1. Video
Video

2. Introduction Video Signal Representation Color Encoding
Computer Video Format
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3. Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.
The term video (from Latin: "I see") commonly refers to several storage formats for moving eye pictures: digital video formats, including DVD, QuickTime, and MPEG-4; and analog videotapes, including VHS and Betamax. Video can be recorded and transmitted in various physical media: in magnetic tape when recorded as PAL or NTSC electric signals by video cameras, or in MPEG-4 or DV digital media when recorded by digital cameras.
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4. Basic Concepts (Video Representation)
Human eye views video
immanent properties of the eye determine essential conditions related to video systems.
Video signal representation consists of 3 aspects:
Visual Representation
objective is to offer the viewer a sense of presence in the scene and of participation in the events portrayed.
Transmission
Video signals are transmitted to the receiver through a single television channel
Digitalization
analog to digital conversion, sampling of gray(color) level, quantization.
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5. aspect ratio
Aspect ratio describes the dimensions of video screens and video picture elements.
All popular video formats are rectilinear, and so can be described by a ratio between width and height.
The screen aspect ratio of a traditional television screen is 4:3, or about 1.33:1. High definition televisions use an aspect ratio of 16:9, or about 1.78:1.
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6. Visual Representation
The televised image should convey the spatial and temporal content of the scene
Vertical detail and viewing distance
Aspect ratio: ratio of picture width and height (4/3 = 1.33 is the conventional aspect ratio).
Viewing angle = viewing distance/picture height
Horizontal detail and picture width
Picture width (conventional TV service ) - 4/3 * picture height
Total detail content of the image
Number of pixels presented separately in the picture height = vertical resolution
Number of pixels in the picture width
= horizontal resolution*aspect ratio
product equals total number of picture elements in the image.
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7. Visual Representation
Perception of Depth
In natural vision, this is determined by angular separation of images received by the two eyes of the viewer
In the flat image of TV, focal length of lenses and changes in depth of focus in a camera influence depth perception.
Luminance and Chrominance
Color-vision - achieved through 3 signals, proportional to the relative intensities of RED, GREEN and BLUE.
Color encoding during transmission uses one LUMINANCE and two CHROMINANCE signals
Temporal Aspect of Resolution
Motion resolution is a rapid succession of slightly different frames. For visual reality, repetition rate must be high enough (a) to guarantee smooth motion and (b) persistence of vision extends over interval between flashes(light cutoff b/w frames).
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8. focal length of lenses
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9. Chrominance
Chrominance (chroma for short), is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal.
Chrominance is usually represented as two color-difference components: U = B'–Y' (blue – luma) and V = R'–Y' (red – luma). Each of these difference components may have scale factors and offsets applied to them, as specified by the applicable video standard.
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10. Luma
luma represents the brightness in an image (the "black and white" or achromatic portion of the image).
Luma is typically paired with chroma. Luma represents the achromatic image without any color, while the chroma components represent the color information.
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11. Visual Representation
Continuity of motion
Motion continuity is achieved at a minimal 15 frames per second; is good at 30 frames/sec; some technologies allow 60 frames/sec.
NTSC standard provides 30 frames/sec Hz repetition rate.
PAL standard provides 25 frames/sec with 25Hz repetition rate.
Flicker effect
Flicker effect is a periodic fluctuation of brightness perception. To avoid this effect, we need 50 refresh cycles/sec. Display devices have a display refresh buffer for this.
Temporal aspect of video bandwidth
depends on rate of the visual system to scan pixels and on human eye scanning capabilities.
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12. Transmission (NTSC) Video bandwidth is computed as follows
700/2 pixels per line X 525 lines per picture X 30 pictures per second
Visible number of lines is 480.
Intermediate delay between frames is
1000ms/30fps = 33.3ms
Display time per line is
33.3ms/525 lines = 63.4 microseconds
The transmitted signal is a composite signal
consists of 4.2Mhz for the basic signal and 5Mhz for the color, intensity and synchronization information.
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13. Color Encoding A camera creates three signals
RGB (red, green and blue)
For transmission of the visual signal, we use three signals
1 luminance (brightness-basic signal) and 2 chrominance (color signals).
In NTSC, luminance and chrominance are interleaved
Goal at receiver
separate luminance from chrominance components
avoid interference between them prior to recovery of primary color signals for display.
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14. Color Encoding RGB signal - for separate signal coding YUV signal
consists of 3 separate signals for red, green and blue colors. Other colors are coded as a combination of primary color. (R+G+B = 1) --> neutral white color.
YUV signal
separate brightness (luminance) component Y and
color information (2 chrominance signals U and V)
Y = 0.3R G B
U = (B-Y) * 0.493
V = (R-Y) * 0.877
Resolution of the luminance component is more important than U,V
Coding ratio of Y, U, V is 4:2:2
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15. Color Encoding(cont.) YIQ signal Composite signal
similar to YUV - used by NTSC format
Y = 0.3R G B
U = 0.60R G B
V = 0.21R -0.52g B
Composite signal
All information is composed into one signal
To decode, need modulation methods for eliminating interference b/w luminance and chrominance components.
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16. Digitalization
Refers to sampling the gray/color level in the picture at MXN array of points.
Once points are sampled, they are quantized into pixels
sampled value is mapped into an integer
quantization level is dependent on number of bits used to represent resulting integer, e.g. 8 bits per pixel or 24 bits per pixel.
Need to create motion when digitizing video
digitize pictures in time
obtain sequence of digital images per second to approximate analog motion video.
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17. Computer Video Format Video Digitizer
A/D converter
Important parameters resulting from a digitizer
digital image resolution
quantization
frame rate
E.g. Parallax X Video - camera takes the NTSC signal and the video board digitizes it. Resulting video has
640X480 pixels spatial resolution
24 bits per pixel resolution
20fps (lower image resolution - more fps)
Output of digital video goes to raster displays with large video RAM memories.
Color lookup table used for presentation of color
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18. Digital Transmission Bandwidth
Bandwidth requirement for images
raw image transmission b/w = size of image = spatial resolution x pixel resolution
compressed image - depends on compression scheme
symbolic image transmission b/w = size of instructions and primitives carrying graphics variables
Bandwidth requirement for video
uncompressed video = image size X frame rate
compressed video - depends on compression scheme
e.g HDTV quality video uncompressed Mbps, compressed using MPEG (34 Mbps with some loss of quality).
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