6 Colours - Colour Models PDF

 Properties of Light “Light” = narrow frequency band of electromagnetic spectrum The electromagnetic spectrum: Red: 3.8x1014 hertz Violet: 7.9x1014 hertz  A ray of light contains many different waves with individual frequencies.  The associated distribution of wavelength int...

 Properties of Light “Light” = narrow frequency band of electromagnetic spectrum The electromagnetic spectrum: Red: 3.8x1014 hertz Violet: 7.9x1014 hertz  A ray of light contains many different waves with individual frequencies.  The associated distribution of wavelength intensities per wavelength is referred to as the spectrum of a given ray or light source.  Colour Fundamentals  Luminance = a measure of the light strength, that is actually perceived by the human eye.  Hue = The nuance: red, blue, green,.....  Saturation - the intensity or purity of a hue  Brightness = a subjective, psychological measure of perceived intensity. Brightness is the relative degree of black or white mixed with a given hue. Humans Only Perceive Relative Brightness  Colour Properties  Psychological colours characteristics:  Dominant frequency (hue, colour),  Brightness =total light energy,  Purity (saturation), how close a light appear to be a pure spectral colour, such as red  Chromaticity, used to refer collectively to the two properties describing colour characteristics: purity and dominant frequency  Colours can be produced by:  a light source (natural or artificial),  chemical pigments.  Intuitive colour concepts:  Shades, tints and tones in scene can be produced by mixing colour pigments (hues) with white and black pigments  Shades - add black pigment to pure colour. The more black pigment, the darker the shade  Tints - add white pigment to the original colour. Making it lighter as more white is added  Tones - produced by adding both black and white pigments  Colour Systems Additive Model (RGB) Subtractive Model (CMYK) - light based model- - Pigment based model - Used by emissive devices: TV set, monitor, projector, Prints (paper based) lighted display, photo camera, scanner. stained glass Complementary Colours subYM  Subtractive  Additive  Orange (between red and subCR  Blue is one-third yellow)cyan-blue  Yellow (red+green) is two-thirds  green-cyanmagenta-red colour  When blue and yellow light are added together, they produce white light  Pair of complementary colours  blue and yellow  green and magenta  red and cyan addRG  Colour Models  Method for explaining the properties or behavior of colour within some particular context  Combine the light from two or more sources with different dominant frequencies and vary the intensity of light to generate a range of additional colours  Primary colours:  3 primaries are sufficient for most purposes  Hues that we choose for the sources  Colour gamut is the set of all colours that we can produce from the primary colours  Complementary colour is two primary colours that produce white:  Red and Cyan, Green and Magenta, Blue and Yellow.  The range of colours that can be produced by a specific method = colour space.  Colour space = abstract mathematical model that describes how colors can be represented/produced.  The RGB Colour Model  Basic theory of RGB colour model  The tristimulus theory of vision. It states that human eyes perceive colour through the stimulation of three visual pigment of the cones of the retina: Red, Green and Blue  Model can be represented by the unit cube defined on R,G and B axes. RGB Colour Model (cont.) ▪ Uses the 3 primary colours (red, green, blue) to generate colours displayed on a monitor. ▪ By adjusting the intensity of each primary component, all colors from the visible light spectrum c a n b e g e n e r a te d. ▪ Ex. colours with 8 bits/channel use values in the range: 0-255 for each pixel => 256 power 3 =16.777.216 colours. RGB Colour Model (cont.) The possible colour combinations of any pixel in an eight-bit graphic (or a 24-bit display).  CMYK Colour Model  The CMYK (Cyan Magenta Yellow black) colour model (process color) is a subtractive colour model, used in colour printing describes the printing process itself.  Colour models for hard-copy devices, such as printers:  Produce a colour picture by coating a paper with colour pigments  Obtain colour patterns on the paper by reflected light, which is a subtractive process  The CMY parameters:  A subtractive color model can be formed with the primary colours: cyan, magenta and yellow  Unit cube representation for the CMY model with white at origin  Printing images suppose image transformation from RGB model to CMYK model, CMYK Colour Model (cont.) The blacK was added to this model because of the following reasons: Black ink is the cheapest one, from the four (C, M, Y, K) inks, The time required for drying the ink is reduced, The text printed on paper must be legible.. Indicate colour intensity: (0-100%) Spot colours (fluorescent, lacquered colours), Derived colour model CcMmYyK – used by Jet printers (photos).  CMYK Colour Model (cont.)  Transformation between RGB and CMY colour spaces:  Transformation matrix of conversion from RGB to CMY  C  1  R   M  = 1 − G        Y  1  B   Transformation matrix of conversion from CMY to RGB  R  1  C  G  = 1 −  M        B  1  Y  HSB Colour Model ▪ HSB (Hue, Saturation Brightness) model is also known as HSV (Hue, Saturation and Value) model. ▪ Together with HSL (Hue, Saturation and Lightness) are the most common cylindrical-coordinate representations of colours. ▪ The two representations rearrange the geometryof RGB in an attempt to be more intuitive and perceptually relevant than the Cartesian (cube) representation.  