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81 | Mansencal and Scharfenberg (2020) - HSV Control Based Study Model 84 |
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86 | Colour Developers Scientific 87 | Plot Entry for 88 | John Hunter - Excellence in 89 | Plotting - 2020: An interactive Jupyter 91 | Widget driving Mansencal and Scharfenberg (2020) HSV 92 | Control Based Study Model, and performing gamut mapping 93 | on scene-referred synthetic imagery. 94 |

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The Cornell 97 | Box is spectrally rendered with Mitsuba 99 | 2: A Retargetable Forward and Inverse (Open Source) 100 | Renderer. 101 |

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Gamut Mapping of Scene-Referred Light Values 111 |

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114 | The Academy 116 | Color Encoding System 117 | (ACES) is a free, open, device-independent color management 118 | and image interchange system used in the production of movies, 119 | series and games. ACES adoption has increased during the last 120 | decade and the system is becoming a successful standard in the 121 | Media and Entertainment Industry. However, amid of the growing 122 | success, ACES users are frequently reporting defects occurring on 123 | their imagery while using the system. 124 |

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The artefacts are often generated by digital still or 135 | motion-picture cameras: their spectral sensitivities do not satisfy 136 | the Maxwell-Ives 138 | criterion (Luther condition). Put another way, they are not 139 | colorimetric and capture the world differently than the Standard 141 | Observer, i.e. a standardised average human viewer. 142 | Ultimately, they produce non-physically realisable values outside 143 | the Spectral 144 | Locus, i.e. the volume bounding the extent of the 145 | Standard Observer sensitivity to the electro-magnetic 146 | spectrum. Colour grading tools used to infuse the creative vision 147 | into the imagery are also source of defect generation. 148 |

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150 | Show Out-of-Gamut Colours 154 |
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156 | The 1976 157 | UCS Chromaticity Diagram on the left highlights 158 | particularly well the non-colorimetric behaviour of the Canon 159 | EOS 5D Mark II. The Spectral Locus is represented by 160 | the largest colour-varying and rounded triangle. 161 | The red triangle defines the primaries of the standard ACES 162 | RGB working colourspace, i.e. ACES AP1, and the gray triangle 163 | that of the display colourspace, i.e. sRGB. 164 |

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166 | Note how the astronaut suits, and the luminous tiles on the 167 | floor, whose colours are outside of the 168 | Spectral Locus, are looking posterised. 169 |

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177 | In the context of this document, and the portion of interest of the 178 | ACES pipeline, the imagery is said to be in a 179 | scene-referred state. A scene-referred image encodes light 180 | values as they were captured at the camera focal plane in a given 181 | scene. The encoded values are intrinsically linear, and 182 | proportional, relatively, to the incident light from the scene to 183 | the camera.

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185 | The A.M.P.A.S formed a new 186 | Virtual 188 | Working Group with the objective of finding strategies to 189 | manage and correct the artefacts. The Working Group is researching 190 | a source-agnostic model that performs the mapping of scene-referred 191 | light values from any source, i.e. any light capturing device, 192 | irrespective of its sensitivities, to a target RGB colourspace 193 | primaries, e.g. ACES AP1. 194 |

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As a contribution to the initiative, the Colour Developers have 197 | designed and built an interactive 199 | Jupyter Widget 200 | to explore the problem space. The widget leverages Colour, an 202 | open-source Python package providing a comprehensive number of 203 | algorithms and datasets for colour science, and Matplotlib.

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206 | As of this writing, the widget implements two gamut mapping study 207 | models via sub-classes: 208 |

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  • Mansencal and Scharfenberg (2020) HSV Control Based Study 210 | Model built on top of the HSV 212 | colourspace
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  • Smith (2020) - RGB Saturation Study Model built on 214 | top of the RGB 216 | colourspace
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220 | Mansencal and Scharfenberg (2020) - HSV Control Based & Smith (2020) RGB Saturation Study Models 223 |
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225 | Mansencal and Scharfenberg (2020) HSV Control Based Study 226 | Model compared with Smith (2020) RGB Saturation 227 | Study Model. Out of the box, Smith (2020) model 228 | produces less hue clumping and more pleasing output. It is 229 | also simpler, faster and more elegant but lacks hue twists 230 | controls. 231 |

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238 | 239 | Various gamut mapping controls are offered such as changing the 240 | compression function and its effect, or, hue twist controls for the 241 | HSV Control Based Study Model. The widget also implement 242 | various viewing controls such as showing the out-of-gamut colours 243 | or image exposure adjustments. 244 |

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246 | Mansencal and Scharfenberg (2020) - HSV Control Based Study Model 249 |
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251 | Mansencal and Scharfenberg (2020) HSV Control Based Study 252 | Model applied onto a photograph. 253 |

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262 | While being order of magnitude slower than commercial applications 263 | such as Davinci 265 | Resolve or The Foundry 267 | Nuke, the interactive widget is extremely useful as it allows 268 | to tap directly into the entire Python scientific ecosystem while 269 | promoting Open Source Software and reproducible research. 270 |

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272 | More information is available on the colour-science's 274 | Gamut Mapping Ramblings repository and ACESCentral. 276 |

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278 | Copyright © 2020 – Colour Developers – colour-developers@colour-science.org 280 |

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