Image Formation and Digital Video Making Pictures Image Formation and Digital Video

Objectives Review how a digital image is created What is digital? Compression

0101010000111100011110001 What is digital? Digital uses on/off values to create an image. The on state is represented by a 1 The off state is represented by a 0 This is known as binary code The Digital Process

Scanning The 3 beams “read” the screen; from left to right and top to bottom There are two types of scanning Interlaced & Progressive Example of Interlace Scanning Odd lines Even lines Final image Odd Lines + Even Lines = 1 Frame

Scan types Interlaced scan Odd and even lines displayed on alternate frames When displaying interlaced video on a progressive scan display, can see “comb effect”

2 fields (1 odd & 1 even) = 1 frame 30 frames = 1 second of video Fields and Frames 2 fields (1 odd & 1 even) = 1 frame 30 frames = 1 second of video

Scan types - Progressive Progressive scan Display all lines on each frame New “fixed-resolution” displays (such as LCD, LED) all use progressive scan

Let’s Do the Math! 1 minute, 34 seconds

Definitions Bit rate Information stored/transmitted per unit time Usually measured in Mbps (Megabits per second) Ranges from < 1 Mbps to > 40 Mbps Resolution Number of pixels per frame Ranges from 160x120 to 1920x1080 FPS (frames per second) Usually 24, 25, 30, or 60

Definitions Resolution Number of pixels per frame Ranges from 160x120 to 1920x1080 The more lines of resolution, the better the image

HDTV vs. SDTV What is HDTV – High Definition Television - Best broadcast quality image - Superior color and resolution -16x9 aspect ratio What is SDTV – Standard Definition Television - Older TV standard - Only 525 lines of resolution - 4x3 aspect ratio

Numbers 1920 x 1080 720 x 480 1280 x 720 1440 x 1080 4K TVs: 4000 lines of resolution Pixels X number of lines of resolution

Which Would You Buy? 29 inch, 1080p $299 32 inch, 1080i $499

Analog vs. Digital Analog is an electronic copy of the original. It is continuous and never skips any part of the signal. Digital is discontinuous. It takes the signal and selects points at equal intervals to represent the original…..this is called sampling

Sampling Samples are taken of an analog signal at equal intervals Larger samples in shorter time frames = lower sample rate Larger samples in shorter time frames = higher sample rate Low Rate High Rate

Compression Once video is in digital format, it makes sense to compress it Store video data as efficiently as possible Maximize quality and minimize storage space and processing resources CODECS are used to compress and decompress files.

Digital Compression LOSSY vs. LOSSLESS

Digital Compression 2 Kinds of Compression: Lossless & Lossy Lossless: Rearranges information rather than getting rid of it Advantage – maintains original file Disadvantage – Makes large files, too large to transfer, stream or download

Compression Lossy: Gets rid of unnecessary information Advantage – Makes file considerably smaller for use on a computer, uploading, streaming etc. Disadvantage – Reduces picture and sound quality What compression you use, depends on the final output of the project: DVD – Lossless Tape - Lossless iPod- Lossy Web- Lossy

Loss of Resolution Original (63 kb) Low (7kb) Very Low (4 kb)

Downloading vs. Streaming media Data must completely download before you can play the whole thing Can watch at the information is being sent to your computer- like watching TV

Close-up details of different JPEG compression ratios Half compression, blurring & halos around sharp edges Max compression, 8-pixel blocks apparent, large distortion in high-frequency areas Uncompressed image (roughness between pixels still visible)

Streaming media Common types include Flash, RealVideo, Quicktime Video is played back in “real time”. Video and audio are compressed to fit inside bandwith

Review What is the difference between interlaced and progressive scanning? Why do we compress files? What are the two types of compression we use? How many frames are there is 1 second of video?