Use of DeviceLink Profiles for graphic industries

Transcription

1 TRICHON Amélie PFE 2006/2007 1

2 SUMMARY Abstract 6 1 PART ONE: BIBLIOGRAPHIC RESEARCH Colorimetry basic knowledge Color and vision CIE color systems Definitions of E Color Management Why do we need Color Management? Definition ICC Profiles ICC organization Definition and interest Rendering intent Content of an ICC profile How to create an ICC Profile ICC DeviceLink Profiles Definition Advantages Disadvantages Dynamic DeviceLink Profile (Dynamic DVLP) Defintition Advantages and disadvantages Black generation UCR and GCR 19 TRICHON Amélie PFE 2006/2007 2

3 1.6.2 Alwan Dynamic Maximum Black generation 20 2 PART TWO: EXPERIMENTAL STUDY Software used Alwan LinkProfiler Alwan CMYK Optimizer Alwan ColorPursuit Tests and Works Description of the project Part 1: Processing of Tiff images Part two: test on PDF 40 3 CONCLUSION 53 4 BIBLIOGRAPHY 55 5 GLOSSARY 57 6 ACKNOWLEDMENTS 58 KEY WORDS 59 ILLUSTRATIONS AND TABLES Figure 1-1: The CIELAB color space 10 Figure 1-2: Color management proposes a way to connect all devices in a graphic chain through a common space 13 Figure 1-3: The architecture of a color management system 14 Figure 1-4: The four ICC rendering intents. On these examples the triangle represents input device gamut and the circle represents the output device gamut. 15 Figure 1-5: The ISOwebcoated profile structure 16 Figure 1-6: Structure of the tables in a profile. The information provided in the header indicates which table should be used (0, 1 or 2). 17 Figure 1-7: On this example, 50% of CMY inks are replaced by 30% of black ink. TRICHON Amélie PFE 2006/2007 3

4 Next the color will be completed by a new percentage of CMY inks. 19 Figure 2-1: screenshots of LinkProfiler interface, arrows show the variable parameters in the tests 24 Figure 2-2: Alwan Color Hub interface 25 Figure 2-3: ICC Profile Processor interface 25 Figure 2-4: CMYK Optimizer Task interface 27 Figure 2-5: CMYK Optimizer DTAC tab interface 27 Figure 2-6: CMYK Optimizer Purity tab interface 28 Figure 2-7: CMYK Optimizer Vector interface 28 Figure 2-8: CMYK Optimizer Action tab interface 28 Figure 2-9: ColorPursuit interface 29 Figure 2-10: In this example, the original file (n 1) is in ISOcoatedv2 color space and the transformed file (n 2) in ISOwebcoated color space. 29 Figure 2-11: Image Comparator window calculates E difference between 2 images 30 Figure 2-12: Twenty color transformations were used to assess probable situations 31 Figure 2-13: CMYK Optimizer Check Only (Preflight) action 32 Figure 2-14: test files 33 Figure 2-15: Schematic diagram of image files processing workflow 34 Figure 2-16a: Average E on VPR 36 Figure 2-17b: Maximum E on VPR with medium GCR 36 Figure 2-18c: % of output colors within E 4 on VPR images 37 Figure 2-19: Average E for TC 3.5 chart with medium GCR 38 Figure 2-20: CMYK on TC 3.5 with the GCR (2.4) settings (series 1) 40 Figure 2-21: PDF test form created for the tests (left: page1; right: page2) 41 Figure 2-22: PDF test form elements 41 Figure 2-23: PDF testform processing workflow 42 Figure 2-24: Medium E 94 on car image from PDF test form with color matching settings 44 Figure 2-25: Medium E 94 on car image from PDF test form with Dynamic Maximum Black settings. 44 Figure 2-26: Medium E 94 on TC 3.5 from PDF test form with color matching settings 45 Figure 2-27: Medium E 94 on Medienkeil from PDF test form with color matching settings 45 TRICHON Amélie PFE 2006/2007 4

5 Figure 2-28: Medium E 94 on TC 3.5 from PDF test form with Dynamic Maximum Black settings 46 Figure 2-29: Medium E 94 on Medienkeil from PDF test form with Dynamic Maximum Black settings 46 Figure 2-30: Positions of the test points on images from page 1 of PDF test form 47 Figure 2-31: Medium E on images from page1 of PDF test form with color matching settings 48 Figure 2-32: Medium E on images from page1 of PDF test form with Dynamic Maximum Black settings 48 Figure 2-33: Results of ink saving test on PDF test form for color matching settings 49 Figure 2-34: Results of ink saving test on PDF test form for color matching settings 49 Figure 2-35: Results of ink saving test on PDF test form for Dynamic Maximum black settings 50 Figure 2-36: Results of ink saving test on PDF test form for Dynamic Maximum black settings 50 Figure 2-37: Results of ink saving test on car image from PDF test form for color matching settings 51 Figure 2-38: Results of ink saving test on car image from PDF test form for Dynamic Maximum Black settings 51 TRICHON Amélie PFE 2006/2007 5

6 Abstract Nowadays ICC color management is widely used. The majority of graphic industry operators use ICC profiles to achieve predictable and consistent colors. Nevertheless, ICC Device Profiles transformations do not take into consideration some printability issues. An ICC Device Profile transformation can convert a source CMYK file to another CMYK destination space for example. Input CMYK pixel values are converted to LAB using the source device profile and then the obtained LAB values are converted to destination CMYK values by means of the destination device profile. This operation can create problems on the press because specific information can be lost. For example, an original black text will be converted in four colors (CMYK) like all the others elements of the document, which will complicate registration on press. To avoid this kind of printability related problems, ICC DeviceLink Profiles (DVLPs) can be used. This is a specific type of ICC profiles, which is built by connecting/concatenating two ICC Device profiles. By using DeviceLink profiles (DVLPs) we can apply color transformations on CMYK data while paying attention to printability parameters like color purity, TAC (Total Area Coverage), black generation etc... ICC DVLPs (DeviceLink Profiles) have been used by the printing industry for some years now to repurpose files in order to adapt incoming data to the actual printing process properties and requirements. These profiles contain a predefined color transformation which s applied to all files and files content without differentiation. Recently, Dynamic DVLP technology has been introduced in the printing industry. Dynamic technology differ from Static (Conventional) DVLP technology in that it takes into account source file content prior to building the optimal DVLP needed for the defined color transformation. A Static DVLP applies the same transformation to all files whereas a Dynamic DVLP will check the content of the file and then optimizes it depending on its content. The aim of this project is to study the interest of Dynamic DVLPs compared to Static (Conventional) DVLPs. This study will try to determine if the technological advance of Dynamic profiles is real or not. The project will look at the benefit of using each type of DVLPs for printers in terms of color matching, print contrast and ink savings. TRICHON Amélie PFE 2006/2007 6

