduce flicker and motion Interlaced scan Grab even lines as a field Grab odd lines as a field Progressive scan: Progressive scan Interlaced Scan: Interlaced Scan Some numbers: Some numbers Commercial Television 29.97 frames per second 2 fields per frame Apparent 60 Hz scan rate Scanning complicates things: Scanning complicates things Film captures the entire frame at once Video presents the top half before the bottom half Webcams may do either… Take care when dealing with interlaced video: Take care when dealing with interlaced video Gulf War Video Analysis Mistakes: Gulf War Video Analysis Mistakes 41/44 or 4/44 or 0/44? Inter-line Flicker: Inter-line Flicker A common problem with Interlace What if we have a minimum width horizontal line? The line is refreshed 30 times per second, not 60! What can we do about it? Interlace complicates EVERYTHING!!: Interlace complicates EVERYTHING!! What does it take to Freeze frame? Fast motion? Slow motion? Reverse play? Motion pictures at 24fps? 3:2 pulldown: 3:2 pulldown 24fps Movie Frames 30fps Video Frames/Fields 30/24=1.25=5/4 60/24 = 2.5 Sometimes called 5:4 pulldown (why?) Advanced Television: Advanced Television Aspect Ratio: Aspect Ratio Aspect Ratio Width/Height 4:3 is conventional television 16:9 for high definition television (HDTV) 16:9 = 1.78:1 Motion pictures 1.85:1 is most common 2.35:1 in some cases Terminator 2, Stargate, Lawrence of Arabia Problems with Aspect Ratio: Problems with Aspect Ratio What if the display AR is less than the production AR? Letterboxing: Letterboxing Simply shrinking vertically to fit I miss the old Letterschlocking FAQ… Cropping: Cropping Just cutting to fit… Pan and Scan: Pan and Scan We can move the window Examples: Examples Yokelvision Widescreen DVD and Letterboxing: DVD and Letterboxing DVD allows for both conventional and letterboxed versions on a disk How do they do this? Why do this do this? Why don’t they do this? DVD Allows… : DVD Allows… Stored wide-screen version (16:9) Wide-screen television support Automatic letterboxing Automatic pan and scan Most disks still: Two copies of the movie Color: Color Broadcast television: YIQ and YUV YIQ – US Television Y = 0.30R + 0.59G + 0.11B (Luminance) I = 0.60R – 0.28G – 0.32B Q = 0.21R – 0.52G + 0.31B YUV – CD’s, compression, European TV Y = 0.30R + 0.59G + 0.11B (Luminance) U = 0.493(B-Y) V = 0.877(R-Y) Why not HLS or something like that?