P1253814550VYUPZ

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???????????????????????? Ian Thomas
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• Inside a digital image
• How colour is defined for images
• How colour is represented in an image

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???????????????????????? Ian Thomas
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Representing colour
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???????????????????????? Ian Thomas
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The Windows colour cube is 6x6x6 with 216
colours.
Blue
Green
Red
Gray line
http//www.webreference.com/dev/graphics/palette.h
tml
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???????????????????????? Ian Thomas
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Green
Blue
Red
We can make the grey line vertical by changing
the direction of the R, G and B axes.
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???????????????????????? Ian Thomas
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Intensity
Hue
Red
Green
Red
Red
Blue
Green
Blue
Green
Blue
Saturation
Transforming from RGB to HSI
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???????????????????????? Ian Thomas
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Converting from RGB to HSI firstly we normalize
the RGB values.
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???????????????????????? Ian Thomas
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Now calculate normalized h, s and i values
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???????????????????????? Ian Thomas
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Now convert h, s and i to more convenient H, S
and I values
http//www.clarkson.edu/class/image_process/RGB_to
_HSI.pdf
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???????????????????????? Ian Thomas
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To convert back from HIS to RGB
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HSV and HLS colour spaces are other ways to
describe colour.
http//escience.anu.edu.au/lecture/cg/Color/printN
otes.en.html
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Saturation of colour
http//escience.anu.edu.au/lecture/cg/Color/printN
otes.en.html
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???????????????????????? Ian Thomas
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Defining colour to an image
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An artists pallette defines the colours used in
a painting.
http//artguildct.org/2005200320March.htm
http//www.arttoheartweb.com/starry_night.htm
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???????????????????????? Ian Thomas
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Van Gogh Robert Rauschenberg
http//www.featurepics.com/editorial/Color-Analyti
cs.aspx
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???????????????????????? Ian Thomas
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Using a palette from the Windows colour cube
Using the best palette for the image - adaptive
palette
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???????????????????????? Ian Thomas
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Image resolution and quantization
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???????????????????????? Ian Thomas
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Resolution is the number of pixels in an image.
128x192
64x48
16x24
32x48
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???????????????????????? Ian Thomas
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Quantization is the number of different colours
for each pixel.
256 levels
4 levels
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???????????????????????? Ian Thomas
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Quantization affects the quality of an image.
64
16
32
256
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???????????????????????? Ian Thomas
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Ways to store images
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Indexed colour means that the image is broken up
into an array of indexes and a palette.
http//en.wikipedia.org/wiki/Bits_per_pixel
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???????????????????????? Ian Thomas
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8 bit indexed colour
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???????????????????????? Ian Thomas
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Highcolour 15 bit - each colour R,G,B has 5
bits. There are a total of 32,768 colours 16
bit - R and B have 5 bits each, but G has 6 bits
because the eye is more sensitive to green. There
are a total of 65,536 colours.
http//en.wikipedia.org/wiki/Highcolour
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???????????????????????? Ian Thomas
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8 bit RGB - Truecolour for example, yellow
(255,255,0) blue (0,0,255) There are a total of
16 million colours.
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???????????????????????? Ian Thomas
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An image can be split into RGB.
red
green
blue
http//homepages.cae.wisc.edu/mccormic/ece533/hw3
.html
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???????????????????????? Ian Thomas
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Or into HSI.
Hue
Saturation
Intensity
http//homepages.cae.wisc.edu/mccormic/ece533/hw3
.html
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???????????????????????? Ian Thomas
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• An image file consists of 2 parts
• a header with information about the image (may
  include the palette)
• the body with the actual image information

Image x size
Image x size
Image file