7 1 PART ONE: BIBLIOGRAPHIC RESEARCH This chapter deals with color management and ICC profiles. It explains what color management is, why we need to use a color management system when we want to reproduce colors and how it functions. Here you will find the definitions of an ICC profile, a Static (Conventional) DeviceLink profile (DVLP) and a Dynamic DeviceLink profile (DVLP). It is essential to be familiar with this part to understand the aim of the project and the practical studies realized. To start, it is important to know how color is created and perceived by human eye so in the first part, you will find reminders about basic colorimetry and color vision. [1] 1.1 Colorimetry basic knowledge In color science, a color is called a color stimulus; it is the result of the combination of three parts: the light source, the object which reflects the light and the human observer which receives the light. We can specify each part of the vision process by its spectral power distribution. In this section you will find the description of the role that the human eye plays in color perception and the methods to measure colors Color and vision A color stimulus is characterized by its spectral power distribution which is the product of the spectral power distribution of the light source and the spectral distribution of the object. But the human eye does not make the difference when it perceives a light which is a mixture of several wavelengths and can not analyze the signal wavelength-by-wavelength. Indeed the human vision system is only sensible to wavelengths between about 400 and 700 nanometers and is based on three kind of retinal photoreceptors (cones), which are sensible to different wavelengths, respectively short, medium and long. This phenomenon explains that we can reproduce any color from a mixture of three primary lights (in appropriate quantities), red, blue and green. So a standard observer can decompose a trichromatic signal to obtain a fourth color. Moreover, the trivalence nature of color vision shows that it is possible to perceive the same color sensation with two color stimuli having different spectral components. In this case, the two color stimuli produce the equivalent effects on the photoreceptors and the two colors appear identical. This is called metamerism. This phenomenon explains why we need to quantify and measure a color very accurately. This is why the CIE (Commission Internationale de l Eclairage) defined different systems to describe a color CIE color systems The first mathematical model proposed by the CIE, is based on three monochromatic TRICHON Amélie PFE 2006/2007 7

8 primary lights: red, green and blue, fixed arbitrarily. L R = 700 nm L G = 546 nm L B = 436 nm But in this model, the white is an equal mixture of the three components but we determine visually that theoretical white point is: 1.1. R= Cd/m 1.3. G=4,591 Cd/m B= 0,059 Cd/m So we determine the RGB system: 1.4. R= 1.5. L R 1.6. G= L V / 4,59 B= L B. 0,059 This system is practical because based on real mechanism of vision but it leads to negative components especially for very saturate colors. So in 1931, the CIE decide to create a new system with imaginary components that are linear functions of the RGB system. This is the CIE XYZ 1931 system: X= 2.77 R G B Y= R G B Z= G B These tristimulus values can also be describe differently by summing the products of the object and light source over the visible wavelengths ( ): TRICHON Amélie PFE 2006/2007 8

9 X = Y = Z = R x d visible visible visible R y d R z d where R represents the spectral reflectance of the reflective object; the spectral power distribution of the light source and X, Y and Z the color matching functions of the CIE Standard Colorimetric Observer. Then we try to build mathematic systems, which can be represented graphically and where the distance between two colors represents the colorimetric distance. If we want representing the XYZ components we need to calculate the reduced x and y coordinates: x = y = X X + Y + Z Y X + Y + Z But the CIEXYZ 1931 system has disadvantages. Indeed the calculated colorimetric differences do not match with the real difference perceived by the observer. The research allows adjusting other systems, more adapted. Among these, we can quote the CIELuv or the CIEL * a * b * 1976, the most used currently. The CIEL * a * b * 1976 system is built from the CIEXYZ 1931 system: L * = 116 Y 3 16 Yo 1 1 X a * 3 Y 3 = 500 Xo Yo 1 1 Y b * 3 Z 3 = 200 Yo Zo 1 With X,Y,Z the CIEXYZ1931 coordinates of the color and Xo,Yo,Zo the CIEXYZ 1931 coordinates of the light source. TRICHON Amélie PFE 2006/2007 9

10 Finally, the sister of the CIELAB system is known as CIELCH system where LCH means Lightness, Chroma and Hue. L * has the same signification than in CIELAB. C * is the chroma coordinate, more the point is far from the center and more the color is saturated. And h is the hue angle, expressed in degrees where 0 is for red, 90 for yellow, 180 for green and 270 for blue. CIELAB and CIELCH systems share the same color space. The only difference is that CIELAB specifies a position on a rectangular grid, although CIELCH use cylindrical coordinates. L * = L * LAB 1 C * ab = (a * + b * 2 ) hab * = arctan( b* a * ) Figure 1-1: The CIELAB color space Definitions of E So, we have shown that there are many different ways to describe and quantify a color. That is why there are also several different methods to quantify a difference between colors. TRICHON Amélie PFE 2006/

11 a) E 76 A difference of color is actually expressed in the CIELAB 1976 system: *2 *2 *2 E76 = L + a + b with Lab coordinates Or *2 *2 *2 E76 = L + C + H with LCh coordinates. Where, for the difference between color 1 and color 2, with x=l *, a *, b * and C * : x = x 2 x 1 1 H * = 2(C * 1 C * 2 ) 2 sin( h 2 ) Warning: in the formula H* h From this definition, CIE defines others E to optimize the matching between visual and calculated perception. b) E94 One is the E94, this formula will use for the tests to calculate color differences: E94 = L * C* + H* k L S L k C S C k H S H 2 With Si: ponderation factor k i : correcting factor depending of observation conditions, typically k i =1 X * = X 2 -X 1 The CIE recommendations are: S L = 1 S C = 1+0,045C * S H = 1+0,015C * TRICHON Amélie PFE 2006/

