8. Colour Management

Phillip M^cKerrow

18/8/01

Human Vision

Colour Models

RGB

Based on Tristimulus theory of vision

CMY

Cyan, Magenta, Yellow - subtractive colour model for printing

HSV

Hue, Saturation, Value - the way people perceive light

Hue is the name of the color, which places the color in its correct position in the spectrum. For example, if a color is described as blue, it is distinguished from yellow, red, green, or other colors.

Saturation refers to the degree of intensity in a color, or a color's strength. A neutral gray is considered to have zero saturation. A saturated red would have a color similar to apple red. Pink is an example of an unsaturated red.

Value (or brightness) describes differences in the intensity of light reflected from or transmitted by a color image. The hue of an object may be blue, but the terms dark and light distinguish the value, or brightness, of one object from another.

YUV 4:2:2

video colour standard

2 Lumanance values for every chroma (sub sampled)

no transparency - alpha channel

hard to render

black is Y=16

R408

new standard for video

YUV based but not sub sampled 4:4:4:4

has an alpha channel

black is Y=0

Gamut

In theory any colour can be made by adding light sources of varying intensity from the 3 primaries (RGB)

CIE colour model - XYZ

CIE chromaticity diagram

Colour Gamut defined on Chromacity Diagrram

C1-C2 - 2 primary colours - all colours along line obtained by mixing proportions of C1 and C2

C3-C4-C5 - 3 primary colours - can only generate colours inside the triangls

C1-C-C2 - complementary colours - white = correct mix of C! and C2

C = white light

C1 = combination of C and CS

Cp is nospectral colour because it is not visible

nonspectral colours are in the purple-magenta range - generate by subtracting Csp from C

Purity of colour C1 is the ration of its distance from C along the line C-Cs to the length C-Cs

RGB colour gamut

coordinates

NTSC standard, CIE Model, Colour Monitor (approx)

R (0.670, 0.330) (0.735, 0.265) (0.628, 0.346)

G (0.210, 0.710) (0.274, 0.717) (0.268, 0.588)

B (0.140, 0.080) (0.167, 0.009) (0.150, 0.070)

Measuring colour gamut

Left is colour range that humans can perceive

Right

• right triange is colour gammut of a macintosh video monitor
• left is gamut of epson photo printer

Measuring gammut with aa spectrophotometer (GretagMacbeth's EyeOne)

Black plot is Viewsonic CRT Display

Peach plot is Apple Cinema Display

Web Safe Colours

A set of 216 colours that appear the same on Windows and Macintosh video monitors

Claim is that if your restrict yourself to these colours then site will look the same on all systems.

Problems

may not handle subtle tone variations in images well

what about new display technology - lcd, plasma

Gamma

Non-linearity between the input voltage to a monitor and the output intensity

Illumination models produce a linear range of intensities - e.g. the RGB colour of (0.25,0.25,0.25) is half the intensity of the colour (0.5, 0.5, 0.5) - usually stored in a linear intensity file so that (64,64,64) is half (128,128,128)

Video monitor is nonlinear

Monitor gamma values are typically in the range 2.0 ..3.0

Typical monitor response curve, Gamma correction curve - Gamma = 2.2

Gamma correction

NTSC Video standard is 2.2

Macintosh monitor 1.8 - an already bright monitor - corrected to 2.2 to make video on Mac look like video

Windows 2.5 = CRT Gamma - no correction done

Halftone

When an output device has a limited intensity range we can create an apparent increase in the number of intensities by using multiple pixels to display each intensity value. Our eyes integrate the fine detail. Used mainly in print media. Cost is reduction in resolution

2*2 pixel grid can display 5 intensity levels

Dithering

techniques for approximating half tones without reducing resolution. basically sharing intensity with neighbour pixels - can produce ghosts particularly with facial features such as hairlines and nose outlines

error-diffusion scheme showing fraction of intensity error that is distributed

Transparency - aplha chanel

Colour Management Systems

Camera, DV camcorder, VHS video tape, printers, video monitors and lcd displays all have different colour characteristics

The role of a colour management system is to transform the image going to/coming from each device so that the colour looks the same on all devices. Usually the colour management system is used to match all devices to the output device so that WIYSIWIG

ColourSync on Macintosh is a fully developed system wide matching system - very robust

ICM is a system for Win98 and 2000 - rather meager system by comparison

Other systems require colour management as part of application

ICC Standard

1993 a standard for describing imaging colour characteristics was developed

Described with a device profile (.icc or .icm file)

Colour model maps colours using a 3D space based on how humans perceive colour - L*a*b

L*a*b has no restrictions on gamut

A device profile is created using a standardised test chart

The image of the chart is compared to a test file and sets of colour transforms are created and stored in the device provile

These transforms describe how the device responds spectrally and what its limitations in digitising colour are

A colour matching module (CMM) uses the device profile to transform colours from one device to the next

The Colour Partnership offers a service to characterise monitors

JPEG files have ICC profiles tagged along with the image data

Colour Matching

Ever wondered why some colours look good together and some don't

One way of determining how to use colours is with a colour wheel

• Primaries - Red, Blue, Yellow
• Secondaries - Orange, green violate
• Tertiaries - blue green (cool cast), yellow green (warm cast)

