PART-TIME SHORT-COURSE Graphic design Session 04: Working method Notes: © Neil Angove | February 2010 | Page 59
TACky print output ? When preparing a document for print, colour-control is not black and white.
Total Ink Limits A colour-managed workflow extends beyond device-calibration, deviceprofiling and application-synchronisation. In order to print a document successfully, adaptations must be made to document CMYK colour values which, of course, translate into dots of ink on a substrate. The offset-litho printing method places certain constraints on allowable densities of colour. The reason for this is that ink is designed to adhere most successfully to the print substrate and less so to other inks. A large concentration of ink in any given area of an impression can cause problems with adhesion and drying. In addition to this, maintaining consistent dense colour throughout a press-run is made more difficult, due to fluctuations in ink-flow, in temperature and humidity and the presence of less-densely inked areas within a document. Different print technologies impose different limitations on maximum achievable ink-concentrations. That maximum is known as the Total Ink Limit ( TIL ) or Total Ink Coverage ( TIC ) or Total Area Coverage ( TAC ). When combining C+M+Y+K in any given area of a document, care must be taken to not exceed the TAC-value for the designated printing process. For example C=100 + M=100 + Y=100 + K=100 results in a TAC of 400%, and is beyond the acceptable limit of most offset-litho presses.
TAC-limit values vary from printer to printer, press to press, ink-type to ink-type substrate to substrate and technology to technology. The list indicated below is intended for general guidance; it is recommended that specific values are acquired directly from the printprofessional contracted for a given job: Newsprint web-offset: TAC 240% – 280% General web-offset : TAC 280% – 310% Uncoated sheet-fed offset : TAC 280% – 300% General coated sheet-fed offset : TAC 300% – 330% Specialist coated sheet-fed offset : TAC 320% – 340%
Most pictures contain dark tones, commonly formed by shadow areas. It is within these areas that ink concentrations are at their greatest and TAC-value limits need to be applied. How black is black ? Within the CMYK colour-space, K is not the darkest black. The darkest possible black will be a combination of C+M+Y+K equivalent to R=0+G=0+B=0. These combinations of C+M+Y+K will differ according to whichever output-device profile is assigned to any given CMYK picture. An indication of this phenomenon is shown below where R0G0B0 is the datum black: Offset-lithography Coated FOGRA39 : C 91 M 79 Y 62 K 97 ; Total ink = 329 % Uncoated FOGRA29 : C 96 M 70 Y 46 K 86 ; Total ink = 298 % Web-coated : C 75 M 68 Y 67 K 90 ; Total ink = 300 % Web-uncoated : C 63 M 52 Y 50 K 95 ; Total ink = 260 % Desk-top bubble-jet Canon i9950 SP1 : C 93 M 81 Y 57 K 93 ; Total ink = 324 % Canon i9950 MP1 : C 89 M 79 Y 61 K 97 ; Total ink = 326 %
DESIGN PRINCIPLES Total ink limits and neutrality PAGE 1 of 4
PART-TIME SHORT-COURSE Graphic design Session 04: Working method Notes: © Neil Angove | February 2010 | Page 60
The TAC-values for blacks on page 59 are automatically constrained by the assigned profile, such that a picture’s darkest shadows – even where they might actually contain the densest black tone – would not exceed the output device’s capability. Rich black Each CMYK black mix listed on page 59 is, in fact, the densest rich black achievable by the respective output-device. Each has a limitation, however, in that the respective device’s total ink limit is reached in each case. This can be a problem where a user creates a rich black manually – e.g. a black graphic within Quark XPress or Adobe InDesign – using these same CMYK numbers. The reason being, that documents created in such applications usually have varying tonal values or finer details in other graphics, even on the same page. A press operator would have difficulties running ink densely enough to maintain such a rich black while, at the same time, running ink less densely to prevent over-inking in areas of finer detail, or to prevent excessive dotgain in lighter tonal values, or to prevent ‘offsetting’ onto other pages. Such a pressoperator’s ideal scenario would be for each CMYK number to be small enough to require the minimal ink-flow. The list of user-defined rich blacks indicated below is intended for general guidance. It is recommended that specific values are acquired directly from the printprofessional contracted for a given job. Note that the total-ink values are considerably less than the maximum TAC-values shown on page 59. The CMY
