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Sistemas De Iluminación Avanzada

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Sistemas De Iluminación Avanzada Sistemas de Iluminaci´onAvanzada Principales T´ecnicas En 1980 se public´oel algoritmo b´asicode ray tracing (T. Whitted). En 1984 se public´oel procedimiento b´asicode radiosidad. En 1984 se dieron a conocer m´etodos est´ocasticosde ray tracing que llevar´ıan a la introducci´onsistem´atica de los m´etodos de Monte Carlo. En 1986 Kajiya public´oun famoso art´ıculoen el que adaptaba procedimientos del campo de la f´ısicaa una formulaci´ongeneral de simulaci´on,lo que se conoci´ocomo la rendering equation, y en el que tambi´en se propon´ıaotro algoritmo importante, path tracing. Hacia 1990 puede decirse que hab´ıados grandes grupos de algoritmos o m´etodos de IG: los basados en ray tracing (que eran ineficaces para computar la iluminaci´onindirecta difusa) y los basados en radiosidad (que eran ineficaces para computar la iluminaci´onindirecta reflejada). De esta situaci´onderivaron nuevos m´etodos que combinaban ambas t´ecnicasen dos o m´as pasos y que se denominaron, en consecuencia, "m´etodos multipaso"(multipass methods). Las aplicaciones m´aseficaces utilizaban ya m´etodos h´ıbridos(como los actuales), que combinaban radiosidad y ray tracing. En 1988 Ward public´oun art´ıculo,ampliado en 1992 (con Heckbert) en el que propon´ıaun m´etodo de recolecci´one interpolaci´onque pas´oa denominarse de radiancia o irradiancia (o irradiance catching). En 1995 Jensen dio a conocer otro m´etodo, photon mapping. Hacia 1997 tambi´ense comenzaron a utilizar sistemas basados en HDRI (High Dynamic Range Im- ages) o “luminaci´onbasada en imagen" a partir de un m´etodo desarrollado por Paul Debevec, en buena parte a partir de los trabajos de Ward, para utilizar una imagen como fuente de luz de la escena y con- seguir as´ıuna representaci´oncrom´aticamucho m´asfiel a la realidad. T´ecnicas Complementarias Entre las m´asrecientes y m´asconocidas est´anlos diversos m´etodos para generar c´austicas. Otra t´ecnicanotable, tambi´en relativamente reciente es Sub-Surface Scattering (abreviada a veces como "SSS.o "3S") que se present´oen Siggraph 2001 por Henrik Wann Jensen (que ya hab´ıaintroduci- do previamente la t´ecnicade proyecci´onde fotones), para simular objetos transl´ucidoscon resultados espectaculares. 1 Clasificaci´onM´etodos Principales Disponibles Desde el punto de vista de su rendimiento los m´etodos disponibles se pueden clasificar en varios grupos. Una clasificaci´onm´aso menos corriente es la que sigue. M´etodos Exactos frente a M´etodos Aproximados. Los m´etodos .exactos"se basan en una resoluci´ondirecta de la rendering equation (mediante m´etodos de Monte Carlo o pseudoMonte Carlo) mientras que los aproximados se basan en una aproximaci´onm´astosca. Ejemplo caracter´ısticode lo primero ser´ıapath tracing en sus diferentes variantes. Ejemplos de lo segundo son Radiosi- dad, Proyecci´onde fotones o Irradiancia. Es decir, todos los principales. Ventajas y Desventajas M´etodos Exactos: Las ventajas de los m´etodos exactos son que producen resultados m´asprecisos, que sus deficiencias s´olose tra- ducen en ruido, que requieren pocos controles y que no requieren gran cantidad de memoria. Las desventajas, que son muy lentos, que no pueden computar determinados efectos (por ejemplo, c´austicas) y que no permiten grabar resultados previos que luego puedan ser refinados. Ventajas y Desventajas M´etodos Aproximados: Las ventajas de los m´etodos aproximados ser´ıanque su mayor o menor precisi´onpuede adaptarse a los diferentes casos, con lo que pueden ser m´asr´apidos,que en algunos casos pueden computar efectos tales como c´austicas, que permiten ajustar los requisitos de calidad y que permiten grabar resultados previos para refinarlos despu´es. Las desventajas, que son menos precisos (aunque la precisi´onpuede aumentarse casi indefinidamente), que sus deficiencias se traducen en varios tipos de artefactos, adem´asde ruido, que pueden requerir ajustes espec´ıficos, que, como consecuencia de esto, requieren m´ascontroles y son m´asdif´ıcilesde utilizar, y que en algunos casos se puede necesitar una gran cantidad de memoria. M´etodos de Trazado frente a M´etodos de Recolecci´on. Los m´etodos de trazado env´ıanrayos desde las luces hacia la escena. Los de recolecci´onenv´ıanrayos desde la c´amara,en funci´onde la resoluci´on.Ambos pueden ser exactos o aproximados. Ejemplos de m´etodos de trazado exacto ser´ıaLight tracing y de m´etodos de trazado aproximado, la proyecci´onde fotones o los estadios iniciales de radiosidad. Ejemplos de m´etodos de recolecci´onser´ıanpath tracing, irradiancia y los estadios finales de radiosidad. Ventajas y Desventajas M´etodos de Trazado: Las ventajas de los m´etodos de trazado ser´ıanque pueden simular con facilidad algunos efectos caracter´ısticos. La desventaja principal es que no tienen en cuenta la resoluci´onni la visibilidad de determinados objetos por lo que se desperdicia potencia de c´alculo. Ventajas y Desventajas M´etodos de Recolecci´on: Las ventajas de los m´etodos de recolecci´onson exactamente las contrarias: son m´aseficientes pues adaptan la potencia de c´alculoa las caracter´ısticasde la escena. Tambi´enpermite simular mayor diversidad de efectos que los anteriores, tales como el efecto de luces de entorno o de objetos luminosos (aunque no c´austicas). 