# Lecture 14 Illumination II – Global Models

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Lecture 14 Illumination II – Global Models
Computer Graphics Lecture 14 Illumination II – Global Models

Global Illumination Extends the Local Illumination Model to include:
Reflection (one object in another) Refraction (Snell’s Law) Transparency (better model) Shadows (at point, check each light source) Antialiasing (usually means supersampling) 6/11/2007 Lecture Notes #14

Wireframe view of a test scene.
Orthographic view from above 6/11/2007 Lecture Notes #14

Test Scene. High quality rendering of test scene. Note :
Mirror and chrome teapot. Shadows on floor. Shiny floor. 6/11/2007 Lecture Notes #14

Locally illuminated test scene.
Ambient term only 6/11/2007 Lecture Notes #14

Locally illuminated test scene.
Phong shading. Ambient and Diffuse terms only Notes : Highlight on wall from light is in the wrong place; screen space interpolation. We cannot illuminate the lights with the light sources – wrong side ! 6/11/2007 Lecture Notes #14

Locally illuminated test scene.
Phong shading. Ambient, diffuse and Specular terms. 6/11/2007 Lecture Notes #14

Locally illuminated test scene.
Flat shading. Note : Mach bands. 6/11/2007 Lecture Notes #14

Locally illuminated test scene.
Gouraud shading. Ambient, diffuse and Specular terms. Note: artefacts on wall. 6/11/2007 Lecture Notes #14

Solution to Gouraud artefacts.
Gouraud shading. Re-triangulated mesh. 6/11/2007 Lecture Notes #14

Comparison. Flat Gouraud Phong Coarser mesh 6/11/2007
Lecture Notes #14

Use Local illumination.
No. In our test scene, we can’t represent : Mirror Chrome teapot. Shiny floor Shadows with local illumination. 6/11/2007 Lecture Notes #14

Kajiya’s Rendering Equation.
Viewer is at point x, looking toward point x. I(x,x) determines amount of light arriving at x from x g(x,x) is a geometry term, = 0 when x is occluded, otherwise = attenuation factor 1/r2 (or 1/(s+k)) (x,x) is the amount of light emitted from x to x. (x,x,x) is the fraction of light reflected and scattered off x to point x from point x The integral S is over all such points (x") on all surfaces. 6/11/2007 Lecture Notes #14

Global illumination. Two methods : View dependent methods.
Calculate the view from the camera with global illumination. Recursive ray-tracing. View independent methods. Solve lighting for the entire scene. Radiosity solution. 6/11/2007 Lecture Notes #14

View dependent methods.
Loop round the pixels….. Good for lighting effects which have a strong dependence on view location : Specular highlights. Reflections from curved surfaces. Only a small number of objects need to be considered at the same time. Poor when many objects need to be considered E.g diffuse interactions (eg. colour bleeding). 6/11/2007 Lecture Notes #14

View independent methods.
Loop round the scene… Good when many (all) objects need to be considered at same time. Diffuse inter-reflections. Poor when shading has strong dependence on view location. Specular reflection. 6/11/2007 Lecture Notes #14

Recall : Ray casting. Involves projecting an imaginary ray from the centre of projection (the viewers eye) through the centre of each pixel into the scene. The first object the ray intersects determines the shade. Scene Window Eyepoint 6/11/2007 Lecture Notes #14

Whitted’s algorithm. Fire off secondary rays from surface that ray intersects. Towards Light Source(s) : shadow rays. L (shadow feelers) In the reflection direction : reflection rays, R In a direction dictated by Snell’s Law : transmitted rays, T 6/11/2007 Lecture Notes #14

Recursive ray tree. Reflection and Transmission Rays spawn other rays.
Shadow rays test only for occlusion. The complete set of rays is called a Ray Tree. Light Source ray determines colour of current object. Viewpoint 6/11/2007 Lecture Notes #14

Recursive ray tree. Reflection and Transmission Rays spawn other rays.
Shadow rays test only for occlusion. The complete set of rays is called a Ray Tree. 6/11/2007 Lecture Notes #14

Test Scene. Ray tree depth 1.
Note only ambient shade on mirror and teapot 6/11/2007 Lecture Notes #14

Test Scene. Ray tree depth 2.
Note only ambient shade on reflection of mirror and teapot. 6/11/2007 Lecture Notes #14

Test Scene. Ray tree depth 3.
Note only ambient shade on reflection of mirror in teapot. 6/11/2007 Lecture Notes #14

Test Scene. Ray tree depth 4.
Note ambient shade on reflection of teapot in reflection of mirror in teapot. 6/11/2007 Lecture Notes #14

Test Scene. Ray tree depth 5. 6/11/2007 Lecture Notes #14

Test Scene. Ray tree depth 6. 6/11/2007 Lecture Notes #14

Test Scene. Ray tree depth 7. 6/11/2007 Lecture Notes #14

When to stop ? Need to know when to stop the recursion.
Can define a fixed depth. Hall introduced adaptive tree depth control. Calculate maximum contribution of a ray to a pixels final value. Multiply contribution of ray’s ancestors down the tree. Stop when below some threshold, perhaps stack overflow. May miss major contribution this way (culled bright pt) 6/11/2007 Lecture Notes #14

0.3 Viewpoint 0.2 6/11/2007 Lecture Notes #14

0.3 0.3 * 0.2 Viewpoint 0.2 * 0.2 0.2 6/11/2007 Lecture Notes #14

Global vs. Local illumination.
In both an object hit by a ray, if lit by a light source, is illuminated by a local illumination model, i.e with specular, diffuse & ambient terms. Global: a reflected ray, a shadow feeler, and a transmission ray (if appropriate) are also cast into the scene. Phong term only reflects light source. Need to adjust local illumination terms to normalise total light values. Inconsistent if local and global specular terms used together as local term spreads light source, global term does not. 6/11/2007 Lecture Notes #14

Increased reflectivity
Phong specular term is held constant Increased transmissivity 6/11/2007 Lecture Notes #14

Incorrect result. Effect of not normalising reflection and transmission – light appears to be created. Reflection & transmission = 100% 6/11/2007 Lecture Notes #14

Problems with Ray-tracing.
A serious problem with Ray tracing is rays are traced from the eye. Refraction is not physically correct. Shadow rays are cast only to light sources Lights reflected in mirrors do not cast shadows Shadows of transparent objects don’t exhibit refraction. Still need local illumination for diffuse shading. 6/11/2007 Lecture Notes #14

Speeding up Ray Tracing
Ray tracing is slow, not real-time. Use appropriate extents for objects. Ray tracing is inherently parallel. Use item buffers – z-ordered lists, store closest object per pixel. Use light buffer – z-ordered list per light ray used for shadowing. 6/11/2007 Lecture Notes #14

Recommended Reading Foley at al. Chapter 16, sections 16.11, and Introductory text Chapter 14 sections 14.6 and 14.7. Most graphics texts cover recursive ray tracing. 6/11/2007 Lecture Notes #14