values may alternatively be applied as an underpin colour-mix, where the user may prefer to overprint K separately, in which case a press operator may prefer to run K at 100% due to the likely presence of text within the same document. Where all four colours are mixed together, the lower K values reflect the greater dot-gain characteristic unique to black ink, particularly where a press operator is likely to run K densely enough for text: Coated FOGRA39 Neutral rich black : C 28 M 21 Y 22 K 95 ; Total ink = 166 % Cool rich black : C 30 K 95 ; Total ink = 125 % Uncoated FOGRA29 Neutral rich black : C 26 M 19 Y 18 K 90 ; Total ink = 153 % Cool rich black : C 30 K 90 ; Total ink = 120 %
How neutral is neutral ? In an ideal print scenario, CMY would be manufactured absolutely purely, such that C=100+M=100+Y=100 would completely absorb all wavelengths of light and reflect nothing back – giving a printed impression which is pure black. In this ideal world, any equal-percentage mix of C+M+Y would produce an absolute neutral tone, e.g. C=50+M=50+Y=50 would result in a perfectly-neutral mid-grey. However, such is not the case; in fact, Cyan generally adds impurity to a colour mix – reflecting higher red-wavelengths than is desirable. As such a higher value of C than M and Y is required to produce neutrality, as can be seen in the colour numbers associated with each black listed on page 59, even with K-values added. Similarly, an absolute mid grey derived from R=128+G=128+B=128 equates to
DESIGN PRINCIPLES Total ink limits and neutrality PAGE 2 of 4
PART-TIME SHORT-COURSE Graphic design Session 04: Working method Notes: © Neil Angove | February 2010 | Page 61
C=66+M=53+Y=53. Note the additional 3% values in M and Y is far exceeded by the additional 16% in C. This Cyan ‘red-hue’ also makes pure blues difficult to achieve, however, Photoshop’s Selective Colour adjustments allow the user to reduce or remove magenta from the Cyan channel, thus compensating for the red-reflecting characteristic.
UCR and GCR The relationship between RGB and CMYK may be further exploited where optimisation of on-press ink-flow and inksaving is desirable. Below is a re-iteration of neutral mid-grey and black and its equivalent CMY ( no K added ) mix: GREY : R 128 G 128 B 128 = C 66 M 53 Y 53 BLACK : R 0 G 0 B 0 = C 100 M 100 Y 100
It has been demonstrated that high inkconcentration mixes are undesirable. A press operator would prefer to use the minimum possible ink to achieve a maximum possible effect. Where the three CMY values combine to form shadow areas of a picture, it is difficult to maintain neutrality during a press-run because of ink-flow fluctuations, consequently a colour-caste may arise. Reducing any one of those colours to ease Coated FOGRA39 press-run operations would lighten the BLACK : R 0 G 0 B 0 = C 91 M 79 Y 62 K 97 shadows undesirably. Increasing their GREY : R 128 G 128 B 128 = C 49 M 38 Y 38 K 20 values would not achieve sufficient tonal WHITE : R 255 G 255 B 255 = C 0 M 0 Y 0 K 0 density. Even the maximum values shown above for CMY, in fact result in a darkUnoated FOGRA29 grey shadow, rather than in black. BLACK : R 0 G 0 B 0 = C 96 M 70 Y 46 K 86 Consequently, K is added to achieve the GREY : R 128 G 128 B 128 = C 51 M 43 Y 40 K 6 desired shadow density, but the result is a WHITE : R 255 G 255 B 255 = C 0 M 0 Y 0 K 0 high concentration of ink, difficult to Where a picture has a colour-caste – i.e. its manage, and which may exceed the TACneutral tones have an undesirable hue limit for a given output device. deviating them away from true neutrality – Introducing sufficient K to achieve a user may correct this deviation by shadow depth, then reducing the CMYidentifying key shadow, mid-tone and mix to achieve a lower combined ink highlight areas and equalising RGB values, density is known as Under Colour Removal using Photoshop’s Levels adjustments: e.g. ( UCR ). An example is given below for a yellow-green caste mid grey of R128 R64 G64 B64 ( 75% grey-tonality ) G136 B119 may be corrected back to ORIGINAL GREY : C 66 M 57 Y 54 K 58 ; Total ink = 235 % neutral R128 G128 B128. The relationship between RGB and CMYK may be usefully exploited where neutrality in shadows, highlights and mid-tones is desirable. Wherever all three RGB numbers are equal in any given pixel value, then that pixel will be absolutely neutral grey. The list below demonstrates a typical RGB – CMYK relationship for ISO 12647-2:2004 [FOGRA] neutral black, mid-grey and white:
UCR GREY : C 53 M 44 Y 41 K 70 ; Total ink = 208 %
DESIGN PRINCIPLES Total ink limits and neutrality PAGE 3 of 4
PART-TIME SHORT-COURSE Graphic design Session 04: Working method Notes: © Neil Angove | February 2010 | Page 62
