MULTIMEDIA SYSTEMS

Transcription

1 1 Department of Computer Engineering, Faculty of Engineering King Mongkut s Institute of Technology Ladkrabang MULTIMEDIA SYSTEMS Pk Pakorn Watanachaturaporn, Wt ht Ph.D. PhD pakorn@live.kmitl.ac.th, pwatanac@gmail.com Chapter 7 Digital 2

2 The main steps Shooting video footage Gathering your media in a video processing environment Editing and sequencing clips Applying special effects Editing sound Applying final color adjustments and balances Rendering Exporting the movie to an appropriate file format with compression Burning the video to a DVD, placing it on a website, or otherwise distributing it 3 Shooting and Capturing Video The art of cinematography Five common ways to get the video footage onto your computer Shoot video on a digital video camera, storing it on the camera s internal hard drive, DVD, or memory card. Then transfer the video file to your computer Shoot video on a digital video camera, storing it on videotape. Then capture the video on your computer Connect a digital video camera to the computer and record the video directly onto the hard drive of the computer Connect an analog video device to the computer and record the video directly onto the hard drive of the computer Copy digital video files from an external source 4

3 Gathering media Non-linear Editor (NLE) generally include the following components A capture window A window id for gathering and organizing ii your media A window for editing clips before placing them in the timeline Source View A window for viewing clips that are in the timeline and applying special effects to them Program View A timeline for sequencing clips, stills, and sound files 5 6

4 7 8

5 Sequencing Placing a clip after another on the timeline with transitions in between 9 Sequencing 10

6 Digital Composition Alpha channels and Alpha blending 11 Digital Composition Alpha channels and Alpha blending 12

7 Digital Composition Alpha channels and Alpha blending 13 Digital Composition Mattes a process of combining two or more image elements into a single, final image An image mask an image whose pixel values determine the opacity levels of the pixels in the video frames to which the matte is applied A luma matte an image matte that uses luminosity values to specify opacity levels An alpha matte an image file which an alpha channel A traveling matte (a track matte) mattes that change, to mask the shapes of moving objects 14

8 15 A travelling matte 16

9 17 Digital Composition Chroma keying the process of eliminating pixels that are of a certain color, within some tolerance 18

10 Keyframes A keyframe is a frame that is marked as a reference point Tweening is the process of making a parameter (i (size/color/position) / ii change gradually from one keyframe to another 19 Timecodes The standard way of denoting time in video editing is SMPTE video timecode, given in terms of hours:minutes:seconds:frames 20

11 Adjusting speed Constant speed change Constant throughout a clip say at 50% speed Changing its duration Variable speed change Without changing its duration 21 Color correction Vector scope Show the hue and saturation of a video frame plotted on circular cua graph Wave form Show how the brightness of the selected frame varies from left to right Waveform Vector scope YCbCr parade RGB parade 22

12 Chip chart Color correction view 23 24

13 Rendering Computational of all effects applied to the video 25 Digital video files Container file format E.g., AVI, MOV, WMV 26

14 With no compression or subsampling NTSC 240 Mb/s HD 1 Gb/s 27 Properties of Codecs Intraframe compression (Spatial compression) Remove redundancies and extraneous information within one frame Interframe compression (Temporal compression) Eliminating redundant or unnecessary information by considering how images change over time Detect how objects move from one frame to another, represent this as a vector, and then use differential encoding along with the motion vector (by a method called motion estimator) Constant bit rate (CBR) encoding / Variable bit rate (VBR) encoding 28

15 Vector quantization Were designed d to fit video within the limitations i i of the low-bandwidth systems Basic methods Create a palette for a frame Divide the frame into areas (e.g., 4 x 4 pixs). Search the code table for the best color match to this area Encode the area by an index into the code table Three best known codes Cinepak originally bundled with Quick Time Indeo Sorenson Quick Time 3, Macromedia Flash (Adobe Flash), YouTube, Yahoo, Google 29 Motion JPEG Compression Apply JPEG compression frame by frame MPEG-A allow markers in the bitstream MPEG-B does not allow markers in the bitstream DV Compression Rely on intraframe compression Take RGB, convert it to YCbCr, downsample, appy DCT, adaptive quantization, variable length encoding Professional versions of DV DVCAM and DVCPRO 30

16 MPEG Compression The first was the work of ITU-T (H.26* series) MPEG codecs cover both audio and video compression First, MPEG versions are numbered MPEG-1, MPEG-2, MPEG2 MPEG-4 MPEG4 Within each of these versions, there are parts which relate to a specific component MPEG-1 has 6 parts MPEG-2 has 9 parts MPEG-4 has 23 parts (and more) 31 MPEG Compression MPEG-1 Using the basic algorithm H.261 Applied mostly to video to be distributed on CDs or the web MPEG-2 Aimed primarily at providing standards for digital television H.262 / H.263 MPEG-4 Object-based coding Was designed to be versatile, with applications ranging from lowdata-rate web distribution to DVDs to high-data-rate HDTV braadcast MPEG-4 AVC (Advanced Video Coding) H

17 33 MPEG Compression Step 1: Divide the sequence of frames into Group of Pictures (GOPs), identifying I, P, and B frames I frame (Intraframe) compressed independently using JPEG compression serve as reference points for the P frame P frame (Interframe or forward predicted frame) compared to its closest previous I or P B frame (Bidirectional frames) compared to its closest I or P frame in both the forward and backward direction 34

18 MPEG Compression Step 2: Divide each frame into macroblocks 35 MPEG Compression Step 3 and 4: for each P and B frame, compare the frame to the related ed I frame to determine a motion vector (or vectors). Record differential values fro P and B frames 36

19 MPEG Compression Step 5: For all frames, compress with JPEG compression 37