Rendering Synthetic Objects into Real World Scenes by Paul Debevec SIGGRAPH 98 Conference Presented By Justin N Rogers for Advanced Comp Graphics Spring 2002 Introduction Realistically adding synthetic objects to realworld scenes is difficult Interplay of light between objects and surroundings must be consistent Should cast shadows on surroundings Should appear in reflections Should refract focus and emit light similarly as real objects would Introduction Current techniques manually model light sources photograph a reference object and use guide for lighting environment Problems with current techniques requires considerable hand refinement hard to simulate effects of indirect illumination from environment Introduction Related Work Reflection Mapping produces realistic results for mirror like objects Disadvantages doesn t account for objects casting light or shadows on the environment Use of geometric models of environment local to object to compute shadows from various light sources Disadvantages requires complete knowledge about each light source in scene Disadvantages doesn t account for diffuse reflection from the scene Introduction Related Work Recent developments have produced algorithms and software packages that realistically simulate lighting Includes indirect lighting with diffuse and specular reflections Recording light measurements Illumination of objects with actual samples of light from real scenes Provides a unified and physically accurate alternative to replicating incidental illumination Difficulties Recording light in scenes is difficult due to high dynamic range that usually exists Due to fact light sources are usually concentrated However the direct light from light sources and indirect light from the environment are important parts of the illumination solution Recording light measurements Conventional imaging equipment is used to derive radiance maps from scenes F stop or f number is used to record the dynamic range of light and form radiance maps f stop refers to the maximum lens apeture f stop also refers to the specific apeture selected for optimal brightness Synthetic objects are illuminated using the radiance maps An omnidirectional radiance map This full dynamic range lighting environment was acquired by photographing a mirrored ball balanced on the cap of a pin sitting on a table The three views of this image are adjusted to a 0 stops b 3 5 stops and c 7 0 stops show that the full dynamic range of the scene has been captured without saturation Illuminating synthetic objects with real light Top row a b c d e With full dynamic range measurements of scene radiance from previous slide Bottom row f g h i j With low dynamic range information from a single photograph of the ball The right sides of images h i j have been brightened by a factor of six to allow qualitative comparison to c d e The high dynamic range measurements of scene radiance are necessary to produce proper lighting on the objects Synthetic objects lit by two different environments a A collection of objects is illuminated using the radiance information from the previous radiance map b The same objects are illuminated by radiance information obtained in an outdoor environment on an overcast day Radiance maps used in illumination are displayed in the upper left hand corner of the images Adding synthetic objects to scenes Scene is broken into three components the distant scene the local scene and the synthetic objects Global illumination is used to simulate the interplay of light between the three components light from the distant scene is ignored Adding synthetic objects to scenes Distant Scene Radiates light towards the local scene and synthetic objects but ignores light reflected to it Local Scene Contains the surfaces that will interact with the synthetic objects Full geometry and reflectance properties must be known to ensure proper interactions Adding synthetic objects to scenes Synthetic Objects May consist of a variety of shapes and materials Should be placed in desired correspondence to the local scene After the three components are modeled and positioned then the global illumination software is used to produce renderings Three Components of General Method Distant Scene light based no reflectance model light Local Scene estimated reflectance model Synthetic Objects known reflectance model Compositing objects into scene Constructing light based model w light probe Light based model of distant scene needs to appear correctly near synthetic objects Used to calculate incident light to illuminate synthetic objects Obtaining radiance map of distant scene photograph a spherical mirror like object near the location of the synthetic object radiance measurements are mapped onto geometry of distant scene Mapping from probe to scene model Correct mapping between coordinates on the ball and ray in the world requires that the ball position relative to the camera the size of the ball and the camera parameters such as its location in the scene and focal length be recorded The data from a single ball image will display some artifacts 1 The camera will be visible 2 The ball interacts with the scene the ball and its support can appear in reflections cast shadows and can reflect light back onto surfaces 3 The ball won t reflect the scene directly behind it and will provide a poor sample of the nearby area Mapping from probe to scene model Problems Careful positioning of the ball and camera will cause these effects to be minimized and they won t have a dramatic impact on the final renderings If the artifacts are significant then the images can be altered by 1 manually in an imageediting software or by 2 combining images of the ball taken from direct angles Mapping from probe to scene model Problems The combination of two images of the ball taken 90 apart serves to eliminate the camera s appearance and helps avoid poor sampling Compositing objects into scene Creating final renderings Synthetic local scene model is created and images of scene are taken from desired viewpoint Software is run to render synthetic objects local scene and distant scene from desired viewpoint Finally the synthetic objects and local scene are composited onto the background image Using a light probe a The background plate of a scene is taken b A light probe records the incident radiance near the desired location of the synthetic objects c A simplified light based model of the distant scene is created The objects on the table which
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