Bài giảng colour management kho tài liệu bách khoa

9/7/16   Color Management Presented by Nguyen Long Giang 2016 Colour management Color Management Presented by Nguyen Long Giang 2016 Content •  Light and colour •  Colour management •  Role of paper in colour management •  Standardised printing Color Management Presented by Nguyen Long Giang 2016 Light and colour Color  Management  -­‐  Nguyen  Long  Giang     9/7/16   Color Management Presented by Nguyen Long Giang 2016 World of Colour - psychological •  Human visual system can discriminate about •  200 hues •  500 levels of brightness •  20 levels of saturation •  € Altogether, we can distinguish million different colours •  However, we only have about 7500 words for different colours •  It is not sure that people see colours in the same way – they may just have learned to call certain colours by certain names Color Management Presented by Nguyen Long Giang 2016 World of colour - technical •  Light is electromagnetic radiation pm nm um m 700 nm 400 nm Color Management cm Presented by Nguyen Long Giang 2016 Illumination and reflection Color  Management  -­‐  Nguyen  Long  Giang     9/7/16   Color Management Presented by Nguyen Long Giang 2016 Illumination affects colours Under incandescent illumination Under daylight illumination Color Management Presented by Nguyen Long Giang 2016 Creating colours RGB 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 0 0 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 0 0 0 0 0 0 CMY 0 0 n Cyan Green 0 0 0 0 0 0 nm Yellow Color Management Presented by Nguyen Long Giang 2016 Colour spaces •  RGB •  used for digital images or for colour displays •  e.g scanners and digital cameras operate in RGB space •  CMYK •  used for four-colour prints Color  Management  -­‐  Nguyen  Long  Giang     9/7/16   Color Management Presented by Nguyen Long Giang 2016 Colour spaces • CIE L*a*b* colour space is the most commonly used device independent (scientific) colour space in the graphic arts industry • Colours are defined by the perception of human visual system • In CIE L*a*b* colour space colours can be defined with chroma, lightness and hue Color Management Presented by Nguyen Long Giang 2016 Three dimensions of colour • Hue • Chroma or saturation • Lightness Color Management Presented by Nguyen Long Giang 2016 How to define colour difference • The most common way to measure color difference is ∆E • It is distance between two colours in CIE Lab – colour space X • We can distinguish colour difference of ~1-2 ∆E • ∆E is not consistent against human perception across the whole gamut ∆E X • Newer formulas are suggested like ∆E2000 •  ∆ E2000 will in the future ISO standards Color  Management  -­‐  Nguyen  Long  Giang     9/7/16   Color Management Presented by Nguyen Long Giang 2016 However - don’t trust your senses Color Management Presented by Nguyen Long Giang 2016 Colour management Color Management Presented by Nguyen Long Giang 2016 The importance of colour in printed products •  Colour is used to impress and strengthen the brand of advertised products or the magazine itself •  Colour accuracy in of utmost importance in advertisements and catalogues of colour critical products such as cosmetics, clothes and furniture Color  Management  -­‐  Nguyen  Long  Giang     9/7/16   Color Management Presented by Nguyen Long Giang 2016 What is colour management •  Standardising how colour is reproduced in different stages of the workflow from camera to proofing and printing •  At the same time colour management ensures that full potential of the materials and processes in question is utilised Color Management Presented by Nguyen Long Giang 2016 The benefits of good colour management •  Quality of printed products increases •  Quality consistency increases •  Time/money savings in prepress through decreased retouching of images and remaking of proofs •  Time/money savings in printing through faster colour ok in makeready Color Management Presented by Nguyen Long Giang 2016 From colour space to colour gamut •  Colour space is a mathematical model to describe the way colour is represented •  RGB, CMYK •  L*a*b* •  Colour gamut is the volume in chosen colour space that a particular device can reproduce •  Colour gamut is usually presented in Lab colour space Color  Management  -­‐  Nguyen  Long  Giang     9/7/16   Color Management Presented by Nguyen Long Giang 2016 Colour gamuts Uncoated Offset Adobe RGB Coated Offset 150b* 100 50 a* -140 -120 -100 -80 -60 -40 -20 20 40 60 80 100 120 -50 -100 Color Management Presented by Nguyen Long Giang 2016 ICC-profiles •  ICC-profiles can •  Describe how a particular device produces colour or •  It can be a abstract colour space (like working RGB colour space) •  There are different kinds of profiles •  •  •  •  input profiles (e.g for scanners and cameras) display profiles (e.g for monitors) output profiles (e.g for proofers and presses) device link profiles (mainly for repurposing cmyk) Color Management Presented by Nguyen Long Giang 2016 ICC output profiles (press profiles) •  ICC output profiles have two functions 1.  