Rendering Synthetic Objects into Real World Scenes by

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 n Realistically adding synthetic objects to realworld scenes is difficult q Interplay of light between objects and surroundings must be consistent n n n Should cast shadows on surroundings Should appear in reflections Should refract, focus, and emit light similarly as real objects would

Introduction n Current techniques q q n manually model light sources photograph a reference object and use guide for lighting environment Problems with current techniques q q requires considerable hand-refinement hard to simulate effects of indirect illumination from environment

Introduction n Related Work q Reflection Mapping produces realistic results for mirror-like objects n q 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 n n Disadvantages – requires complete knowledge about each light source in scene Disadvantages – doesn’t account for diffuse reflection from the scene

Introduction n Related Work q Recent developments have produced algorithms and software packages that realistically simulate lighting n Includes indirect lighting with diffuse and specular reflections

Recording light measurements n Illumination of objects with actual samples of light from real scenes q n Provides a unified and physically accurate alternative to replicating incidental illumination Difficulties q Recording light in scenes is difficult due to high dynamic range that usually exists. n n 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 n Conventional imaging equipment is used to derive radiance maps from scenes q F-stop or f-number is used to record the dynamic range of light and form radiance maps n n q 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 n n 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. q light from the distant scene is ignored

Adding synthetic objects to scenes n Distant Scene q n Radiates light towards the local scene and synthetic objects, but ignores light reflected to it Local Scene q q 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 n Synthetic Objects q q n 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 n q Light-based model of distant scene needs to appear correctly near synthetic objects. n q Used to calculate incident light to illuminate synthetic objects Obtaining radiance map of distant scene n n 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 n n 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 n n 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 n 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 n Creating final renderings q q q 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 were not explicitly modeled, become projected onto the table. (d)Synthetic objects and a BRDF model of the local scene are added to the light-based model of the distant scene. A global illumination solution of the model is computed with light coming from the distant scene and interacting with the local scene and synthetic objects. Light reflected back to the distant scene is ignored. Finally the results from this rendering are composited into the background plate from (a) to achieve the final result.

Rendering with a Combined Probe Image The full dynamic range environment map shown above was assembled from two light probe images taken 90° apart. As a result, the only visible artifact is a small amount of probe support visible on the floor. The map is shown at -4. 5, 0, and +4. 5 stops.

Rendering with a Combined Probe Image The rendering was produced using the lighting information from the previous slide. It exhibits diffuse and specular reflections, shadows from different sources of light, reflections, and caustics.

Improving quality with differential rendering n The method presented thus far requires that geometry and material properties of the local scene be modeled properly. n If the model is inaccurate, the appearance of the local scene will not be consistent with the adjacent distant scene. Differential rendering introduces a method for greatly reducing such effects

Improving quality with differential rendering n n LSb – local scene in the light-based model LSnoobj – local scene without synthetic objects The error in the rendered local scene is: Errls = LSnoobj –LSb. This error results from the difference between the BRDF characteristics of the actual local scene as compared to the modeled local scene. LSobj – local scene with synthetic objects

Improving quality with differential rendering n n We can compensate for error by computing the final rendering as LSfinal = LSobj - Errls or LSfinal = LSb + (LSobj – LSnoobj) When LSobj and LSnoobj are the same the final rendering is equivalent to LSb. When LSobj is darker than LSnoobj , light is subtracted from the background to form shadows and viceversa.

Improving quality with differential rendering Incorrect results can still be produced depending on the amount of error in the estimated local scene BRDF and inaccuracies in the light-based model of the distant scene. An alternative approach is to adjust for relative error in the local scene. LSfinal = LSb(LSobj /LSnoobj).

BRDF n n Materials interact with light in different ways, and different materials have different appearances given the same lighting conditions. The reflectance properties of a surface are described by a reflectance function, which models the interaction of light reflecting at a surface. The bi-directional reflectance distribution function (BRDF) is the most general expression of reflectance of a material The BRDF is defined as the ratio between differential radiance reflected in an exitant direction, and incident irradiance through a differential solid angle

BRDF

Estimating the local scene BRDF 1. Assume a reflectance model for the local scene 2. Choose approximate initial values for the parameters of the reflectance model 3. Compute a global illumination solution for the local scene with the current parameters using the observed lighting configuration 4. Compare the appearance of the rendered local scene to its actual appearance in one or more views

Estimating the local scene BRDF 5. If the renderings aren’t consistent, adjust the parameters of the reflectance return to step 3 n Assuming a diffuse-only model of the local scene in step 1 makes the adjustment in step 5 straightforward

Estimating the local scene BRDF

Estimating the local scene BRDF We use the global illumination software to render each patch as a perfectly diffuse reflector, then compare the resulting radiance to the observed value. Dividing the two quantities produces the next estimate for the diffuse reflection coefficient p‘d. If there is no interreflection within the local scene, then the p‘d estimates will make the renderings consistent. Interreflection requires that the algorithm be iterated until there is convergence.

Compositing Results

Compositing Results

Conclusion n A general framework for adding new object into light-based models with correct illumination has been presented. The method uses high dynamic range images of real scene radiance to realistically illuminate synthetic objects. A practical instance of the method was presented used a light probe to record incident illumination in the vicinity of synthetic objects