12 In practical, we consider that if: E94 < 1, there are no perceptible differences for human eye 1 < E94 < 2, it is very difficult to see a difference, only a trained eye can detect something 2 < E94 < 4, the difference is perceptible for human eye E 94> 4, we perceive a difference of color. E76 and E94 are the two mode of calculation available in Alwan s software but there are other formulas. c) E CMC (Color Mesurment Committee) ECMC = * L lsl 2 C + cs C * 2 H + SH * 2 If L*<16: S L =0,511 If L* 16: 0,04L * S L = S 1+ 0,017L * C = 0, ,0698C* and 1+ 0,013C * And we often find l=2 and c=1 TRICHON Amélie PFE 2006/

13 1.2 Color Management Why do we need Color Management? In a graphic chain an original is scanned, displayed, proofed and printed with different devices at every stage of the process. All these devices have different behaviors: as we know every imaging device has its own colorimetric characteristics and is usually used with different system: RGB for displays, digital cameras and scanners and CMYK for proofers and printers. Moreover final file can be created in RGB, in CMYK or use several color spaces in the same document in some specific cases. (cf. figure 1-2) Figure 1-2: Color management proposes a way to connect all devices in a graphic chain through a common space We absolutely need to know how to manage the different characteristics of these because if we give the same data to two different devices, two printers for example, we will not print the same color because each device has its own interpretation of the data [2], working with multiple system components necessitates a way to get predictable, consistent color [3]. On the other hand, we need to have a common translator for colors, a system which is device-independent and which will be able to quantify and compensate for any device variability. This is the role of color management. TRICHON Amélie PFE 2006/

14 1.2.2 Definition A color management system is defined as a set of methods that allows us to get consistent, predictable and reproducible colors. Side benefits of using a color management system can be the optimization of color separations and ink usage and the reduction of production costs. Every color management system is composed of several components: the reference color space called PCS ( Profile Connection Space), usually the LAB space, an input and an output space, an input profile, an output profile and a CMM (Color Management Module) which is the calculator. The tool that realizes these operations is described in ICC profiles. [4] (Cf. figure 1-3) INPUT PROFILE OUTPUT PROFILE INPUT SPACE CMM PCS CMM OUTPUT SPACE Figure 1-3: The architecture of a color management system 1.3 ICC Profiles ICC organization Eight companies founded the ICC or International Color Consortium in 1993: Adobe, Agfa, Apple, Kodak, Taligent, Microsoft, Sun and Silicon Graphics. Since 1993 more than 70 companies joined them. The ICC objective is color exchange standardization. It is a regulator body that supervises color management protocols between software vendors, equipment manufacturers and users. They established specifications for color transformations between devices. These specifications are applied in a special file format called ICC profile Definition and interest An ICC profile is a file that allows controlling color transformations. It interprets the different color pixels values, RGB for example and works out what color they actually refer to. The profile must accompany the image to allow a correct interpretation of the device dependent values. That is why every device must have a profile [5], [6], [7]. An ICC profile has a standard format and is neither vendor nor platform dependent. TRICHON Amélie PFE 2006/

15 1.3.3 Rendering intent Generally, device gamuts are not large enough to reproduce all the colors we want. That is why ICC profiles provide four ways to still do the transformations. This paragraph will describe the different ways. [8] Perceptual rendering intent (0): This mode preserves the relationship between colors but it deviates the original colors of the image. It allows having a good contrast in the image. This mode is well adapted for image or photography. Relative colorimetric rendering intent (1) In this mode, only the values, which are outside of the destination gamut, are changed. The others values are preserved. Absolute colorimetric rendering intent (1) This mode is almost the same as the relative mode except that in this case media (paper) color is taken into account Saturation rendering intent (2) Maximum saturation is assigned to each value. This mode is not widely used. Figure 1-4: The four ICC rendering intents. On these examples the triangle represents input device gamut and the circle represents the output device gamut. TRICHON Amélie PFE 2006/

16 1.3.4 Content of an ICC profile An ICC profile consists of two parts: the header and the tags (Cf. figure 1-5). The header contains information about the building of the profile: who, when, which color space, size of the file, the CMM used, type of device concerned, which table should be used. This is a standardized part which has a fixed size. The tags form the body of the profile. They refer to tables, which contain the data useful for the conversions. The size of tags varies depending on the type of device concerned by the profile (monitor, printer, proofer, scanner ) and the author of the profile. We can find two types of tags: required tags that are describing by ICC specifications for each type of profile [9] and are necessary to operate the profile. In addition you can also have optional tags that are not required to have a valid profile but are recognized by ICC too. tags header Figure 1-5: The ISOwebcoated profile structure Each profile contains 6 tables: 3 from source space to PCS and 3 from PCS to destination space. We find a table for each rendering intent: perceptual, saturation and colorimetric. We can notice that absolute and relative colorimetric mode use the same table, the only difference is the use or not of the media color (white of the paper). [2], [10] TRICHON Amélie PFE 2006/

17 Source PCS PCS Destination Perceptual A 2 B 0 B 2 A 0 Colorimetric A 2 B 1 B 2 A 1 Saturation A 2 B 2 B 2 A 2 Figure 1-6: Structure of the tables in a profile. The information provided in the header indicates which table should be used (0, 1 or 2). The profile can be embedded in the image with all formats (eps, PDF, Tiff, jpeg, bmp, gif, png, pict) except the PS format because PS has its own color profiles How to create an ICC Profile We need three steps to achieve accurate color called the Three Cs : calibration characterization conversion. [11] Calibration: We should make sure that all adjustments of devices are compliant with established specifications. We should create defined and repeatable conditions: anything that can alter colors of the image must be identified and locked-down. Characterization It is the creation of profile strictly speaking. We should now study the response of the output device depending on input values. We use a test chart: we send a reasonable sampling of color patches to the device and we measure the color we really obtain. The collected data can be used to create the corresponding tables of profiles Conversion This is the use of profiles that allows converting a file from one color space to another. TRICHON Amélie PFE 2006/

