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We describe how environmental context can help determine whether oxygen (O2) detected in extrasolar planetary observations is more likely to have a biological source. Here we provide an in-depth, interdisciplinary example of O2 biosignature identification and observation, which serves as the prototype for the development of a general framework for biosignature assessment. Photosynthetically generated O2 is a potentially strong biosignature, and at high abundance, it was originally thought to be an unambiguous indicator for life. However, as a biosignature, O2 faces two major challenges: (1) it was only present at high abundance for a relatively short period of Earth's history and (2) we now know of several potential planetary mechanisms that can generate abundant O2 without life being present. Consequently, our ability to interpret both the presence and absence of O2 in an exoplanetary spectrum relies on understanding the environmental context. Here we examine the coevolution of life with the early Earth's environment to identify how the interplay of sources and sinks may have suppressed O2 release into the atmosphere for several billion years, producing a false negative for biologically generated O2. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. We review the most recent knowledge of false positives for O2, planetary processes that may generate abundant atmospheric O2 without a biosphere. We provide examples of how future photometric, spectroscopic, and time-dependent observations of O2 and other aspects of the planetary environment can be used to rule out false positives and thereby increase our confidence that any observed O2 is indeed a biosignature. These insights will guide and inform the development of future exoplanet characterization missions. Key Words: Biosignatures-Oxygenic photosynthesis-Exoplanets-Planetary atmospheres. Astrobiology 18, 630-662.

In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets-Biosignatures-Habitability markers-Photosynthesis-Planetary surfaces-Atmospheres-Spectroscopy-Cryptic biospheres-False positives. Astrobiology 18, 663-708.

Here we review how environmental context can be used to interpret whether O2is a biosignature in extrasolar planetary observations. This paper builds onthe overview of current biosignature research discussed in Schwieterman et al.(2017), and provides an in-depth, interdisciplinary example of biosignatureidentification and observation that serves as a basis for the development ofthe general framework for biosignature assessment described in Catling et al.,(2017). O2 is a potentially strong biosignature that was originally thought tobe an unambiguous indicator for life at high-abundance. We describe thecoevolution of life with the early Earth's environment, and how the interplayof sources and sinks in the planetary environment may have resulted insuppression of O2 release into the atmosphere for several billion years, afalse negative for biologically generated O2. False positives may also bepossible, with recent research showing potential mechanisms in exoplanetenvironments that may generate relatively high abundances of atmospheric O2without a biosphere being present. These studies suggest that planetarycharacteristics that may enhance false negatives should be considered whenselecting targets for biosignature searches. Similarly our ability to interpretO2 observed in an exoplanetary atmosphere is also crucially dependent onenvironmental context to rule out false positive mechanisms. We describe futurephotometric, spectroscopic and time-dependent observations of O2 and theplanetary environment that could increase our confidence that any observed O2is a biosignature, and help discriminate it from potential false positives. Byobserving and understanding O2 in its planetary context we can increase ourconfidence in the remote detection of life, and provide a model forbiosignature development for other proposed biosignatures.

In the coming years and decades, advanced space- and ground-basedobservatories will allow an unprecedented opportunity to probe the atmospheresand surfaces of potentially habitable exoplanets for signatures of life. Lifeon Earth, through its gaseous products and reflectance and scatteringproperties, has left its fingerprint on the spectrum of our planet. Aided bythe universality of the laws of physics and chemistry, we turn to Earth'sbiosphere, both in the present and through geologic time, for analog signaturesthat will aid in the search for life elsewhere. Considering the insights gainedfrom modern and ancient Earth, and the broader array of hypothetical exoplanetpossibilities, we have compiled a state-of-the-art overview of our currentunderstanding of potential exoplanet biosignatures including gaseous, surface,and temporal biosignatures. We additionally survey biogenic spectral featuresthat are well-known in the specialist literature but have not yet been robustlyvetted in the context of exoplanet biosignatures. We briefly review advances inassessing biosignature plausibility, including novel methods for determiningchemical disequilibrium from remotely obtainable data and assessment tools fordetermining the minimum biomass required for a given atmospheric signature. Wefocus particularly on advances made since the seminal review by Des Marais etal. (2002). The purpose of this work is not to propose new biosignaturesstrategies, a goal left to companion papers in this series, but to review thecurrent literature, draw meaningful connections between seemingly disparateareas, and clear the way for a path forward.

ABSTRACT In this publication we will look at the different methods presented over the past few decades which attempt to recreate digital paintings. While previous surveys concentrate on the broader subject of non-photorealistic rendering, the focus of this paper is firmly placed on painterly rendering techniques. We compare different methods used to produce different output painting styles such as abstract, colour pencil, watercolour, oriental, oil and pastel. Whereas some methods demand a high level of interaction using a skilled artist, others require simple parameters provided by a user with little or no artistic experience. Many methods attempt to provide more automation with the use of varying forms of reference data. This reference data can range from still photographs, video, 3D polygonal meshes or even 3D point clouds. The techniques presented here endeavour to provide tools and styles that are not traditionally available to an artist. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]