44:["$","title",null,{"children":"couleur RGBA dans un SVG | msc59 | Observable"}] 4c:T545,# couleur RGBA dans un SVG Il existe de [nombreuses normes de codage des couleurs](https://fr.wikipedia.org/wiki/Codage_informatique_des_couleurs). Les normes RGB et RGBA sont utilisées pour les affichages sur écran et relèvent de la *synthèse additive*, d’autres normes relèvent de la *synthèse soustractive* telles CMY et CMYK (Cyan-Magenta-Yellow-blacK) pour l’impression en trichromie et en quadrichromie,respectivement. Dans la norme RGBA, une couleur par un **quadruplet d'octets**. Chacun de ces 4 octets est un **entier non signé compris entre `0=0x00` et `255=0xff`**. ${tex.block`\texttt{0x00}=0\times16+0=0 \qquad;\qquad \texttt{0xff}=15\times16+15=255`} * les trois premiers octets sont les composantes `red`, `green` et `blue`; * le dernier est la composante dite `alpha` qui indique la transparence (de transparent `0x00` à opaque `0xff`) * lorsque ce canal `alpha` est absent, le triplet d'octets restant est une couleur RGB, considérée comme opaque (tout se passe comme si le canal `alpha` valait `0xff`) En ce sens l'espace des couleurs RGBA est la **produit cartésien** ${tex.block`[0..255]^4 = [0..255] \times [0..255] \times [0..255] \times [0..255]`} où l'on désigne par ${tex.block`[a..b] = \{a,a+1,\dots,b\}`} l'ensemble des ${tex`b-a+1`} entiers compris au sens large entre ${tex`a`} et ${tex`b`}.4d:T8aa,htl.html``4e:[] 48:{"id":"1cd6e7cf1e9cae3b","slug":"couleur-rgba-dans-un-svg","title":"couleur RGBA dans un SVG","thumbnail":"588ac00b7138c8376cf211129855202dbcf5baa1611948c88b95e54b9b63bfef","access_level":"public","create_time":"$D2021-10-26T18:40:52.172Z","update_time":"$D2021-11-30T15:05:25.778Z","view_time":null,"first_public_time":"$D2021-10-26T20:24:07.699Z","has_unpublished_edits":null,"fork":{"id":"7012061a800e7057","slug":"simple-svg","title":"Simple SVG","owner":{"id":"f35c755083683fe5","login":"observablehq","name":"Observable"}},"stars":0,"starred":false,"owner":{"type":"team","id":"51e9e6bbb85f05f1","login":"msc59","name":"","avatar_url":"https://avatars-next.observableusercontent.com/70c6ba5df36a92dfd587104f6df90d4ab8b3bf8af09ad73b132ca42ec0256306","tier":"starter_2024","bio":"No one to follow \n/\nAnd nothing to teach \n/\nExcept that the goal \n/\nFalls short of the reach\n/\nLeonard Cohen\n","followed_by_you":false,"follows_you":false},"version":212,"publish_version":null,"role":null,"authored":false,"authors":[{"type":"individual","id":"0f6586efe16fde16","login":"msc59","name":"","avatar_url":"https://avatars-next.observableusercontent.com/70c6ba5df36a92dfd587104f6df90d4ab8b3bf8af09ad73b132ca42ec0256306","tier":null,"bio":"Tente d'enseigner un peu de maths à de futurs informaticiens - IUT Lille.\n\n// No one to follow \n// And nothing to teach \n// Except that the goal \n// Falls short of the reach\n// L. Cohen","followed_by_you":false,"follows_you":false}],"members":[{"type":"individual","id":"0f6586efe16fde16","login":"msc59","name":"","avatar_url":"https://avatars-next.observableusercontent.com/70c6ba5df36a92dfd587104f6df90d4ab8b3bf8af09ad73b132ca42ec0256306","tier":null,"role":"editor"}],"comments":[],"custom_thumbnail":false,"trashed":false,"lapsed":null,"subscribed":null,"forks":1,"collection":null,"type":"notebook","body":{"titling":"auto","title":"couleur RGBA dans un SVG","cells":[{"id":0,"value":"$4c","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":6,"value":"$4d","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":31,"value":"viewof rgba = verticalSliders({\n names: [\"r\", \"g\", \"b\",\"a\"],\n labels:[\"red\", \"green\", \"blue\",\"alpha\"],\n maxs: [255,255,255,255],\n values: [31,56,85,255], // RGB 31 56 85 -> RAL5003\n steps: [1,1,1,1]\n})","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":49,"value":"rgbastring=\"#\"+((((256+rgba.r)*256 + rgba.g)*256 + rgba.b)*256 + rgba.a).toString(16).slice(1,9)","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":33,"value":"import {verticalSliders} from '@tomlarkworthy/vertical-sliders'","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":29,"value":"import {form} from \"@tomlarkworthy/mutable-form-input\"","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"}],"files":"$W4e","stdlib":"1"}} 24:null