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Tutoriel - Modifier la radiom\u00e9trie de l'orthophoto CIGAL 2015 - DataGrandEst<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.datagrandest.fr\/site\/formations\/tutoriel-modifier-la-radiometrie-de-lorthophoto-cigal-2015\/\" \/>\n<meta property=\"og:locale\" content=\"fr_FR\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Tutoriel - Modifier la radiom\u00e9trie de l'orthophoto CIGAL 2015 - DataGrandEst\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.datagrandest.fr\/site\/formations\/tutoriel-modifier-la-radiometrie-de-lorthophoto-cigal-2015\/\" \/>\n<meta property=\"og:site_name\" content=\"DataGrandEst\" \/>\n<meta property=\"article:modified_time\" content=\"2025-12-16T09:13:58+00:00\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Dur\u00e9e de lecture estim\u00e9e\" \/>\n\t<meta name=\"twitter:data1\" content=\"3 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"WebPage\",\"@id\":\"https:\\\/\\\/www.datagrandest.fr\\\/site\\\/formations\\\/tutoriel-modifier-la-radiometrie-de-lorthophoto-cigal-2015\\\/\",\"url\":\"https:\\\/\\\/www.datagrandest.fr\\\/site\\\/formations\\\/tutoriel-modifier-la-radiometrie-de-lorthophoto-cigal-2015\\\/\",\"name\":\"Tutoriel - 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\r\n\r\nAu d\u00e9part, les courbes de transfert sont des droites et ne comportent que deux points (0,0) (255,255). Il s\u2019agit d\u2019ajouter des points d\u2019inflexion (breakpoints) dans les courbes.\r\n\r\n \r\n\r\nPremier ajustement\u00a0:\r\n\r\nVisualiser les valeurs mini et maxi prises par les pixels dans les 3 bandes. Dans l\u2019image d\u2019origine quasiment aucun pixel ne d\u00e9passe la valeur 220, il est donc possible d\u2019\u00e9tirer l\u2019histogramme, pour ce faire j\u2019ajoute un point d\u2019inflexion (exemple de la bande rouge\u00a0ci-dessous : 218,250).\r\n\r\nLes points d\u2019inflexions sont ainsi \u00e0 ajouter dans les 3 bandes pour tenir compte \u00e0 la fois des valeurs min et max r\u00e9ellement prises par les pixels. Dans un premier temps, les points d\u2019inflexions ajout\u00e9s sont identiques dans les 3 bandes.\r\n\r\n![\"\"](\"https:\/\/web.archive.org\/web\/20250322214130im_\/https:\/\/www.datagrandest.fr\/portail\/cigal\/documents\/document\/ltable_ortho_2015.png\")
\r\n\r\nV\u00e9rifier dans les zones tr\u00e8s blanches et les zones sombres (ombres) de l\u2019image si l\u2019application des courbes ne g\u00e9n\u00e8re pas une perte d\u2019information trop importante par effet de saturation.\r\n\r\n \r\n\r\nSecond ajustement\u00a0: r\u00e9\u00e9quilibrage des couleurs\r\n\r\nV\u00e9rifier si les couleurs sont \u00e9quilibr\u00e9es en contr\u00f4lant les valeurs RGB sur quelques pixels gris de l\u2019image (dans la voirie notamment), les valeurs devraient similaires dans les 3 bandes. Si ce n\u2019est pas le cas, il convient d\u2019ajuster les points d\u2019inflexion pour r\u00e9duire la pr\u00e9dominance de l\u2019une ou l\u2019autre bande en r\u00e9duisant la valeur maximale sur cette bande \u00e0 245 ou 250 au lieu de 255 par exemple, ce qui aura pour effet de tasser l\u2019histogramme. Cela permet de r\u00e9duire l\u2019effet de voile jaune visible sur l\u2019ortho CIGAL par exemple.\r\n\r\nTout cela est assez th\u00e9orique, ce qui compte aussi c\u2019est de r\u00e9aliser quelques impressions papier \u00e0 diff\u00e9rentes \u00e9chelles pour v\u00e9rifier \u00e0 l\u2019\u0153il le rendu final.\r\n\r\nUne fois que tout est r\u00e9gl\u00e9 comme il convient (pour l\u2019instant, il ne s\u2019agit que d\u2019affichage \u00e0 l\u2019\u00e9cran et d\u2019impressions),\u00a0il s\u2019agit de calculer concr\u00e8tement la nouvelle image dont la radiom\u00e9trie est modifi\u00e9e gr\u00e2ce \u00e0 l\u2019outil LUT Stretch d\u2019ERDAS.\r\n