To separate images for printing To simulate printing in proofing Color  Management  -­‐  Nguyen  Long  Giang     9/7/16   Color Management Presented by Nguyen Long Giang 2016 UCR, GCR, TAC •  UCR (under color removal) and GCR (grey component replacement) are methods to determine, how black is reproduced RGB to CMYK conversion •  UCR,GCR, TAC are defined in ICC profile •  •  •  UCR € black replaces CMY in neutral (achromatic) tones GCR € black replaces CMY also in chromatic tones TAC (total area coverage) € defines the maximum ink coverage Color Management Presented by Nguyen Long Giang 2016 Black separation Isocoated v2 300 C M Y K Maximum GCR C 0% M 33% Y 42% K 10% 7% 36% 45% 1% Color Management Presented by Nguyen Long Giang 2016 Gamut mapping with ICC-profiles original's gamut • printer's gamut 100 Different rendering intents •  Perceptual •  Relative colorimetric •  Absolute colorimetric 50 b* •  Saturation -100 -50 50 100 •  PerceptuaI •  “Moves" also the colours inside colour gamut in order to maintain the image as natural as possible -50 •  -100 a* Relative •  Does not move colours inside the gamut Color  Management  -­‐  Nguyen  Long  Giang     9/7/16   Color Management Presented by Nguyen Long Giang 2016 Gamut mapping – an example CMYK RGB Perceptual Relative Color Management Presented by Nguyen Long Giang 2016 History of proofing Flat-bed proofing Analog proofers 1st generation of digital proofers Modern digital proofers Color Management Presented by Nguyen Long Giang 2016 Proofing •  Inkjet proofers use more than colour to produce colours •  CMYK + light cyan + light magenta + different blacks •  Proofing uses absolute colorimetric € white point of the ICC profile is taken into account Color  Management  -­‐  Nguyen  Long  Giang     9/7/16   Color Management Presented by Nguyen Long Giang 2016 Proofing •  How to match paper shade and light tones in proofing •  Use same paper in proofing as in printing •  Use proofing paper with similar shade and optical brightener amounts •  Simulate paper shade in proofing with ink •  Most common way in modern inkjet proofing b* a* -7 -5 -3 -1 -1 Proofing paper shade -3 -5 Color Management Printing paper shade Presented by Nguyen Long Giang 2016 Simplified workflow Color Management Presented by Nguyen Long Giang 2016 Role of paper in colour management Color  Management  -­‐  Nguyen  Long  Giang   10   9/7/16   Color Management Presented by Nguyen Long Giang 2016 Significance of paper in colour management Dot gain Colour gamut / paper shade 100 b* 50 -100 -50 50 100 -50 -100 a* Influenced mainly by paper! Color Management Presented by Nguyen Long Giang 2016 Colour gamut •  Paper defines reachable print density level i.e colour gamut •  smooth and dense paper can reach higher print densities than rough and porous Coated paper Uncoated paper Color Management Presented by Nguyen Long Giang 2016 Paper shade and luminance MWC 96 Hi-brite LWC 95 WFC PT1 94 MWC MFC Hi-brite LWC Std LWC 93 MFC * L SC WFC 92 PT3 -6 -4 -2 -2 91 90 89 -4 SC Std LWC Spektrolino D50, 2°, white backing -6 Color  Management  -­‐  Nguyen  Long  Giang   11   9/7/16   Color Management Presented by Nguyen Long Giang 2016 Paper – the 5th colour Color Management Presented by Nguyen Long Giang 2016 Dot gain (TVI, Tone Value Increase) •  Dot gain is an increase in the area of the halftone dot 40% dot on plate 60% dot on paper € 20% dot gain Dot gain curves Color Management Presented by Nguyen Long Giang 2016 Influence of paper on TVI •  Surface of the paper (mainly roughness) defines dot gain properties of the paper Double coated fine paper High brightness LWC Film coated LWC Color  Management  -­‐  Nguyen  Long  Giang   12   9/7/16   Color Management Presented by Nguyen Long Giang 2016 Roughness and dot gain 4,5 Matt • Matte-coated in the standard actually refers to silk grades (semimatt) •  Difference between silk and glossy grades is much smaller than with "true" matt grades Roughness - PPS 3,5 2,5 Silk 1,5 Glossy 0,5 0 20 40 Paper gloss, % Color Management 60 80 Presented by Nguyen Long Giang 2016 Paper classification – shade vs dot gain properties Ink demand (dot gain - TVI) increases UWF SC MFC Std LWC Hi-brite LWC MWC WFC Paper shade and brightness Color Management Presented by Nguyen Long Giang 2016 Paper classification in ISO 12647-2 •  Classification in 2004 version is almost the same as in 1996 version •  New developments at ISO TC 130 coming up Color  Management  -­‐  Nguyen  Long  Giang   13   9/7/16   Color Management Presented by Nguyen Long Giang 2016 ECI Standard ICC-profiles Color Management Presented by Nguyen Long Giang 2016 Classification examples Color Management Presented by Nguyen Long Giang 2016 Standardised printing Color  Management  -­‐  Nguyen  Long  Giang   14   9/7/16   Color Management Presented by Nguyen Long Giang 2016 ISO 12647-2:2004 Paper groups Lab targets for CMYK, RGB 3.  Dot gain targets 4.  Tolerances for Lab, dot gain Color Management Presented by Nguyen Long Giang 2016 Paper classification in ISO 12647-2 •  Classification in 2004 version is almost the same as in 1996 version •  New developments at ISO TC 130 coming up Color Management Presented by Nguyen Long Giang 2016 ISO 12647-2:2004 Paper groups Lab targets for CMYK, RGB 3.  Dot gain targets 4.  Tolerances for Lab, dot gain Color  Management  -­‐  Nguyen  Long  Giang   15   9/7/16   Color Management Presented by Nguyen Long Giang 2016 Printing to Lab-targets •  Using Lab-targets to calculate correct print density 12 10 b -120 -100 -80 -60 -40 -2 -20 0 80 100 120 -40 -60 -80 -100 -120 a Color Management Presented by Nguyen Long Giang 2016 ISO 12647-2:2004 Paper groups Lab targets for CMYK, RGB 3.  Dot gain targets 4.  Tolerances for Lab, dot gain Color Management Presented by Nguyen Long Giang 2016 Dot gain •  Dot gain is influenced by •  Printing ink •  Ink concentration, tack, fount emulsion •  Paper •  Roughness, porosity € ink demand •  Press conditions •  Nip pressure, construction, etc •  Print density level •  Printing plate € screen ruling / algorhitms •  Process disturbancies •  Doubling, slurring Color  Management  -­‐  Nguyen  Long  Giang   16   9/7/16   Color Management Presented by Nguyen Long Giang 2016 Dot gain vs screen ruling •  30% Dot gain increases with increasing screen ruling 25% •  Difference can be compensated in CTP dot gain 20% 15% 10% 5% 0% 0% 20% 40% 60% 80% 100% dotpercentage 52 l/cm 130 l/inch Color Management 60 l/cm 150 l/inch 70 l/cm 175 l/inch 80 l/cm 200 l/inch Presented by Nguyen Long Giang 90 l/cm 225 l/inch 2016 CTP compensation •  Not only level but also shape of the dot gain curve is adjusted •  Usually iterative process to find average level of a certain press •  However paper and ink changes should only affect level •  should be quite easy to implementate Color Management Presented by Nguyen Long Giang 2016 Earlier case - after compensation •  Not a perfect match 30 % •  Some fine tuning needed 25 % dot gain 20 % 15 % 10 % 5% 0% 0% 52 l/cm 130 l/inch 20 % 40 % 60 % dot percentage 60 l/cm 150 l/inch 70 l/cm 175 l/inch 80 % 80 l/cm 200 l/inch Color  Management  -­‐  Nguyen  Long  Giang   100 % 90 l/cm 225 l/inch 17   9/7/16   Color Management Presented by Nguyen Long Giang 2016 How to control dot gain in CM workflow dot gain is = influenced Color Management Presented by Nguyen Long Giang 2016 Standardised printing Find target Lab-values (ISO 12647-2) and corresponding print density levels Compensate dot gain (TVI) according to ISO 12647-2 K70 CMY70 K50 CMY50 K30 CMY30 Print to pre-defined print densities and fine-tune print result by using gray balance patches Color Management Presented by Nguyen Long Giang 2016 Some examples on ink adjustments Color  Management  -­‐  Nguyen  Long  Giang   18   9/7/16   Color Management Density too low Color Management Dot gain too low Color Management Target print conditions Presented by Nguyen Long Giang Normal density 2016 Density too high Presented by Nguyen Long Giang Normal dot gain 2016 Dot gain too high Presented by Nguyen Long Giang Magenta dot gain too high 2016 Magenta print density decreased Color  Management  -­‐  Nguyen  Long  Giang   19   ... through faster colour ok in makeready Color Management Presented by Nguyen Long Giang 2016 From colour space to colour gamut •  Colour space is a mathematical model to describe the way colour is represented... time colour management ensures that full potential of the materials and processes in question is utilised Color Management Presented by Nguyen Long Giang 2016 The benefits of good colour management. .. CMYK •  used for four -colour prints Color Management  -­‐  Nguyen  Long  Giang     9/7/16   Color Management Presented by Nguyen Long Giang 2016 Colour spaces • CIE L*a*b* colour space is the