Upload
brianna-fitzpatrick
View
228
Download
1
Tags:
Embed Size (px)
Citation preview
4.1Si23_03
SI23Introduction to Computer
Graphics
SI23Introduction to Computer
Graphics
Lecture 4 – Colour Models
4.2Si23_03
Colour ModelsColour Models
In order to help us specify colour, a number of attempts have been made to define colour spaces– A formal method of representing
the visual sensations of colour A colour will be defined by its
position in this space
4.3Si23_03
RGB Colour ModelRGB Colour Model
0,0,0Black
1,0,0Red
0,1,0Green
0,0,1Blue
1,1,1White
1,0,1Magenta
0,1,1Cyan
1,1,0Yellow
3D cube withRGB device signalsaligned with axesof cube
Black at origin;red, green, blue atend of axes;grey values alonglong diagonal
4.4Si23_03
RGB CubeRGB Cube
4.5Si23_03
RGB Colour ModelRGB Colour Model
Advantages– relates easily to CRT operation– easy to implement
Disadvantages– RGB values generally not transferable
between devices (no standard `red’ phosphor)
– not perceptually uniform (colours close together near white are distinguishable, but not true near black)
– not intuitive - eg where is skin colour?
4.6Si23_03
Hue, Lightness, Saturation Model
Hue, Lightness, Saturation Model
The Hue, Lightness, Saturation (HLS) colour model is more intuitive
RedCyan
YellowGreen
MagentaBlue
Hues are placedon a hexagon,with red at 0,green at 120, andblue at 240 degrees
4.7Si23_03
Hue, Lightness, Saturation Model
Hue, Lightness, Saturation Model
RedCyan
YellowGreen
MagentaBlue
Lightness
1.0White
Black0.0
Lightness axis goesfrom 0.0 to 1.0
4.8Si23_03
Hue, Lightness, Saturation Model
Hue, Lightness, Saturation Model
RedCyan
YellowGreen
MagentaBlue
Lightness
1.0White
Black0.0
Saturation is givenby the distancefrom the central axis
4.9Si23_03
Hue, Lightness, Saturation Model
Hue, Lightness, Saturation Model
RedCyan
YellowGreen
MagentaBlue
Lightness
1.0White
Black0.0
L
S
H
The final modelis a double hexagonalcone
4.10Si23_03
Hue, Lightness, Saturation Model
Hue, Lightness, Saturation Model
This model is commonly used in computer graphics
Advantages– intuitive: choose hue, vary
lightness, vary saturation Disadvantages
– directly related to RGB (cube stood on end) so different on different monitors
– not perceptually uniform
4.11Si23_03
Hue, Saturation and Value (HSV)
Hue, Saturation and Value (HSV)
Alternative method is known as HSV
Both HLS and HSV widely used in computer graphics
RedCyan
YellowGreen
MagentaBlue
Black
White
V
S
H
4.12Si23_03
CIE Colour SystemCIE Colour System
Commission Internationale de L’Eclairage (CIE) have developed the most accurate colour specification system
Recall (lecture 3, slide 11/12) the colour matching experiments where observers matched test lights of each wavelength against combinations of R (700nm), G (546nm), B (436nm) - and averaged to give the `CIE standard observer’
4.13Si23_03
Colour MatchingColour Matching
This gives set of three curves showing relative amounts of each primary required at each wavelength
400 500 600 700
B G R
relativeintensity
Note curves (esp red) go negative! This means thatthe primary light had to be repositioned so as to addto the test light. Cyan at 500nm for example cannot bematched by blue and green, but must be diluted with red.
4.14Si23_03
Tristimulus functions Fx, Fy, Fz produced as a combination of the R,G,B matching functions
Colour of any wavelengthis now matched by positiveamounts of Fx, Fy, Fz
Fy matches the combinedresponse of cones - iethe luminance efficiency ofthe eye
400 700
CIEt-fun
Fx() = 0.49R() + 0.31G() + 0.20B()Fy () = 0.18R() + 0.81 G() + 0.01B()Fz () = 0.00R() + 0.01G() + 0.99B()
Fz
CIE Tristimulus FunctionsCIE Tristimulus Functions
FyFx
4.15Si23_03
Tristimulus ValuesTristimulus Values
Colour of light in CIE is measured by tristimulus values X,Y,Z
FxFy
Fz
400 700
CIEt-fun
400 700
Lightsource
X = 25.3Y =19.6Z =13.3
For each , take light source power, multiply by CIE Fx-value and addover all wavelengths to get X; similarly for Y and Z.
Tristimulus values
4.16Si23_03
CIE Colour SpaceCIE Colour Space
Thus any colour can be uniquely defined in terms of co-ordinates (X,Y,Z)
The set of all visible colours forms a cone shape
For convenience, it is common to take a slice through the cone to get a 2D colour diagram - slicing so that X+Y+Z=1
x = X / ( X+Y+Z )
y = Y / ( X+Y+Z)
4.17Si23_03
4.18Si23_03
CIE Colour DiagramCIE Colour Diagram
700
600500
400
520
red
green
cyan
purple
y1.0
x1.0
0
pure colourslie on perimeter
white at centre dot
colours becomemore saturated onmoving from whiteto boundary
axes are outsidethe horseshoe
4.19Si23_03
CIE Colour DiagramCIE Colour Diagram
700
600500
400
520
red
green
cyan
purple
y1.0
x1.0
0
Combinations of twocolours, A and B, represented as straightline between A and Bin diagram.
A
B
4.20Si23_03
Colour GamutsColour Gamuts
700
600500
400
520
red
green
cyan
purple
y1.0
x1.0
0
indicates colourof RGB phosphors
indicates thecolour gamut -set of coloursthat can be produced by thatmonitor
G
R
B
4.21Si23_03
CIE Colour DiagramCIE Colour Diagram
700
600500
400
520
red
green
cyan
purple
y1.0
x1.0
0
W
pure colourLine from purecolour to whitegives effect ofdesaturating thecolour
4.22Si23_03
CIELUV SpaceCIELUV Space
The CIE 1931 space is not wholly satisfactory– X, Y, Z primaries
are imaginary sources
– Space is non-uniform
Led to publication in 1976 of the CIELUV uniform colour spacee
Cross section throughCIELUV model
4.23Si23_03
Colour Naming SystemColour Naming System
4.24Si23_03
Web ColoursWeb Colours
For Web pages, colour specified as R,G,B values in range 0-255– Specified in the form
#RRGGBB where RR GG BB are hexadecimal values
– Thus (255,0,0) = #ff0000
– Also with names
Netscape defined set of 216 (6x6x6) web safe colours