cs 101 – sept. 14 review huffman code image representation –b/w and color schemes –file size...

CS 101 – Sept. 14

• Review Huffman code

• Image representation– B/W and color schemes

– File size issues

Huffman Code

• We’re given set of letters used in message, and their frequencies.

Here’s a 2nd example:– Ex. P=5, N=10, D=10, L=15, A=20, S=20, E=30

• Arrange frequencies in order• Group the letters in pairs, always looking for the

smallest sum of frequences Create a tree!

Images

• Fundamental unit is pixel

• Size = usually 8 bits

• Scheme = grayscale, range 0-255

• Dimensions given as (horiz vert)

– Ex. 400 300 120,000 pixels

– Note that an 8-bit pixel = 1 byte

• Aspect ratio

– Ex. 4 to 3

– When changing size, ratio shouldn’t change

Properties of rep’n

• “Sampling & Quantizing”

• Resolution of image– total number of pixels in image

• Dynamic range– How many shades of gray

• To reduce file size– Reduce either # of pixels, or # bits/pixel

Resolution

• here is a (edited) digitized image with a resolution of 272 x 416

Picture resolution determines both the amount of detail as well as its storage requirements

Resolution

• notice the changes when the resolution is reduced (136 x 208)


Resolution

• notice more changes when the resolution is reduced (68 x 104)


Dynamic Range

• Here is an intensity or graylevel image with 256 levels (i.e., 0 to 255 scale)

DYNAMIC RANGE refers the number of values for the measuring scale used in quantizing

Dynamic Range



Dynamic Range

• Here is an intensity or graylevel image with 2 levels (i.e., 0 to 1 scale or a binary image)

• Dithering can help


Don’t overdo it

• Too little resolution: pixelated

• Too few bits per pixel: sharp edges, cheap– Extreme case is “binary image”

• Note that n bits per pixel gives 2n values in dynamic range. 0 is black, 2n – 1 is white– Examples: n = 8, 4, 2, 1

B/W vs. Color

• B/W: usually 1 byte (8 bits) per pixel– Each pixel = grayscale number 0-255– Ex. 180 is a brighter shade of gray

• Color: usually 3 bytes (24 bits) per pixel– Each pixel has three values, each 0-255– Ex. (200, 50, 128) = ?

Color rep’ns

• RGB – system based on light

• CMY – based on printing

• HSB – based on art

• Indexed color – a swatch to save space

RGB system

• Based on primary colors for light

• Each pixel has (red, green, blue) values.

• Examplesblack = (0, 0, 0)

purple = (75, 0, 100)

white = (255, 255, 255)

• How about (x, x, x) or (0, 0, x) ?

RGB examplesColor R G B

black 0 0 0

white 255 255 255

red 255 0 0

green 0 255 0

blue 0 0 255

cyan 0 255 255

magenta 255 0 255

yellow 255 255 0

cs 101 – sept. 14 review huffman code image representation –b/w and color schemes –file size...

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