18 1.4 ICC DeviceLink Profiles Definition An ICC DeviceLink profile, or DVLP, is a special kind of profile recognized by the ICC specification, which connects two profiles and their settings together. In a DVPL there is no PCS, the original and the final spaces are directly linked. So it provides a way to avoid the problems created when we convert 4-component data like CMYK in a 3- component color space like LAB and then we reconvert in CMYK again. A DVLP is a unidirectional conversion without using a PCS. [12] Advantages A DVLP is often used for CMYK to CMYK data repurposing. It provides better control of special printability features of CMYK printing. By linking directly CMYK data, DVLPs provide a way to manage preservation of pure colors and to have a more accurate control on output GCR and TAC. Purity preservation is very useful for one-color-text or linework elements which remain easy and well registered. Improved GCR and TAC control help achieve better printability and possibly ink savings because on one hand black ink is cheaper than color inks and on the other hand DVLP allows the reduction of the quantity of ink on paper. Reminder: the TAC or Total Area Coverage is the maximal amount of overprinting ink that we can put on a media. In theory, maximum TAC is 400% but in practice, to have a better printability on the press, the maximum inking has to be limited to lower values. Some software used to create DVLPs allows you also to preserve secondary colors, achromatic colors (CMK, CYK, MYK) and 100% solid colors but these options are not provided in all software. By using a DVLP we reduce the number of operations: we need only one DVLP to convert a file from one space to another. With Device profiles two profiles are needed Disadvantages As it was said before, a DVLP is a unidirectional transformation so if you want to step back the simplest way is to make a copy of the original file. Moreover, currently DVLPs are not supported by all RIPs and applications like Adobe Photoshop. For more information about DVLP please read [2], [10] and Alwan s documentation available on their web site. TRICHON Amélie PFE 2006/

19 1.5 Dynamic DeviceLink Profile (Dynamic DVLP) Defintition The main difference between Static and Dynamic DVLP is that Alwan Dynamic DVLPs are built on the fly after image or page analysis and take into account characteristics of:. source image/page color space, black generation and TAC - destination color space or device profile - software separation settings Dynamic DVLPs aim is to optimize color separations for a given file and desination process and to ensure improved printability, improved contrast, possibly reduction of costs, while maintaining color Advantages and disadvantages The purpose of this study is also to find out the advantages as well as the advantages of Dynamic DVLPs. 1.6 Black generation UCR and GCR A fundamental part of the color conversion process is the black printer generation. We can use several algorithms for that. The most common are called UCR (Under Color Removal) applied to neutral colors only and GCR (Gray Component Replacement) applied to all colors. The aim of the operation is to replace CMY inks by black ink to extend the gamut of colors achievable in dark areas, facilitate color control on the press and save ink. Often calculations are based on addition of density. There are many ways to calculate black generation but for GCR, we obtain always a monotonic increasing function (cf. figure 1-7) Figure 1-7: On this example, 50% of CMY inks are replaced by 30% of black ink. Next the color will be completed by a new percentage of CMY inks. TRICHON Amélie PFE 2006/

20 Examples of equations you can find: a) Equation of Jonhson (1985) The total black z is given by the following formula [13]: c a z = c a 1 a 1 a + ma(1 K ) K K With m: fraction of CMY color density replaced by equivalent density of black c: total density a: three-color density (CMY) K: convergent point of the additive Yule s diagram b) Black printer model 2: The following equation includes a UCR concept too; the CMYK values after replacement are given by [14]: k ' = b min(c,m,y) c ' = c uk ' m ' = m uk ' y ' = y uk ' With 0 b 1 and 0 u 1 In this model, b and u represents respectively a black rate and a UCR rate and c,m,y, k the original dot area values Alwan Dynamic Maximum Black generation Alwan Dynamic Maximum Black option aims to use more black (and less CMY) than what is possible with conventional GCR. Alwan Dynamic Maximum Black algorithms are not public, but the princple as explained to me is the following: Dynamic Maximum Black option consists of finding, for each color (that is to say each Lab combination) represented in the destination profile the C-M-Y-K output values which will have the optimal combination of max (K), min (CMY), min ( E), TRICHON Amélie PFE 2006/

21 among all the possible solutions. This model seams to be more efficient for ink savings than conventional UCR/GCR but is not always very smooth depending on the output profile. This is the reason why Alwan strongly recommends the use of Dynamic Maximum Black option only with high quality ICC profiles and preferably, the actual printing process profile. TRICHON Amélie PFE 2006/

22 2 PART TWO: EXPERIMENTAL STUDY 2.1 Software used In this part you will find a description of the software developed by Alwan Color Expertise that I used for my study. You will find a summary of the applications and the main functions of the two software. For more complete information about software, please visit Alwan website or contact them Alwan LinkProfiler a) Definition Alwan LinkProfiler is a profiling software that allows to create Static (Conventional) DVLPs between two CMYK spaces. It allows repurposing CMYK files prepared for one CMYK space to another CMYK space. For example, it allows changing paper or press before printing. [15] b) Features LinkProfiler includes features such as dot gain correction, TAC and black generation adjustments and preservation of purities. So no additional device profiles are necessary for different kind of papers and presses (cf. figure 2-1). c) Specific features used for the test Among all the available features of the software, here are the one used in the tests and their significance: - Separation tab: you will find GCR (1.7), a medium GCR and GCR(1.0), an Dynamic Maximum Black as described in section I.6.1 and I.6.2 for each case you can define the TAC you want to obtain, the K start value representing the lightness of the color for the CIE-L value where the black generation begins; the K max value representing the maximum amount of black in the output separation. - Purity tab: options primary colors, secondary colors and 100% solid colors were used. Primary colors means that an input pure color value (C, Y, M or K) will remain a pure color on the output, but the value can change: a 60% of cyan on the input can become 63% of cyan on the output but there is no color contamination. Secondary colors is based on the same principle and applies to two pure colors. A mixture of yellow and magenta will remain a mixture of yellow and magenta on the output. 100% solid colors means that input solid areas stay unchanged after the conversion (a 100% black text will stay 100% black text). So all these options are very practical to maintain and control printability on the press. TRICHON Amélie PFE 2006/