Grey Component Replacement ( GCR ) also has the effect of reducing the concentration of ink and may be applied to any neutral tone within a picture. Where the CMY values combine to form greys, a press operator may prefer to replace that grey component with black in order to maintain all neutral tones more easily. Because K is tonally more dense than C, M, or Y, adding K reduces the concentration of ink – making the press operation easier. The extent to which K replaces CMY greys varies according to: the relative density of grey in any given tone – e.g. light grey = less K replacement than dark grey; the method by which a replacement is calculated – percentage removal of C, M, or Y replaced by equivalent percentage of K; or datum curve – where K replaces C, M, or Y at applicable tonal-density points; the intensity of K – from light where small amounts of K replace CMY, retaining most chromaticity ( colour ), to maximum where great amounts of K replace CMY, resulting in least chromaticity and high neutrality. There are various schools of thought about which method of GCR is preferred, however a prevailing consensus exists where ease of press operations, coupled with excellence in print reproduction is a desirable objective. Generally, if low inkdensities were achievable without compromising chromaticity and neutrality, then such a colour-managed workflow is to be recommended. Photoshop offers various tools which make such a working method possible – most
notably GCR and UCR dialogues within Colour Settings and Levels, Curves and Selective Colour within Image > Adjustments. Such colour-management is most effective when applied on a picture-by-picture basis, making individual picture optimisation an important component of a colour-managed workflow. How white is white ? Achieving neutrality in white is also desirable when correcting a colour-caste. There are, typically, two kinds of white, or highlight, in a picture: Specular – derived from high concentrations of light, e.g. pinpoint light sources and high-key reflection points White surface – where some tonality and/ or texture is expected Specular highlights are often reproduced as the whitest-possible white, where no RGB or CMYK values exist. The effect allows the natural paper white to be revealed in such parts of a picture : SPECULAR WHITE : R 255 G 255 B 255 / C 0 M 0 Y 0 K 0
White-surface highlights require some RGB or CMYK values. The minimum recommended values are indicated in the list below. Most offset-litho printers would have difficulty printing a halftone screen less than 5%: Offset-lithography : WHITE SURFACE Coated FOGRA39 : R 241 G 241 B 241 / C 6 M 5 Y 5 K 0 Uncoated FOGRA29 : R 237 G 237 B 237 / C 7 M 5 Y 5 K 0 Desk-top bubble-jet : WHITE SURFACE Canon i9950 SP1 : R 240 G 240 B 240 / C 4 M 5 Y 4 K 0 Canon i9950 MP1 : R 235 G 235 B 235 / C 4 M 4 Y 4 K 0
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PART-TIME SHORT-COURSE Graphic design Session 05: Rendering with intent Notes: © Neil Angove | February 2010 | Page 63
Optimising pictures Maximising the chromatic and tonal ranges and removing colour-castes from the neutral areas of a picture may be achieved using Levels and Selective Colour image adjustments in Photoshop, Black, white and neutral points When R, G and B values are equal for a given cluster of pixels, their consequent tonality forms a true neutral. Adjusting shadows and highlights to achieve equal RGB values will remove an unwanted colour-caste throughout a picture’s tonal range. Consider the following points : Where is the darkest tonal value ? Should the darkest tone be neutral and not contain a false colour-caste ? What should be the TAC-value in this darkest tone with respect to the capability of the output device ? Do the highlights comprise Surface whites and / or Specular whites ? Should a Surface-white highlight-tone be truly neutral and not contain a false colour-caste ? What should be the minimum printable ink percentage in this highlight-tone with respect to the capability of the output device ? What is the chromatic characteristic of the picture : saturated or ‘colourful’ ; neutral ; high colour-caste ? What is the tonal charactersistic of the picture : greater trend to the mid-tones ; to the shadows, or ; to the highlight areas ? Consequently, depending on the nature of the picture, optimising its tonal-range will enhance its chromaticity and tonal-clarity, and will reduce the amount of ink and its drying-time in the print-process. A suggested method for optimising images with neutral shadow and highlight areas is indicated below. ( For pictures with a high
colour-caste [ e.g sunsets ], applying sample points in shadow and highlight areas is useful for determining TAC-value and Surface-whites. ) NEUTRAL BLACK AND WHITE POINTS. Using Adobe Photoshop for RGB source image : 01. select Colour Sampler Tool. Select ‘Sample Size: 3 by 3 Average’ ( top-left of screen ) 02. select Window > Info 03. select Image > Adjustments > Threshold 04. drag slider to left until only the largest cluster of darkest pixels remain visible. Remember location. Cancel Threshold window ( do not OK ) 05. using the Colour Sampler Tool, click a #1 sample point within that largest cluster of darkest pixels. Info palette shows sampled colour values for the pixels at that point. Click the ‘eye-dropper’ icon below #1 in this palette to select sampled colour value