Images and colour Colour - colours - colour spaces - colour models Raster data - image...

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Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi- channel) images - colour lookup tables - operations on images
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Transcript of Images and colour Colour - colours - colour spaces - colour models Raster data - image...

Page 1: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Images and colour

Colour

- colours

- colour spaces

- colour models

Raster data

- image representations

- single and multi-band (multi-channel) images

- colour lookup tables

- operations on images

Page 2: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Colour images

• Colour images have two components:– raster data - an array of pixels;– colour model - a description of how pixels are mapped to

colours.

• A pixel is defined in terms of its components in a particular colour space

Page 3: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Colour spectrum – visible light

400 nm 700 nm

Ultraviolet Infrared

Page 4: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Colour spaces

• A colour space represents a system for measuring colours

• Most colours can be represented using three colour components

• They are called the primary colours (or the primaries)

Page 5: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Colour spaces

• There are many colour spaces.

• The choice of a particular space depends on the context in which we want to describe colours. The four most common colour spaces are:– RGB– HSV– CMY– XYZ

Page 6: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

RGB

• Primaries: Red - Green - Blue• Similar to colours detected by colour receptors in the

eye• Also used in TV technology

White

Black

Blue Cyan

Green

Magenta

Red Yellow

Page 7: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

RGB

Page 8: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

RGB – an additive system

Page 9: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

HSV

• Primaries: Hue - Saturation - Value• Colour space related to subjective description of

colours

Black

Green Yellow

Red

MagentaBlue

Cyan White

Page 10: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

HSV

Page 11: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

CMY

• Primaries: Cyan - Magenta – Yellow

• Used in printing technology• Mixing is subtractive

Page 12: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

CMY – subtractive system

Page 13: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

CIE XYZ

• Primaries: X, Y, Z• Based on colour perception by humans• Device independent

• The most common representation of the CIE XYZ space is the CIE chromacity diagram

Page 14: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

CIE XYZ

Chromacity diagram

Page 15: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Monochrome images

• Images represented by a single primary - a value

• Show only shades of grey

• Binary images use only two shades of grey: black and white.

Page 16: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Vector notation for colours

[ Primary1 Primary2 Primary 3 ]

[ R G B ] yellow = [ 1 1 0 ]

orange = [ 1 0.5 0 ]

pink = [ 1 0.7 0.7]

[ H S V ] pink = [ 0 0.3 1 ]

[ C M Y ] red = [ 0 1 1 ]

Page 17: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Colour space conversion

• Colours can be converted from one space to another

• Conversion from RGB to CMY:

[ C M Y ] = [ 1 1 1 ] - [ R G B ]

• Example: Convert green from RGB to CMY

[ C M Y ] = [ 1 1 1 ] - [ 0 1 0 ] = [ 1 0 1 ]

Page 18: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Conversion from RGB to XYZ

• Each of the R, G and B primaries is a weighted sum of X, Y and Z primaries

• Weights expressed in matrix notation, e.g.

0.41 0.21 0.02

0.36 0.71 0.120.18 0.07 0.95

Page 19: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Conversion from RGB to XYZ

• Conversion implemented as a matrix multiplication

[ X Y Z ] = [ R G B ] ·

0.584 0.311 0.047

0.188 0.614 0.1030.179 0.075 0.939

Page 20: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Raster data

• Colour• - colours• - colour spaces• - colour models • Raster data• - image representations• - single and multi-band (multi-channel) images• - colour lookup tables• - operations on images

Page 21: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Raster data - pixel structure

• Raster data - an image

• Pixels arranged as a rectangular array

• The structure of a pixel depends on – the colour space – the colour model

Page 22: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Raster array Pixel structure

Page 23: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Raster data - pixel structure

• Binary images– pixel represented by a single bit

• Monochrome images– pixel represented by a single byte (a one-dimensional

vector)

• Colour images– usually represented by a 3-dimensional vector– examples [ R G B], [H S V] or [C M Y]

Page 24: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Representations: Java terminology

• Normal representation– the amount of each primary defined by a number in the

range [0,1] – Example: pink (RGB): [1.0 0.5 0.5]

• Logical representation– Pixel representation as a vector

• Physical representation– depicted by the Colour Model

Page 25: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Colour channel

• Colour channel - a component of a colour vector• RGB: red channel, green channel and blue channel

• A pixel vector can have more than three channels

• Examples– alpha channel (often used to describe transparency of a

pixel)– z channel (in 3D graphics, the depth of the pixel, used in

hidden surface removal)

Page 26: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Colour models

A colour model describes how pixels are mapped into colours.

Page 27: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Direct Colour (True Colour)

• Image is an array of vectors

• Each vector directly encodes values of the three primaries

Page 28: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Packed Colour Model (Packed Array)

• Image is an array of values, each encoding a colour• Examples:

– 4-byte integer aaaaaaaa bbbbbbbb gggggggg rrrrrrrr

– 1-byte integer

rrrgggbb

Page 29: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Indexed Colour Model

• The most commonly used representation on smaller workstations

• A pixel value (or a value of a pixel component) is an index (a pointer) to a table containing colour definitions

Page 30: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Colour Map

• Synonyms:– Colour Lookup Table– CLUT– LUT

• Each location in a LUT stores a colour definition for a pixel with a given value

Page 31: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Colour mapping for 1-byte pixels

Colour mapR

G

B

R

GB

Cell

Pixel value

128

128

0255255

Page 32: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Colour mapping for 3-dimensional pixel vectors

Colour map

R

G

B

R

GB

Red

Green

Blue

Page 33: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Operations on images:Changing pixel colours

• Changing pixel colours is very easy within the Indexed Colour Model

• A raster array containing pixel values (or pixel vectors) stays unchanged.

• Only colour definitions in the LUT are changing

Page 34: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Colour mapping functions: Grey level mapping

Input

Output R

Input

Output G

Input

Output B

Page 35: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Inverse mapping

Input

Output R

Input

Output G

Input

Output B

Page 36: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

“Hot colours” mapping

Input

Output R

Input

Output G

Input

Output B

Page 37: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Changing pixel values: Convolution

• Convolution: a spatial filtering technique• calculates a weighted average of several pixels lying

within a small neighbourhood, normally 3x3 square,

• The 3x3 neighbourhood is called a kernel. • The effects depend on the values of weights.

Page 38: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Examples of smoothing kernels

1/9

1/9

1/9 1/9

1/9

1/9 1/9 1/9 1/10

1/10 1/10

1/10

1/10

1/10

1/10

1/5

1/10

1/9

1/8

1/16 1/16

1/8

1/8

1/8

1/16

1/4

1/16

Page 39: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Original image Image after smoothing

Page 40: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Examples of sharpening kernels

-1

-1

9 -1

-1

-1 -1 -1 -1

0 0

-1

-1

-1

0

5

0

-1

-2

1 1

-2

-2

-2

1

5

1

Page 41: Images and colour Colour - colours - colour spaces - colour models Raster data - image representations - single and multi-band (multi-channel) images -

Original image Image after sharpening