images, sound, and multimedia - computer sciencelin/comp089h/lec/multimedia.pdf · • images,...
TRANSCRIPT
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Images, Sound, and Multimedia
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No Surprises • Images, Sound, Music, and Movies – It’s all numbers – Binary Numbers
• Today we’ll discuss how multimedia is converted to and from numbers, and processed on a computer
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Colors • In a computer’s memory colors are represented
as a combination of three-additive primary colors (red, green, blue)
• Nearly every color that you perceive can be generated by combining these colors
• Under normal conditions, humans can discern about 100 shades of a color (Just noticeable differences)
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Visual Sensitivity A light sensitive organ; the highest bandwidth channel into the brain.
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Trichromatic Sensitivity • Two kinds of cells, rods and cones
• Three types of cones red, green, and blue
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Computer Colors • Combining what we know
– Only 3 colors are needed to stimulate the perception of nearly all others – Roughly 100 shades of each are
perceivable • Three one-byte channels (r, g, b) each with
256 levels • Alternatives: 15-bit color, 5-bits per r, g, b
(banding is sometimes noticeable)
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Some Preliminaries • Download the Python Imaging Library from
http://www.pythonware.com/products/pil
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Image Compression • There are 2 common types of image
compression – Lossy - (.jpg) – Higher compression rate (fewer bits/pixel)
• Good for “Natural images” – Lossless - (.bmp, .png) – Lower compression rate (more bits/pixel – Best for “Graphics Arts Images” – Preserves Sharp lines
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Image Compression Example
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Sound by Numbers • Sound is a result of pressure waves
propagating through the air
• You perceive sound as the intensity and frequency of these pressure waves
• How are these “waves” represented in a computer
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Sound Perception
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Some Quick and Dirty Sound
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Audio Sampling • The sampling rate, defines
the number of samples per second taken from a continuous signal to make a discrete (digital) signal.
• It is measured in hertz (Hz) • The inverse of the sampling
frequency is the sampling period or sampling interval, which is the time between samples.
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Common Sampling Rates
• The table above gives the common sampling rates used in many audio applica7ons
• The highest reproducible audible frequency by any format is 1/2 of its sampling rate
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Making a Sound
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A Square Wave Tone
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Noise
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More to Come… • Digital Sound and other • Graphics, images, etc