23 To have an exhaustive description of LinkProfiler features, please refer to this report appendices or to Alwan documentation. d) LinkProfiler Interface: TRICHON Amélie PFE 2006/

24 Figure 2-1: screenshots of LinkProfiler interface, arrows show the variable parameters in the tests e) Application of DeviceLink profiles LinkProfiler allows the creation of DVLPs but does not allow applying these profiles to a file. This is why it is necessary to combine LinkProfiler with a file processing software or a RIP. I choose another software from Alwan ColorHub framework : ICC Profile Processor. We access to ICC Profile Processor by lauching Alwan ColorHub and choosing ICC Profile Processor Task in the Queue settings tab (cf. figure 2.2). DVLPs created with LinkProfiler can be selected in CMYK Color processing settings, from Default CMYK profile pop-up menu (cf. figure 2-3). TRICHON Amélie PFE 2006/

25 Figure 2-2: Alwan Color Hub interface Figure 2-3: ICC Profile Processor interface TRICHON Amélie PFE 2006/

26 2.1.2 Alwan CMYK Optimizer a) Definition CMYK Optimizer is a color and separation Preflight and Optimization software. Like LinkProfiler, CMYK Optimizer can create CMYK to CMYK ICC DVLPs. Globally, this software has the same aim as LinkProfiler but it is more sophisticated. It includes a file handling and management framework (Alwan ColorHub), offers more features and options for a sophisticated color management like managing vectors and bitmaps separately etc and the more important difference is its Dynamic TAC and color calculations. [16] b) Specific features The main difference between CMYK Optimizer and LinkProfiler DVLPs is the Dynamic nature of CMYK Optimizer DVLPs. Indeed, on the contrary of LinkProfiler DVLPs that, like all conventional DVLPs, apply the same CMYK to CMYK transformation to all input data, CMYK Optimizer does an analysis of the file to be processed and of the destination color space and profile before applying transformations. Depending on the settings of the Separation tab, DTAC (Dynamic TAC) tab and the original file separation, an optimal output TAC is calculated for each images or page file. DTAC parameters are the following: - Filter image noise : removes noise from images before TAC analysis. - Tolerate excess TAC up to : represents an area of high TAC that will be tolerated even if its TAC is superior to the target nominal TAC. Generally, this surface is small and corresponds usually to relatively small details. For example, if you print a portrait on a newspaper (TAC = 240), you can tolerate that part of the subject, such as the eyes pupils, have a TAC superior to Keep nominal TAC up to : defines the maximum area that can have the nominal TAC value. Beyond this surface, output TAC can be set to decrease below nominal TAC to avoid set-off or rippling problems. So if the file contains a very large dark area, the output TAC can be decreased accordingly. This technological advance allows adjusting and optimizing the transformation of the original file. The aim of the study is to determine if the dynamic effect has a real effect in practice or not. TRICHON Amélie PFE 2006/

27 c) CMYK Optimizer Interface Figure 2-4: CMYK Optimizer Task interface Figure 2-5: CMYK Optimizer DTAC tab interface TRICHON Amélie PFE 2006/

28 Figure 2-6: CMYK Optimizer Purity tab interface Figure 2-7: CMYK Optimizer Vector interface Figure 2-8: CMYK Optimizer Action tab interface TRICHON Amélie PFE 2006/

29 2.1.3 Alwan ColorPursuit a) Definition ColorPursuit is software that allows comparing two files numerically without printing them. You can determine E between an original file and a transformed file. This software will be used to evaluate color differences in files. [17] b) Comparison Process to open a widget and load an image to compare two images Figure 2-9: ColorPursuit interface ColorPursuit allows you to open a color or an image widget, ie a window where you can choose a color or an image and compare it ( E) with another color or image. You can also build a workflow by linking widgets, assigning the relevant ICC Profiles and RI (Rendering Intents) to simulate the result of a succession of color transformation in a workflow. For this study, all transformations will be done using LinkProfiler (Static DVLPs) and CMYK Optimizer (Dynamic DVLPs) before comparing the results using ColorPursuit. ColorPursuit does not apply any color transformation to the files. Figure 2-10: In this example, the original file (n 1) is in ISOcoatedv2 color space and the transformed file (n 2) in ISOwebcoated color space. If the ICC profiles are embedded in the files they are automatically detected and used by ColorPursuit. If loaded files do not contain embedded profiles, you should choose TRICHON Amélie PFE 2006/

30 the relevant profile in the profiles list to obtain the good softproof and calculations. Figure 2-11: Image Comparator window calculates E difference between 2 images Image Comparator calculated average E, maximum E and the percentage of pixels having a E less than the specified maximum E. 2.2 Tests and Works Description of the project a) Methodology The aim of this project is to study the interest of Dynamic DVLPs compared to Static (Conventional) DVLPs. This study will try to determine if the technological advance of Dynamic profiles is real or not. The project will look at the benefit of using each type of DVLPs for printers in terms of color matching, print contrast and ink savings. The project has been divided in three experimental parts: TRICHON Amélie PFE 2006/

