Week 15 -Wednesday What did we talk about last time? Review up to - PowerPoint PPT Presentation

Week 15 -Wednesday

 What did we talk about last time?  Review up to Exam 2

 We already talked about reflections!  Environment mapping was our solution  But it only works for distant objects

 The reflected object can be copied, moved to reflection space and rendered there  Lighting must also be reflected  Or the viewpoint can be reflected

 This problem can by solved by using the stencil buffer  The stencil buffer is set to areas where a reflector is present  Then the reflector scene is rendered with stenciling on

 Ray tracing can be used to create general reflections  Environment mapping can be used for recursive reflections in curved surfaces  To do so, render the scene repeatedly in 6 directions for each reflective object

 Transmittance is the amount of light that passes through a sample  When the materials are all a uniform thickness, a simple color filter can be used  Otherwise, the Beer-Lambert Law must be used to compute the affect on the light d  T = e - α cd

 Refraction is how a wave bends when it the medium it is traveling through changes  Examples with light:  Pencil looks bent in water  Mirages

 The amount of refraction is governed by Snell's Law  It relates the angle of incidence and the angle of refraction of light by the equation: θ sin n = 1 1 θ sin n 2 2

 Light is focused by reflective or refractive surfaces  A caustic is the curve or surface of concentrated light  The name comes from the Greek for burning Refractive: Reflective:

 Subsurface scattering is where light enters an object bounces around and exits at a different point than it entered  Causes:  Foreign Particles (pearls)  Discontinuities (air bubbles)  Density variations  Structural changes

 To create a realistic scene, it is necessary for light to bounce between surfaces many times  This causes subtle effects in how light and shadow interact  This also causes certain lighting effects such as color bleeding (where the color of an object is projected onto nearby surfaces)

 Radiosity simulates this  Turn on the light sources and allow the environmental light to reach equilibrium (stable state)  While the light is in stable state, each surface may be treated a light source  The equilibrium is found by forming a square matrix out of form factors for each patch times the patch’s reflectivity  Gaussian Elimination on the resulting matrix gives the exitance (color) of the patch in question

 Trace rays from the camera through the screen to the closest object, the intersection point .  For each intersection point, rays are traced:  A ray to each light source  If the object is shiny, a reflection ray  If the object is not opaque, a refraction ray  Opaque objects can block the rays, while transparent objects attenuate the light  Repeat recursively until all points on the screen are calculated

 Classical ray tracing is relatively fast but performs poorly for environmental lighting and diffuse interreflections  In Monte Carlo ray tracing, ray directions are randomly chosen, weighted by the BRDF  This is called importance sampling .  Monte Carlo ray tracing gets excellent results but takes a huge amount of time

 Full global illumination is expensive  If the scene and lighting are static, much can be precomputed  Simple surface prelighting uses a radiosity render to determine diffuse lighting ahead of time  Directional surface prelighting stores directional lighting information that can be used for specular effects  Much more expensive in memory  Volume information can be precomputed to light dynamic objects

 Global illumination algorithms precompute various quantities other than lighting  Often, a measure of how much parts of a scene block light are computed  Bent normal, occlusion factor  These precomputed occlusion quantities can be applied to changing light in a scene  Create a more realistic appearance than precomputed lighting alone

 Precomputed ambient occlusion factors are only valid on stationary objects  Example: a racetrack  For moving objects (like a car), ambient occlusion can be computed on a large flat plane  This works for rigid objects, but deformable objects would need many precomputed poses  Example: a human  Also can be used to model occlusion effects of objects on each other

 The total effect of dynamic lighting conditions can be precomputed and approximated  This method is trying to take into account all possible lightings from all possible angles  Computing all this stuff is difficult, but a compact representation is even more difficult  Spherical harmonics is a way of storing the data, sampled in many directions  Storage requirements can be large  Results generally hold only for distant lights and diffuse shading

 We can imagine all the different rendering techniques as sitting on a spectrum reaching from purely appearance based to purely physically based Sprites Billboards Appearance Physically Based Based Layers Triangles Global Lightfields illumination

 When objects are close to the viewer, small changes in viewing location can have big effects  When objects are far away, the effect is much smaller  As you know by now, a skybox is a large mesh containing the entire scene  Some skyboxes look crappy because there isn't enough resolution screen resolution  Minimum texture resolution (per cube face) = tan(fov/2)

 If you are trying to recreate a complex scene from reality, you can take millions of pictures from of it from many possible angles  Then, you can use interpolation and warping techniques to stitch them together  Huge data storage requirements  Each photograph must be catalogued based on location and orientation  High realism output!

 A sprite is an image that moves around the screen  Sprites were the basis of most old 2D video games (back when those existed, before the advent of Flash)  By putting sprites in layers, it is possible to make a compelling scene  Sequencing sprites can achieve animation

 Applying sprites to 3D gives billboarding  Billboarding is orienting a textured polygon based on view direction  Billboarding can be effective for objects without solid surfaces  If the object is supposed to exist in the world, it needs to change as the world changes  For small sprites (such as particles) the billboard's surface normal can be the negation of the view plane normal  Larger sprites should have different normals that point the billboard directly at the viewpoint

In a particle system, many small, separate objects are controlled using some algorithm  Applications:   Fire  Smoke  Explosions  Water Particle systems refer more to the animation than to the rendering  Particles can be points or lines or billboards  Modern GPUs can generate and render particles in hardware 

 An impostor is a billboard created on the fly by rendering a complex object to a texture  Then, the impostor can be rendered more cheaply  This technique should be used to speed up the rendering of far away objects  The resolution of the texture should be at least: object size screen resolution ⋅ ⋅ 2 distance tan(fov/2)

 Impostors have to be recomputed for different viewing angles  Certain kinds of models (trees are a great example) can be approximated by a cloud of billboards  Finding a visually realistic set of cutouts is one part of the problem  The rendering overhead of overdrawing is another  Billboards may need to be sorted if transparency effects are important

 Image processing takes an input image and manipulates it  Blurring  Edge detection  Color correction  Tone mapping  Much of this can be done on the GPU

 Lens flare is an effect caused by bright light hitting the crystalline structure of a camera lens  The bloom effect is where bright areas spill over into other areas  Depth of field simulates a camera's behavior of blurring objects based on how far they are from the focal plane  Motion blur blurs fast moving objects  Fog can be implemented by blending a color based on distance of an object from the viewpoint

 Most of the work we've focused on all semester is doing rendering that in some way mirrors the natural world  However, a wide area of rendering is non- photorealistic rendering (NPR)  Goals:  Simplified technical drawings  Simulating artistic styles

 The most common form of NPR in video games is toon shading  Also called cel shading  The goal is to render 3D models as if they were cartoons  Shading is often done with either a single color or a two tone (color and shading) approach  Then a thick black silhouette is added around edges

 The color is often determined by the dot product n · l (surface normal dot light vector)  If negative, the surface should be darkened  Otherwise, it's some flat color  Or a threshold other than 0 can be used  A more complex system uses a one dimensional texture indexed into with the dot product Highlight Normal Shadow