Evaluating the Use of the Perceptual Reference Medium Gamut in ICC PrintingWorkflows

The International Color Consortium (ICC) specifies a standard profile format and associated color management architecture. In the version 4 of its specifications, it adopted a Perceptual Reference Medium and an associated gamut (PRMG) for perceptual workflows. The goals of this change were improved interoperability and more pleasing results. In this paper we start evaluating the effect of using of the PRMG on the quality of printed images. A series of psychophysical experiments were conducted with a large set of sRGB images. A small number of expert observers found that they tended to prefer the quality of prints obtained via a v4 workflow when printing with a printing system having a medium sized gamut. However, when printing with a system having a gamut similar to the PRMG, they did not find any significant preference. They also found that the use of v4 led to a better consistency of the rendering of the prints when using two printing systems with different gamuts. None of the algorithms were strongly preferred by the observers of a larger panel. Introduction The International Color Consortium (ICC) framework is widely used in color management workflows [1, 2]. The underlying architecture is based around a reference color space (the Profile Connection Space or PCS) and profiles which embody color transforms that convert between device encodings and this reference colorimetry. A Color Management Module (CMM) provides the mathematical engine to perform the profile-to-profile transformations, allowing input and output transforms to be paired arbitrarily at run time even though they are created independently. ICC profiles incorporate color rendering and re-rendering algorithms, expressed as rendering intents. Four color rendering intents are specified by the ICC: absolute colorimetric, mediarelative colorimetric, perceptual and saturation [1]. Each one represents a different color reproduction goal. The profile creator has the responsibility to select appropriate color rendering algorithms for each of the intents. In this paper, we focus on the perceptual rendering intent. The goal of this intent is to produce a pleasing reproduction of an original on a destination output medium, compensating for differences in viewing conditions and gamuts between source and reproduction. It is also called preferred reproduction: “It aims to maximize the correspondence of the reproduction with preconceived ideas of how a given image should look according to an individual whereby this criterion encompassed contrast, the lack of artifacts, sharpness, etc. ” [3] The perceptual intent is useful for general reproduction of pictorial images, when the input and output media are substantially different and it is not required to exactly maintain image colorimetry. In a perceptual workflow, the image is re-rendered from the source encoding to the PCS by the source profile perceptual intent transform and re-rendered from the PCS to the destination encoding by the destination profile perceptual intent, changing the appearance of the image content, as necessary to produce a pleasing reproduction for the output medium [4]. The ICC profile format has undergone continued evolution since its first publication, and a number of recent developments have made significant improvements to the range of workflows supported and removed considerable ambiguity from the specification [5]. One such development was the specification of a reference color gamut, the Perceptual Reference Medium Gamut (PRMG) adopted in 2005 in the first amendment to the version 4 of the ICC specifications (v4). It was defined as an unambiguous reference gamut to render, or re-render, to and from [1,6]. The shape of the PRMG is similar to that of a gamut of a printing system, thus quite different in shape from a reference display gamut. Hence the rendering embedded in an RGB source profile can be quite complex. Koh et al. have created an sRGB v4 profile embedding a perceptual rendering transform from the sRGB to the PRMG [7]. Since in a v4 perceptual workflow the image is rendered or rerendered to the reference gamut, the re-rendering in a v4 output profile perceptual intent should in principle be less complex than in a v2 profile [7]. The ICC expects more pleasing results for most images when combined with any correctly constructed v4 output profile using the perceptual rendering intent [8]. Furthermore, final images printed on different devices should be very similar since “they all tap the same input-side re-rendering”. The goal of this paper is to evaluate the consequences of the use of the PRMG, in an ICC perceptual workflow, on the quality of the printed images. The aim of the perceptual intent is to produce pleasing reproductions, and therefore the quality of this transform is evaluated by asking observers to judge the reproduction of pictorial images in a psychophysical experiment [3]. In this paper we briefly discuss the use of the PRMG in perceptual intent workflows together with the different aspects of the present evaluation. We then present the results of several psychophysical experiments and discuss the future work required to complete the evaluation. Perceptual Intent and PRMG The Perceptual Reference Medium Gamut [6] includes the great majority of surface colors that might be encountered in color reproduction. The reference medium has white and black points having a neutral reflectance of 89 %, and 0.3% respectively. The maximum C*ab chroma values of the reference gamut at a range of different L* lightnesses are specified in ISO 12640-3 [6]. The PRMG was incorporated in the v4 of the ICC specification in 2005, with a means to identify whether the PRMG was used through the rig0 tag. The adoption of the PRMG is still not widespread. On the input side, an sRGB v4 preference profile [7–9] and a ROMM RGB v4 profile [10] are available on the ICC website [11]. Unfortunately for professional photographers, no v4 profile for the widely-used Adobe RGB (1998) encoding is available yet [12]. On the output side, while several vendors have internally implemented profile-making tools that use the PRMG in perceptual transforms, currently there seems to be no available commercial application explicitly using the PRMG. Perceptual Intent: V2 versus V4 There are several significant differences between the v2 and the v4 perceptual intent workflows. In a v2 workflow, the input transform rendering intent is ambiguous since in a v2 input profile only a single rendering intent is required. This intent is nominally perceptual, with the goal of producing “the CIE colorimetry which will produce the desired color appearance if rendered on a reference imaging media and viewed in a reference viewing environment. This reference corresponds to an ideal reflection print viewed in an ANSI standard viewing booth.” [13]. In practice v2 input profiles often incorporate minimal rendering to the ICC PCS and are closer to a colorimetric rendering. Most of the rerendering thus has to be performed by the output profile. Therefore the quality of a v2 workflow depends to a considerable extent on the color transform of the output profile. Two key issues are, first that the output profile does not have a defined gamut to re-render from (and therefore the profile maker has to select an arbitrary one within the PCS), and second that multiple output profiles needed for all the printing systems settings and media combinations might be complex to build and therefore require significant resources. Because the source gamut is unknown when the destination profile is created, the quality of the output profile perceptual intent also depends on the nominal gamut which the transform renders from, and the difference between this and the actual gamut of the source medium. In a v4 perceptual workflow, the Perceptual Reference Medium Gamut is used as an intermediate gamut to render to and from: first the input transform re-renders the image color data from the input color space (e.g. sRGB) to the PRMG. Then the output transform re-renders the image color data from the PRMG to the output color space (e.g. CMYK). While building elaborated input profiles require resources, the multiple output profiles are expected to be easier to build by the ICC: “Simple media white and black scaling can accommodate differences in dynamic range between an original and a reproduction and (to some extent) differences in color gamut size. In cases where color gamut shapes are roughly similar, and gamut size differences correlate with white and black point differences, media-relative colorimetric plus black point compensation may produce excellent perceptual rendering” [14], see also [15, 16]. Therefore the quality of a v4 perceptual workflow has a strong dependence on the perceptual intent of the input profile. Perceptual Rendering Intent Transforms The exact content of ICC rendering transforms is not defined, it is vendor specific as Koh et al. explain: “State-of-theart color re-rendering algorithms are closely guarded trade secrets [...] Also, in some cases, the color re-rendering transforms produced by the proprietary algorithms are manually optimized (tweaked). Such optimizations can be included in profiles” [7]. A perceptual re-rendering transform of a typical v4 input profile