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Computer Graphics Inf4/MSc Computer Graphics Lecture 14 Illumination II – Global Models.

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Presentation on theme: "Computer Graphics Inf4/MSc Computer Graphics Lecture 14 Illumination II – Global Models."— Presentation transcript:

1 Computer Graphics Inf4/MSc Computer Graphics Lecture 14 Illumination II – Global Models

2 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #142 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)

3 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #143 Wireframe view of a test scene. Orthographic view from above

4 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #144 Test Scene. High quality rendering of test scene. Note : Mirror and chrome teapot. Shadows on floor. Shiny floor.

5 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #145 Locally illuminated test scene. Ambient term only

6 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #146 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 !

7 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #147 Locally illuminated test scene. Phong shading. Ambient, diffuse and Specular terms.

8 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #148 Locally illuminated test scene. Flat shading. Note : Mach bands.

9 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #149 Locally illuminated test scene. Gouraud shading. Ambient, diffuse and Specular terms. Note: artefacts on wall.

10 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1410 Solution to Gouraud artefacts. Gouraud shading. Re-triangulated mesh.

11 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1411 Comparison. FlatGouraudPhong Coarser mesh

12 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1412 Use Local illumination. No. In our test scene, we can’t represent : –Mirror –Chrome teapot. –Shiny floor –Shadows with local illumination.

13 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1413 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/r 2 (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.

14 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1414 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.

15 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1415 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).

16 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1416 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.

17 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1417 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

18 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1418 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  

19 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1419 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. Viewpoint Light Source ray determines colour of current object.

20 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1420 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.

21 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1421 Test Scene. Ray tree depth 1. Note only ambient shade on mirror and teapot

22 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1422 Test Scene. Ray tree depth 2. Note only ambient shade on reflection of mirror and teapot.

23 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1423 Test Scene. Ray tree depth 3. Note only ambient shade on reflection of mirror in teapot.

24 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1424 Test Scene. Ray tree depth 4. Note ambient shade on reflection of teapot in reflection of mirror in teapot.

25 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1425 Test Scene. Ray tree depth 5.

26 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1426 Test Scene. Ray tree depth 6.

27 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1427 Test Scene. Ray tree depth 7.

28 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1428 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)

29 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1429 Adaptive tree depth control. Viewpoint 0.3 0.2

30 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1430 Adaptive tree depth control. Viewpoint 0.3 * 0.2 0.2 * 0.2 0.3 0.2

31 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1431 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.

32 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1432 Increased reflectivity Increased transmissivity Phong specular term is held constant

33 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1433 Incorrect result. Effect of not normalising reflection and transmission – light appears to be created. –Reflection & transmission = 100%

34 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1434 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.

35 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1435 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.

36 Computer Graphics Inf4/MSc 6/11/2007Lecture Notes #1436 Recommended Reading Foley at al. Chapter 16, sections 16.11, 16.12 and 16.12.5. Introductory text Chapter 14 sections 14.6 and 14.7. Most graphics texts cover recursive ray tracing.


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