Cool colours on the left of red and green and warm colours on the right of red and green

Cool colours have a strong blue undertone

Warm colours have a yellow or red undertone

White and black neutralise the true colours

Use arrows to determine colour mixes

Making a colour wheel by adding % of primaries

Colour wheel program

People

Have different skin colours

so different colours work with their skin colours

Margaret has fair skin with a blue undertone

best complemented by cool, clear colours

pale and palin versus radiant

These skin colours are refered to as seasons - winter, spring. summer, autumn

Mary and Kim both have green eyes and bluue hair but

Mary has pink skin and looks best in cool colours with blue undertones

Kim has ivory skin with golden highlights and looks best in warm colours with yellow undertones

Tone of your skin comes from 3 pigments - melamin (brown), carotene (yellow) and hemoglobin (red)

A combination of these gives you your skin tone - colour under your skin

A person can wear almost any colour, it's the shade and intensity that determine whether the colours look goog

Spring - clear, delicate or bright with yellow undertones

Autumn - stronger with orange and gole undertones

Winter - clear, vivid, or icy with blue undertones

Summer - cool and soft with blue underones

Another place to look at colour matching is interior decorating

Top left shows compatible and complementary shades

Psychology of Colour

Colours effect people differently

Artists are sensitive to the link between colours and experience

Colour creates illusions that change an object's appearance

Cool colours suggest calm and serenity

Warm colours tend to be energetic and stimulating

Blue

cool clear colours of sea and sky lifts the spirits

preference of most adults - prominant shade of society's apparel

Navy blue has authority and dignity

Soft blues have a calming effect

Not usually associated with eating

the colour of denim jeans

Green

spacious colours of nature create tranquility

cool, comfortable appeal

creates neutral backgrounds

Forest green appeals to the very wealthy

• mixed with white displays clean, well bred elegance

neutralised and softened to olive combines well with brights

Olive green is used in the cammouflage patterns that are often in fashion with teenagers

Neutral

create peace and calm in harmony

create dramatic impact in contrast with splashes of vibrant colour or black

Purple

exciting and opulent, suggests wealth and royalty

create a feeling of energy, an exotic colour

difficult to combine with other colours

Lavender (purple lightened with white) is associated with delicate old age

Orchid (a red based light purple) can increase feelings of illness and nausea

Red

strong, sensuous, warm, romantic, sensual, agressive

creates pure drama

red rooms make food look very appealing and encourage eating

increases a person's pulse and awareness of surroundings

men responf favourably to yellow-based red

blue-based reds attract women's attention

Red is an advancing colour, a colour to be noticed

will focus attention

Brown

no-nonsence colour, scincere

more casual than grey, navy and black

can appear dirty and dull

Yellow

adds light and life

create cheerful energy - sunny, clean and fresh

highly visible

when used in small amounts on dark backgrounds it makes them seem more lively (pop)

as a dominant colour it creates anxiety and encourages negative responses

• inhibits minor muscle movement
• people tend to lose control of their temper more readily
• the most schizophrenic colour
• time passes more slowly

Yellow and black is a warning colour combination

Web Banner Adds

A way of attracting viewers to your site - to lure them away from the site they are on

a good add is concise

a single, focused, clear image

distill message down to essentials, eliminate distractions

must deliver message in less than 10 seconds

has to say something interesting - e.g. Free stuff, Special offer

aim is to get a gut level response - lets see what this is about

clearly indicates your brand - you want them to remember you

Include head shots - we respond positively to people's faces

Colour

Strong, clean use of colour to create focus

Use only one or two colours - multiple colours will fragment

Pick specific shades of each colour that work well together

Colour Illusions

1. Light colours advance and seem large, dark colours recede and reduce

Which rectangle looks largest?

2. Bright colours advance and seem larger

Which inner square do you see first as you look at the page?

3. Light colours placed near dark colours make dark colours seem darker and dark colours make light colours seem lighter

Which black inner rectangle looks most intense?

Which red inner rectangle looks whitest?

4. Intense colours stand out against duller backgrounds

Which inner rectangle is most prominent?

5. Large areas of dull colours are needed to balance small areas of bright colour

Which rectangle looks slimest?

6. an uninterupted flow of colour lengthens and slims shape, a horizontal division of colour shortens and widens a shape

Which rectangle looks longest and slimest?

References

Images in this lecture were scanned from the following:

1. Adobe e-zine
2. Apple Computer, 1999. Managing Color With ColorSync, ADC Developer Library.
3. Gupil, A.L. Color manual for artists, Reinhold, New York
4. Guild, T. and Wilhide, E. 1992. Tricia Guild on Color, Conran Octopus, London
5. Hearn, D. and Baker, M.P. 1997. 2nd edition, Computer Graphics, C version, Prentice Hall
6. Jackson, C. 1980. Colour me beautiful, Little Hills Press
7. Jackson, C. and Lulow, K. 1984. Colour for men, Little Hills Press
8. Reid, D. 1999. Colour management and Colorimetery for Video-Rate Microscopy, Advanced Imaging, October, pp 34- 37.
9. Reid, D. 2001. Digital Displays: Surpassing Traditional CRTs, Advanced Imaging, June, pp 38-41
10. Tate, S.L. and Edwards, M.S. 1984. The fashion coloring book, Harper and Row