options, e.g. RGB, CMYK, or Total Ink 06. select Image > Adjustments > Threshold 07. drag slider to right until only the largest cluster of lightest pixels remain visible. Remember location. Cancel Threshold window ( do not OK ) 08. using the Colour Sampler Tool, click a #2 sample point within that largest cluster of lightest pixels. Info palette shows sampled colour values for the pixels at that point. Click the ‘eye-dropper’ icon below #2 in this palette to select sampled colour value options, e.g. RGB, CMYK, or Total Ink 09. within Info palette Click the ‘eye-dropper’ icon below #1 then select RGB value 10. select Image > Adjustments > Levels 11. ‘Channel’ gives options for selecting either Red, Green or Blue channel in isolation from each other. Select relevant colour Channel. Using the left-hand Input Levels or Output Levels slider, adjust pertinent colour in order to result in equal values for Red, Green and Blue as indicated at #1 within the Info palette. Select each Channel in turn until all values are equal, resulting in an absolute neutral shadow within the largest cluster of darkest pixels
DESIGN for PRINT Picture optimisation PAGE 1 of 2
PART-TIME SHORT-COURSE Graphic design Session 05: Rendering with intent Notes: © Neil Angove | February 2010 | Page 64
12. Refer to RGB levels indicated at #2 in the Info palette. In
15b. keeping Levels window open as part of the same
the Levels window, ‘Channel’ gives options for selecting
process as adjusting shadow and highlight values,
either the Red, Green or Blue channel in isolation from
select RGB combined Channel to adjust all colours in the
each other. Select relevant colour Channel. Using the
mid-tone range. Adjust Input Levels mid-point slider from
right-hand Input Levels or Output Levels slider, adjust per-
reference value 1.00 to the right. Suggested values range
tinent colour in order to result in equal values for Red,
between 1.00 and 0.50 according to a user’s preference )
Green and Blue as indicated at #2 within the Info palette. Select each Channel in turn until all values are equal, resulting in an absolute neutral white surface highlight within the largest cluster of lightest pixels 13. keeping Levels window open, within Info palette click the ‘eye-dropper’ icon below #1 then select ∑ (Total Ink) value 14. within Levels window, select RGB combined Channel to adjust all colours at #1 in order to result in a ∑ no greater than the maximum Total Ink percentage for the output device ( usually, 300% – 320% [see p 59] ). 13. keeping Levels window open, within Info palette click the ‘eye-dropper’ icon below #2 then select CMYK value 14. within Levels window, select RGB combined Channel to
For high-chromaticity ( colourful or saturated ) images, it is advisable to reduce the risk of ‘muddying’ bright colours due to the uniquely-higher dot-gain characteristic of K. The technique below is best applied to CMYK pictures. ( RGB adjustments to the ‘Black’ channel [ which relates to the darkto-light L-axis in L*,a*,b* ] requires more subtle adjustments.) An effective method for CMYK pictures is as follows: 01. select Image > Adjustments > Selective Colour
adjust all colours at #2 in order to result in CMYK values
02. select Method: Absolute ( for RGB select Relative )
which achieve the minimum recommended percentage for
03. ‘Colours’ gives options for selecting chromatic tonal-
the output device in at least three of the CMYK values for
ranges of either Reds, Yellows, Greens, Cyans, Blues,
that given output device ( usually, 4% – 5% [see p 62] ).
Magentas, Whites, Neutrals or Blacks in isolation from
For images where tones cluster towards the shadow- or mid-tones, it is advisable to reduce the risk of clipping within subtle tonal transitions, as follows: 15a. keeping Levels window open as part of the same process as adjusting shadow and highlight values, select RGB combined Channel to adjust all colours in the mid-tone range. Adjust Input Levels mid-point slider from reference value 1.00 to the left. Suggested values range between 1.00 and 1.50 according to a user’s preference
( Alternatively, for high-key images where tones cluster towards highlights, it is advisable to intensify the mid-tone range to reduce the risk of clipping within subtle highlight tonal transitions, as follows:
each other. The Cyan, Magenta, Yellow and Black channels affect the degree of influence each of these fourcolour-process primaries has upon each chromatic tonal-range of the pertinent picture 04. select each chromatic tonal-range option at a time, beginning with Red. Do not apply this method to Whites, Neutrals or Blacks. Move the Black process-colour slider to the left to reduce or remove its influence from each colour. CMYK pictures may require the full removal of Black ( –100 ) to result in a highly-saturated chromatic range. RGB pictures require less-robust Black reduction
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