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Lecture

Week 10

5 Pages
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Department
Computer Science
Course Code
Computer Science 1033A/B
Professor
Pamela Glatt

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Description
COMPSCI – WEEK 10 Warm up questions • which of the following colour sampling methods offers 0% compression? ◦ 4:4:4 – must keep everything ◦ 4:2:2 – 30% ◦ 4:2:0 ◦ 4:1:1 – 50% • name three types of data transfer methods you can use to move your digital video from your camcorder to your computer ◦ USB ◦ Firewire ◦ Thunderbolt • how many scan lines are there on an analog TV from the 1970s? ◦ 480 • what does 720p mean? ◦ p = # of scan lines Editing video • before digital video: ◦ had to copy from one tape to another tape ◦ had to load up tapes on a machine to copy ◦ loss of quality after each copy made • now: ◦ can easily move clips around ◦ no loss of quality ◦ slight compression occurs in the camcorder when the video is captures ▪ Q: what is the name of the most common form of compression in the camcorder: • DV25 Things to think about before exporting video • where will I be putting my video? ◦ On the web (bandwidth is an issue) ◦ on CD-rom 9playback speed is an issue) ◦ DVD video (must be in mpeg2 format) • who is my audience? ◦ Will they be on different platforms ◦ how old will their computer be, how old will their CD or DVD player be? ▪ Don't compress until the very end • Will I still need to edit it later on? Should I compress it at all? ◦ On a CD, the thing doesn't spin fast enough to get the information out Why compress?An example: • assume we have a video that is? ◦ 1140 x 1080 pixels – 1, 555, 200 pixels per frame ◦ 24-bit colour ◦ 30 fps ◦ 1 second long ◦ audio is stereo, so 2 channels ◦ audio is 48, 000Hz and 16 bit = 48, 0000 x 1 sec x 16 bits/samples x 2 channels = ▪ 1, 536, 000 bits • Video ◦ 1, 555, 200 x 24 bits per pixel (for colour) – 37, 324, 800 bits per frame ◦ 37, 324, 800 x 30 frames per second x 1 sec = 1, 119, 744, 000 bits / 8 = 139, 968, 000 bytes = 133 MB • Audio ◦ 1, 536, 000 bits / 8 = 188 KB • Total ◦ 133 MB + 188 KB = ~133 MB -> lots of storage for just 1 second ◦ THUS a DVD could hold 35 seconds of uncompressed video • full colour = 24 bit colour Data rate • amount of video processed per second • average data rate = file size/length in seconds of video • our example -> 133 MB per second • consider a 48X speed CD rom ◦ average playback rate is 7MB/s ◦ our video would be VERY CHOPPY • consider a video that is 100MB and 10 seconds long? ◦ Q: what would it's data rate be? ▪ 10 MB/second ▪ it would be choppy on our CD player • consider a video that is 100MB and 33 seconds long, it would playback at 3 MB per second and be fine on our CD player • NOTE: problems occur not just because of file size, but also because of data rate General compression strategies • Lower the frame size of the video ◦ if it was 640 x 480, change it to 320 x 240, less pixels • Lower the frame rate of the video ◦ changing the frame rate by 1/2 (say 20fps to 10fps) will generally 1/2 the file size ◦ Q: what type of video would you NOT want to do this on? ▪ Where there are sports because there is a lot of action • Pick a codec that does higher compression ◦ Codec ▪ Codec is a piece of code that compresses video or audio as it is created (exporting from the editing software) and then when displaying it to the user deccompresses the video or audio (also stands for (coder/decoder) ▪ there are LOTS of codecs. You have to use the same one to decompress that was used to compress a video ▪ most common codecs are: • Sorenson Spark • H.264 • DivX ▪ can sometimes get a piece of video on your machine and then not be able to play it, this is because you are missing the appropriate codec! ▪ For example: if you take your friends camera and plug it into your computer and the jpgs will display, but the videos won't play—you are missing the codec ▪ codecs sometimes depend on the file format ▪ need appropriate codec for specific videos to work • Pick a codec that does higher compression ◦ QuickTime and AVI use different codecs so you can pick one that gives better compression • Lower the picture quality of the video ◦ Sorenson codec lets you set a limit on the data rate ◦ some let you set the quality • Lower the colour depth ◦ not popular because video looks best at 24-bit colour (unless is it a cartoon) ◦ some compressors won't compress colour • Play with the audio ◦ unfortunately the audio is usually not the problem, so compressing it more won't usually help much Compression concepts • Temporal compression vs. Spatial compression ◦ Spatial ▪ compresses each frame individually ▪ uses the same tech
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