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Week 9

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Computer Science
Computer Science 1033A/B
Pamela Glatt

COMP SCI – WEEK 9 Warm up questions • how many frames per second should we have when building an animation for display on a computer: ◦ 12-15 • which type of animation uses frames: ◦ cel based ◦ path based ◦ both ◦ neither • if an animation is 40 frames long and the fps is 5, how long with the animation take to play? ◦ 40/5 = 8 seconds • what was the first ◦ full length animated feature film? ▪ Snow White ◦ full length completely computer generated film? ▪ Toy Story What is video? • Asequence of still images (photohgraphs) that create the illusion of movement when played in succession • Q: what is each still image called? ◦ Frame • Frames per second ◦ Movies—24 to 30 fps ◦ Computer displayed video—12 to 15 fps • digital video—each frame is a bitmapped graphic, stored as 0s and 1s Alittle history • digital video often adheres to standards for TV broadcasting • regular analog TV broadcasting began in the United States in 1939 using the NTSC standard • Q: what is the standard for broadcasting in Britain? ◦ PAL • NTSC frame rate was originally 30 fps but went down to *29.97* to accommodate for colour information • experiments with High Definition TV began in the late 40s and 50s, but it wasn't adopted by a single station till 1996 • before 1996ALLTV was broadcast using interlaced display and fields • the originalANALOG video choices made about TV display (frames per second, frames per size, etc...) affect the standards that were picked for DIGITAL video! • Choices in 1939 effect how we convert videos today How did the original TV display work? • Our eyes see phosphor dots on the screen • an electron beam (gun) activates the dots. The gun scans through the dots horizontally • a complete scan is when the gun starts at the top left and scans several times horizontally will it gets to the bottom right • the scan only draws every OTHER line (1, 3, 5,....479) then starts back at the top and draws the even line (2, 4, ...480) ◦ thus two passes ◦ each pass is called a field ◦ the process is called interlaced display • this way it can cheat the eye, while the phosphor dots are disappearing, it is drawing the line underneath NTSC standards • the frame size of NTSC standard DV frame is 720 pixels by 480 pixels ◦ for PAL – 720 x 576 pixels ◦ frame aspect ratio is 4:3 (4 across, 3 up) ◦ pixels are distorted (not square) because 720:480 is actually 3:2 ratio, thus must change the pixel aspect ratio • high definition for NTSC ◦ 1140 x 1080 ◦ 1280 x 720 ◦ frame aspect ratio is 16:9 ◦ 1440:1080 – ratio is (4:3) 1.333 (pixels are not square) ◦ 1280:720 – ratio is 16:9 (pixels are square) TVs • older TVs – 480 scan lines • new plasma, flat screen – 720 or 1080 lines ◦ progressive NOT interlaced • smaller TVs are more crisp Sampling and quantizing of motion • since each frame is just an image: ◦ each frame is sampling into a discrete samples and each sample becomes a pixel – sampling proccess ▪ Remember: • more samples mean better quality • more samples means bigger file sizes ◦ each pixel get assigned a colour, maybe just 2 colours (black and white -> 1 bit colour) or maybe 16 million colour (24 bit colour) – quantizing process Displaying digital video • can display video on: ◦ computer -> don't need to worry about NTSC standards ◦ TV ◦ DV • not all digital video must conform to NTSC standards. Digital video that will primarily be played on a computer does not have to conform, BUT digital video that will be used in DVD playback needs to conform Displaying digital video on a computer • even though it doesn't have to be, digital video that is displayed on a computer is still very tied to analog TV standards • Q: what are the typical sizes of video that you imbed into your web pages? ◦ (4:3) ◦ 640 x 480 ◦ 320 x 240 -->this size is good for iPods Capturing the video • assuming we want to play our video on TV or on the computer, HOW do we capture the video to then later display it? Usually use: camcorders • if we use an: ◦ analog camcorder: we must convert it to digital BEFORE we can put it on our computer to edit. To convert analog video to digital video we need a Video Capture card ◦ digital video camcorder: no need to convert Digital video camcorders • most digital video camcorders (DV Camcorders), do a little bit of compression right inside the camera. DV25 is the most common DV compression used by today's camcorders ◦ DV25 format specs: ▪ pixel dimension is 720 x 489 (note, this is 3:2 ratio) ▪ frame aspect ratio either 4:3 or 16:9 ▪ data rate: 25 mega buts per second (that's why it is DV25) ▪ frame rate: 29.97 fps ▪ colour sampling: YUV 4:1:1 Colour compression in the camera • for still images RGB is commonly used • for video the model is YUV (YIQ) or TBbCr (for MPEG compression) • Y -> luminance (brightness) • UV -> (CbCr) chrominance (colour/hue)
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