HSB Colour Model (cont.) Describes three fundamental characteristics of colours: Hue = the colour reflected or transmitted through an object. It is measured as the colour location on the colour wheel (0 ° -360 °). Saturation = the intensity or colour purity. It is the rapport of gray and the hue, measured as a percentage; the values are in the range 0% (gray) to 100% (fully saturated). Brightness - the lightness or darkness of the colour, measured as a percentage from 0% (black) to 100% (white).  HSB Colour Model  Hue is the most obvious characteristic of a colour.  Saturation is the purity of a colour:  High chroma colours look rich and full,  Low chroma colours look dull and grayish,  Sometimes chroma is called saturation.  Brightness is the lightness or darkness of a colour  Sometimes light colours are called tints, and  Dark colours are called shades.  HSB Colour Model (cont.)  Interface for selecting colours often use a colour model based on intuitive concepts rather than a set of primary colours.  Derived by relating the HSB parameters to the direction in the RGB cube,  Obtain a colour hexagon by viewing the RGB cube along the diagonal from the white vertex to the origin.  Transformation RGB->HSB  To move from RGB space to HSB space:  Can we use a matrix? No, it’s non-linear. min = the minimum R, G, or B value max = the maximum R, G, or B value  0 if max = min   g −b +   60 max − min 0 if max = r and g ≥ b   g −b 0max − min if max = 0 60 + 360 if max = r and g < b s= h= max − min  max otherwise   b−r 60 + 120  if max = g  max − min   r−g  60 + 240 if max = b v = max  max − min  HLS Colour Model  HLS colour model  Another model based on intuitive colour parameter  The colour space has the double-cone representation:  It uses hue (H), lightness (L) and saturation (S) as parameters.  YIQ and Related Colour Models  YIQ is the NTSC colour encoding for forming the composite video signal.  YIQ parameters:  Y= luminance  Calculated Y from the RGB equations: Y = 0.299 R + 0.587 G + 0.114 B  Chromaticity information (hue and purity) is incorporated with I and Q parameters, respectively.  Calculated by subtracting the luminance from the red and blue components of colour. I =R – Y Q = B – Y  Separate luminance or brightness from colour, because we perceive brightness ranges better than colour.  YIQ and Related Colour Models (cont.)  Transformation between RGB and YIQ colour spaces:  Transformation matrix of conversion from RGB to YIQ Y   0.299 0.587 0.114   R   I  =  0.701 − 0.587 − 0.114 ⋅ G        Q  − 0.299 − 0.587 0.886   B   Transformation matrix of conversion from YIQ to RGB:  Obtain from the inverse matrix of the RGB to YIQ conversion  R  1 1 0  Y  G  = 1 − 0.509 − 0.194 ⋅  I         B  1 0 1  Q   Lab Colour Model  Lab model uses human perception of colours to generate them.  Contains numerical values that describe all the nuances that can be recognized by the human eyes,  L = lightness (0-100); a = the green-red (+127, -128), b = blue- yellow component (+127, -128)  It is a device independent model, and describes how the colour looks and not how the colour is produced. Grayscale Colour Model  Grayscale model generates gray tones.  An image created by using 8 bits/pixel may contain 256 (8-bit gray) tones.  Each pixel has a brightness value between 0 (black) - 255 (white).  The values can be measured as percentages of black ink intensity (0% - White, 100% -Black). Comparison RGB CMY YIQ HSB HSL CMYK  Colour Selection and Applications  Graphical package provides colour capabilities in a way that aid users in making colour selections.  For example, it contains sliders and colour wheels for RGB components instead of numerical values.  Colour applications guidelines:  Displaying blue pattern next to a red pattern can cause eye fatigue=>  Prevent by separating these colour or by using colours from one-half or less of the colour hexagon in the HSV model.  Smaller number of colours produces a better looking display,  Tints and shades tend to blend better than pure hues,  Gray or complement of one of the foreground colour is usually best for background.  | COLOUR PROPERTY CSS syntax allows you to configure colors in a variety of ways: colour name: p { color: red;} hexadecimal colour value: p { color: #FF0000; } hexadecimal shorthand colour value (web-safe colours): p { color: #F00; } decimal color value (RGB triplet): p { color: rgb(255,0,0); } HSL (Hue, Saturation, and Lightness) - p { color: hsl(0, 100%, 50%); } CSS3; http://www.w3.org/TR/css3-color/#hsl-color https://meyerweb.com/eric/css/colors/ - configuring color values using different notations https://color.adobe.com/create - colour selection wheel  Colour Wheel ▪ A colour wheel = the visual representation of colours, arranged according to their chromatic relationship. ▪ Primar y colours: Colours at their basic essence; those colours that cannot be created by mixing others. ▪ Secondar y colours: Those colors achieved by a mixture of two primaries. ▪ Tertiary colours: Those colours achieved by a mixture of primary and secondar y hues. ▪ https://color.adobe.com/create  Colour Conversion  When does it occur?  when displaying an image on a monitor or a projector,  when capturing an image with a scanner, a digital camera or a camcorder,  When printing the image.  Is it achieved with quality loss.  It is done by a colour management system,  Uses colour profiles,  Profile - mathematical description of the colour space of the device.