2 M´etodos Dependientes del Punto de Vista frente a M´etodos Inde- pendientes del Punto de Vista. Los m´etodos de trazado son en general independientes del punto de vista mientras que los m´etodos de recolec- ci´onpermiten ambas alternativas. Las ventajas y desventajas son obvias. Los m´etodos independientes del punto de vista no necesitan ser recalcu- lados cuando se cambia el punto de vista. Pero, por esto mismo, son poco eficaces si el punto de vista no se va a cambiar demasiado. Y con los dependientes del punto de vista pasa lo contrario. ¿Cu´alUtilizar? De todo esto se desprende que la mejor soluci´ones, si es posible, utilizar sistemas h´ıbridos que utilicen m´as de un m´etodo para llegar a un resultado ´optimo.Esto debe permitir entender mejor la estrategia de plug-ins como Mental Ray o VRay En la pr´actica,la mayor´ıade los m´etodos comerciales son h´ıbridosy utilizan una combinaci´onde las t´ecnicascitadas. Tambi´enhay que decir que los mejores resultados se encuentran en plug-ins que se instalan como complementos sobre las plataformas de simulaci´onm´asconocidas. As´ı,Mental Ray utiliza b´asicamente photon mapping pero con sistemas adicionales de recolecci´on(final gath- ering). Vray, FinalRender o Brazil, que son algunos de los motores de render m´aspopulares (en 2006), utilizan combinaciones diversas de photon mapping e irradiance caching. En el propio motor de render de Max se encuentra diversas combinaciones, desde m´etodos b´asicosde proyecci´onde rayos (light tracer) hasta diversas formas de aplicaci´onde radiosidad (directa o con recolecci´on)aunque menos eficientes que las de los sistemas citados. C´austicas Cuando los rayos luminosos que se emiten desde un punto, atraviesan una lente o un medio imperfecto, forman un haz que no converge hacia un punto imagen sino que se dispersa en torno a una curva cuyas propiedades geom´etricasfueron estudiadas por primera vez en el siglo XVII (por Tschirnhausen, en 1682, pero tambi´enpor Huygens y, posteriormente, por Lagrange, en el XVIII y Quetelet y Cayley en el XIX, entre otros). Los patrones de luz que se forman cuando una fuente de luz atraviesa una botella o un vaso, o los que se forman en el fondo de una piscina o sobre su superficie, son ejemplos caracter´ısticosde c´austicas.Las c´austicasno se pueden simular con los m´etodos corrientes de c´alculode iluminaci´onavanzada como ray tracing o radiosidad. Sin embargo s´ıes posible simularlas con m´etodos espec´ıficos, particularmente adecuados para generar estos efectos, como ocurre con los m´etodos basados en la emisi´onde fotones. 3 T´ecnicasde Iluminaci´onde Algunos Motores Render Yafray Iluminaci´onglobal completa Yafray permite iluminar escenas mediante un sistema de iluminaci´onglobal completo, empleando aproxi- maciones Montecarlo y Quasimontecarlo. Para solucionar problemas de iluminaci´onen espacios interiores, dispone de un sistema de mapeado de fotones (PhotonMapping), que junto con el cach´ede irradiaci´on (Irradiance Cache) y refinamiento de sombras, permite obtener resultados de calidad. Iluminaci´onSkydome Este sistema de iluminaci´onse basa exclusivamente en la luz que proviene de un cielo emisor, con el c´alculo de sombras suaves que ello conlleva. Iluminaci´onHDRI La iluminaci´onse basa en la informaci´onde luz contenida en una imagen del tipo HDR. Puede usarse con Skydome o GI completa. C´austicas Con YafRay podemos simular los efectos de iluminaci´oncausados por la distorsi´onde luz que provocan los materiales reflectantes y transmisores como el cristal o los espejos. DOF Real Con esta caracter´ıstica,es posible reproducir el efecto de enfoque natural de las lentes. Tras enfocar un punto en la escena, comienzan a desenfocarse progresivamente, los objetos que se alejan del mismo. Reflexiones borrosas (blurry/glossy) Cuando una superficie no es un reflector perfecto, se produce una distorsi´onen el reflejo que crece seg´un se aleja el objeto reflejado. Esta distorsi´ones causada por una microsc´opicasuperficie rugosa. http://www.yafray.org/index.php?s=10 Kerkythea Classic Ray Tracing. Path Tracing (Kajiya). Bidirectional Path Tracing (Veach Guibas). Metropolis Light Transport (Kelemen, Kalos et al.). Photon Mapping (Jensen) [mesh maps, photon maps, final gathering, irradiance caching, caustics]. 4 Diffuse Interreflection (Ward). Depth Rendering. Mask Rendering. Clay Rendering. http://www.kerkythea.net/joomla/index.php?option=com_content&task=view&id=14&Itemid=48 3DELIGHT Ray tracing hider to trace primary rays instead of using REYES. Global Illumination (including photon mapping, final gathering and high Dynamic range lighting and rendering).
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