31 Part1 assessment of converted and optimized Tiff images using ColoPursuit calculations Part2 assessment of converted and optimized PDF documents using ColoPursuit calculations as well as visual assessment and SpectroEye measurements on produced proofs Each experimental part and measurement series involved up to 20 different color transformations chosen from plausible real life situations in a multinational prepress or press environment. Each color transformation has an associated number to facilitate its referencement in graphics. Applied color transformations are the following: 1 isocoated_v2_ec i ISOwebcoate d 2 isocoated_v2_ec i ISOnewspaper26V4 3 isocoated_v2_ec i SWOP2006_Coated5v2 4 isocoated_v2_ec i Japan Color 2001 Coate d 5 ISOwebcoate d ISOnewspaper26V4 6 ISOwebcoate d SWOP2006_Coated5v2 7 ISOwebcoate d Japan Color 2001 Coate d 8 ISOwebcoate d isocoated_v2_ec i 9 ISOnewspaper26V4 ISOwebcoate d 10 ISOnewspaper26V4 SWOP2006_Coated5v2 11 ISOnewspaper26V4 Japan Color 2001 Coate d 12 ISOnewspaper26V4 isocoated_v2_ec i 13 SWOP2006_Coated 5 v 2 ISOwebcoate d 14 SWOP2006_Coated 5 v 2 ISOnewspaper26V4 15 SWOP2006_Coated 5 v 2 Japan Color 2001 Coate d 16 SWOP2006_Coated 5 v 2 isocoated_v2_ec i 17 Japan Color 2001 Coate d ISOwebcoate d 18 Japan Color 2001 Coate d ISOnewspaper26V4 19 Japan Color 2001 Coate d SWOP2006_Coated5v2 20 Japan Color 2001 Coate d isocoated_v2_ec i Figure 2-12: Twenty color transformations were used to assess probable situations b) Criteria of comparison i. Colorimetric accuracy assessment TRICHON Amélie PFE 2006/

32 To evaluate these transformations color accuracy, converted files were compared with their corresponding originals using: Part 1: color managed images are assessed using ColorPursuit software as described in section Part 2: color managed PDFs are assessed in 2 ways. 1- using ColorPursuit. 2- by producing proofs that were measured with GMB Spectroeye. ii. Ink savings calculations All Ink consumption and savings were done using CMYK Optimizer Ink Statistics manager intrerface and data base. To evaluate Dynamic profiles ink savings, ink statistics reports generated by CMYK Optimizer have been used (cf. figure 2-14). When Ink Consumption Statistics is chosen in CMYK Optimizer action tab (cf. figure 2-8), ink statistics including individual CMYK inks consumption before and after optimization are generated for each processed file. To evaluate conventional DVLPs ink savings, files that were processed using LinkProfiler DVLPs and ICC Profile Processor were analyzed using CMYK Optimizer Check Only (Preflight) mode.(figure 2-13) This option allows the preflighting of a file without applying any transformation to it. Corresponding preflighting report and log contain color and separation information about the file including its ink characteristics and consumption. Figure 2-13: CMYK Optimizer Check Only (Preflight) action TRICHON Amélie PFE 2006/

33 Figure 2.9 Extract of a report. Ink amounts and consumption statistics are in bold Part 1: Processing of Tiff images a) Test files Two types of images were used: GMB TC 3.5 chart including 432 patches was used for colorimetric measurements. Parts of Ugra VPR (Visual Print Reference) were selected and used for measurement and visual assessments (cf. figure 2-14). Figure 2-14: test files TRICHON Amélie PFE 2006/

34 b) Test workflow processing Figure 2-15: Schematic diagram of image files processing workflow c) Settings For this study, four series of tests with different separation options were performed: Series 1 Medium GCR Series 2 Heavy GCR Series 3 Maximum GCR Series 4 Dynamic Maximum Black (Alwan intelligent Black generation) TRICHON Amélie PFE 2006/

35 Aim of settings 1 & 2 was to study the comparative effect of different levels of conventional GCR on the color accuracy for each type of DVLP transformation, Static and Dynamic. Aim of series 3 and 4 was to study the comparative effect of Dynamic Maximum Black generation on ink savings for each type of DVLP transformation, Static and Dynamic. A maximum of 20 different Transformations were used for the test series as listed in figure A complete description of the used settings can be found in Appendix tables 1 to 5d, p.3 to 15. d) Results and interpretation i. Colorimetric accuracy assessment File Measurements: For each converted file, ColorPursuit was used to calculate the E94 difference between original and converted CMYK files. The aim of the measurements is to determine which transformation (Static or Dynamic) gives more color accurate results. ColorPursuit software calculates and displays average E94, maximum E94 as well as the percentage (in number) of pixels that have a E94 which is lower than the specified E limit. In this analysis E limit has been set to 4 because it seems to correspond to an accepted average E by the industry as well as by Standards (ISO /7). Results of series 1 (Medium GCR): Terminology: In this report, the following terms found on the graphs refer to the corresponding ICC profile/color space: Coated = ISOcoated v2 Web = ISOwebcoated SWOP = swop 2006 coated 5v News = ISOnewspaper 26v2 Japan = Japan color 2001 coated RGB = Adobe RGB 1998 TRICHON Amélie PFE 2006/

36 VPR (Visual Print Reference) images analysis: Globally, we can say that a trend emerges: for 11 cases among 20, dynamic transformations give a better (lower) E than the static/conventional transformation. E average is lower by 1.1 to 2.6, Emax is lower by 2.0 to 4.1 units and number of in gamut colors (having an individual color shift < E 4) is 3.8% to 50.8% higher with the dynamic technology for these images, depending on the color transformation. This result is most probably due to the content dependant nature of dynamic transformations which convert colors and adapt TAC taking into account differences between strongly inked images and weekly inked images, and differences between input and output color gamuts capabilities. Figure 2-16a: Average E on VPR Figure 2-17b: Maximum E on VPR with medium GCR TRICHON Amélie PFE 2006/

37 Figure 2-18c: % of output colors within E 4 on VPR images The same observations can be done for Heavy GCR color separation. For complete results, please refer to Appendix. TC 3.5 chart conversions analysis: There is hardly any difference between the two methods except for one case, Coated to News (2), where the dynamic transformation of the chart gives a slightly better result ( E average lower by 1.2) than conventional transformation, cf. figure This result which is contradictory with the images assessment result was investigated. Two possible reasons can explain this result. The first reason would be that TC 3.5 chart contains 400% TAC patches which inhibits in some cases input Dynamic TAC according to Alwan, the second being that the chart patches are too small to activate output Dynamic TAC which is surface dependant. With both dynamic processes deactivated, similar and even identical results would be expected from the two systems. In order to test the threshold of Dynamic processing, Nominal TAC area (see DTAC tab figure 1-5 page 27) was decreased until a difference was measured between Static and Dynamic processing. A difference was noticed for a setting of 1cm2 which indeed corresponds approximately to the TC 3.5 patches surface. So it seems that the assumption was probably true: the parameter Keep nominal TAC up to is behind the TRICHON Amélie PFE 2006/

38 fact that in TC 3.5 chart tests, we did not see the Dynamic effect of CMYK Optimizer. This seems to confirm that the surface of 25 cm2 was not adapted for the size of TC 3.5 patches but is probably adapted for more conventional files received by printers everyday. This may be confirmed by the following tests done on PDF test forms (para 2.2.3). Figure 2-19: Average E for TC 3.5 chart with medium GCR Conclusions of color matching assessment of images: VPR images: For VPR images, it has been possible to clearly establish that Dynamic DVLPs achieve better color accuracy than conventional DVLPs. Average and maximum E as well as gamut mapping of out of gamut colors were in favor of CMYK Optimizer Dynamic technology. TC 3.5: The conclusion regarding TC 3.5 chart is more reserved as there is hardly any colorimetric difference between the two systems except for severe gamut mapping situations like for Newspaper output. TRICHON Amélie PFE 2006/

39 ii. Ink Savings: Measurements: As described in section a.ii), CMYK Optimizer reports ink consumption of each image file including cyan, magenta, yellow and black inks separately. It compares original and converted files ink consumption and determines three values: K = K final -K initial where K i is the percentage of black plate ink coverage. CMY = CMY final -CMY initial where CMY i is the sum of percentage of cyan, yellow and magenta plates ink coverage. CMYK = CMYK final -CMYK initial where CMYK i is the sum of percentage of cyan, yellow, magenta and black plates ink coverage. Results for TC 3.5 chart analysis: TC 3.5 chart output TAC and ink savings assessment confirms in a way the colorimetric assessments. There is no clear difference between the two methods except for the transformations 2, 5, 14 and 18 (ISOnewspaper26v4 destination) which are un favor of Dynamic DVLPs. For newspaper output, TAC reduction and ink savings are higher by 7% and 3% respectively with Dynamic DVLPs. We may explain this small difference by noting that DTAC parameter applied to dynamic transformations is not effective on a test chart having 400% values and relatively small areas for dark colors, hence we cannot see the full dynamic effect on such a file. If we want to increase the effect of DTAC, we must choose a Keep nominal TAC up to value which is under 1 cm2. Despite this image specific limitation, we can observe that CMYK Optimizer achieves higher ink savings with a small gamut output profile, than do conventional DVLPs. However, we observe that the decrease of inking on the studied surface is not systematic. TRICHON Amélie PFE 2006/

40 Figure 2-20: CMYK on TC 3.5 with the GCR (2.4) settings (series 1) These results in terms of color accuracy and ink savings have been found to be practically identical with all levels of GCR: Heavy GCR, Maximum GCR and Dynamic Maximum Black Part two: test on PDF a) Testing file A PDF/X A3 test form has been created using Adobe InDesign software. The document is composed of two pages with parts for numerical evaluation (charts, control strip) and parts for visual evaluation (vignettes, images) as shown on figure To simulate a real life situation where multi-page PDF documents contain images and pages produced with different color profiles and separation options, we decided to separate each set of images with a different color profile. The test document hence contains 4 sets of 3 VPR images converted from their source RGB color space to a predefined output CMYK color space, and a car magazine cover page having an unknown color profile and separation characteristics. All PDF content characteristics and color management parameters can be found in the Appendix. TRICHON Amélie PFE 2006/

41 Figure 2-21: PDF test form created for the tests (left: page1; right: page2) Figure 2-22: PDF test form elements TRICHON Amélie PFE 2006/

42 We know that Static DVLPs loaded in a RIP will always apply the same color conversion regardless of the content of the input files. Alwan Dynamic technology applies context dependant color conversion to input files in order to generate Dynamic DVLPs on the fly prior to color conversion. The context parameters are: - input image/page color space, TAC and GCR - chosen output color space, TAC and GCR - software custom settings This will be tested in this study as our document includes elements of different color spaces and separation characteristics as shown in figure The first column of parts 1, 2, 3, 4 and parts 5, 7 and 8 will be used for numerical evaluation and the second and third columns of parts 1, 2, 3, 4 and part 6 will be used for a visual evaluation. (Cf. figure 2-22) b) Process Figure 2-23: PDF testform processing workflow TRICHON Amélie PFE 2006/

43 c) Settings Two modes of colorimetric transformations were tested in this part: the first called Color Matching with parameters of series 2 and the second called Dynamic Black generation - with the parameters of series 4 as described in part 2.2.The complete parameters used for applying DVLPs are in the Appendix in Tables 6 and 7. The aim of color matching settings is to achive maximum color accuracy hence minimum E. The aim of Dynamic Maximum Black settings is to save maximum ink on the press. The original PDF contains 3 different CMYK color spaces and an undefined color space. The output color space for the color transformations has been arbitrarily chosen to be Fogra39/ISOcoatedv2. So for the two sets of transformations, numbers 1 to 3 refer to conventional/static transformations and numbers 4 to 6 refer to Dynamic transformations. Once color managed and optimized, the files were proofed on an Epson SP 4000 inkjet digital printer, with EPSON Ultra Chrome inks, semi-mat contract proofing paper and Fogra39/ISOcoatedv2 simulation. The inkjet printer has been calibrated and characterized before proofing. Proofs were controlled and validated using Ugra/Fogra media wedge and ISO tolerances and then used for measurements and visual evaluations. d) Measurements To evaluate the performance of each type of DVLPs, two different methods were used to assess the achieved color accuracy of each one: 1- calculations done using ColorPursuit. 2- measurements using two spectrophotometers: X-Rite EyeOne/iO and Xrite 968 Evaluation of ink savings was done using Alwan ColorHub Ink Statistics Manager. e) Results and interpretation of color matching i. Colorimetric accuracy: Color Pursuit: test on files The same methodology that was used to test TIFF images in part 1 of this study, was used for PDF test files. However, for PDF files the results differed significantly from single images tests (cf. figure 2-24 and 2-25). Indeed we can see from the results shown below that CMYK Optimizer does take into TRICHON Amélie PFE 2006/

44 consideration the input colour space and separation of the different elements of the PDF page and adapts its optimization accordingly. Figure 2-24: Medium E 94 on car image from PDF test form with color matching settings Figure 2-25: Medium E 94 on car image from PDF test form with Dynamic Maximum Black settings. Moreover our other evaluation criteria confirmed this trend. Maximum E 94 and percentage of pixels which have E 94<4 after colour transformation were also in favour of Dynamic technology. We can note that we have the same level of E 94 with the two types of settings (Colour Matching and Ink Savings) so we can say that Ink savings do not affect reproduction colorimetric accuracy of optimized files. For more complete results, please refer to Fig. 9 and 15 in appendix. TRICHON Amélie PFE 2006/

45 Eye One Io: proof measurement The Eye One Io measures automatically the colorimetric values (CIELAB) of each patch on a chart and gives the results in a text file. It was used on TC 3.5 chart and Medienkeil control strip. Then, GMB Mesure tool 5.0 software was used to compare the measurements obtained from optimized files and those obtained from the original. The software gives the average E and the associated standard deviation for the total chart, the best 90% patches and the worst 10%. It gives the maximum E on the total chart and the best 90% patches too. The results show in each case that Dynamic DVLPs E is always lower than Static DVLP as you can see on figures 2-26 to Medium DeltaE 94 on TC 3.5 from PDF test form DeltaE total best 90% worst 10% Transformations with color matching settings static dynamic Figure 2-26: Medium E 94 on TC 3.5 from PDF test form with color matching settings Medium DeltaE 94 on Medienkeil chart from PDF test form 8 DeltaE total best 90% worst 10% Transformations with color matching settings static dynamic Figure 2-27: Medium E 94 on Medienkeil from PDF test form with color matching settings TRICHON Amélie PFE 2006/

46 Medium DeltaE 94 on TC 3.5 from PDF test form 7 6 DeltaE static dynamic 0 total best 90% worst 10% transformations with Dynamic Maximum Black settings Figure 2-28: Medium E 94 on TC 3.5 from PDF test form with Dynamic Maximum Black settings 8 7 Medium DeltaE 94 on Medienkeil chart from PDF test form DeltaE total best 90% worst 10% transformations with Dynamic Maximum Black settings static dynamic Figure 2-29: Medium E 94 on Medienkeil from PDF test form with Dynamic Maximum Black settings For more complete results, please refer to Fig. 10, 11, 16 and 17 in appendix. Manual spectrophotometer: proof measurement To confirm the results obtained from the charts, CIELAB values were collected from the printed images of the first column of PDF test form and from the graduated areas of the page. TRICHON Amélie PFE 2006/

47 The tested points are the following: five in the image (green, orange, yellow, red and blue) and three in each graduated strip (left, middle and right the upper part of the strip) Figure 2-30: Positions of the test points on images from page 1 of PDF test form For each part of the page, the average has been done and reported on a graph with the following notation: RGB: images from line 1 ISOcoated: images from line 2 SWOP: images from line 3 JAPAN: images from line 4 Graduated: graduated stripes The following figure gives medium E between original proof and optimized reproductions for the two types of settings (cf. figures 2-31 and 2-32) TRICHON Amélie PFE 2006/

48 Delta E Medium delta E for color matching settings on PDF test form RGB ISOcoated Swop images of page 1 of the PDF test form Japan Graduated static dynamic Figure 2-31: Medium E on images from page1 of PDF test form with color matching settings Delta E Figure 2-32: Medium E on images from page1 of PDF test form with Dynamic Maximum Black settings Medium delta E for Dynamic Maximum Black settings on PDF test form RGB ISOcoated Swop Japan Graduated static images of page 1 of the PDF test form dynamic We can note that there is no significant difference of colorimetric accuracy when the original file is in RGB but for all the other cases, the advantage of Dynamic DVLPs is clear even if the right profile is used with Link Profiler like in case B (ISOcoated input). For more complete results, please refer to Fig. 12 and Fig. 18 in appendix. TRICHON Amélie PFE 2006/

49 ii. Ink savings: We notice that in all cases, Dynamic DVLPs give a lower level of inking than Static DVLP (cf. figures 2-33 to 2-36) Moreover, we can see that Colour Matching settings do not save ink, but this is understandable because it is not the first aim of this setting. With Dynamic Maximum black we can see that the differences of inking level are significant between Static and Dynamic processing and, as seen in the last paragraph, saving more ink does not produce any additional colorimetric distortion. Delta CMYK total on page 1 of PDF test form 20 % of ink static dynamic 0 ISOcoated output SWOP output JAPAN output Transformations with color matching settings Figure 2-33: Results of ink saving test on PDF test form for color matching settings Figure 2-34: Results of ink saving test on PDF test form for color matching settings TRICHON Amélie PFE 2006/

50 Figure 2-35: Results of ink saving test on PDF test form for Dynamic Maximum black settings Delta CMYK total on page 2 of PDF test form 10 % of ink static dynamic -10 ISOcoated output SWOP output JAPAN output Transformations with Dynamic Maxim um Black settings Figure 2-36: Results of ink saving test on PDF test form for Dynamic Maximum black settings If we observe one image in particular, for example, the image of the car - having an undefined CMYK colour space - we can see that not only we save more ink with the dynamic processing but, as found previously (cf. figure 2-24 and 2-25) we were able to obtain better colorimetric accuracy as well. Moreover, this conclusion was confirmed by the visual evaluations (cf. next part iii) TRICHON Amélie PFE 2006/

51 Figure 2-37: Results of ink saving test on car image from PDF test form for color matching settings Figure 2-38: Results of ink saving test on car image from PDF test form for Dynamic Maximum Black settings For more complete results, please refer to Fig. 13, 14, 19 and 20 in appendix. iii. Visual evaluation: A sample group of seven persons were asked to observe the proofs, compare Static optimization with Dynamic optimization and say if they see a difference. If they did, they had to say which proof is closest to the original proof. In a large majority of proofs (7/10), observers did see a difference between the two processing. For only 1 proof, some observers found that the Static DVLP proof was more faithful to the original than the Dynamic DVLP proof. The observers assessments can be summarized as follows: TRICHON Amélie PFE 2006/