# Computer Graphics. Lecture 14 Bump-mapping, Global Illumination (1)

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1 Computer Graphics Lecture 14 Bump-mapping, Global Illumination (1)

2 Today - Bump mapping - Displacement mapping - Global Illumination Radiosity

3 Bump Mapping - A method to increase the realism of 3D objects without editing their geometry - By adding high resolution bump maps - In a way similar to texture mapping

4 What's Missing? What's the difference between a real brick wall and a photograph of the wall texture-mapped onto a plane? No shadows between the bricks

5 This is what we want

6 How do we achieve this? High resolution brick wall? - Difficult to prepare the model - Very costly to handle during run-time

7 Bump Mapping Use textures to alter the surface normal but not the geometry Done during the rasterization stage Can synthesize realistic images without using high resolution meshes Sphere w/diffuse Texture Swirly Bump Map Sphere w/diffuse Texture & Bump Map

8 Preparing Data Prepare a bump map whose values change from 0 to 1 Prepare a mapping of vertices to uv coordinates Cylinder w/texture Map & Bump Map

9 Procedure During rasterization, compute the barycentric coordinates of the pixel and compute its uv coordinates by interpolation Lookup the bump map and compute the finite difference of the bump map (Fu, Fv) = (df/du, df/dv) u= α u + β u2 + γ u3 v = α v + β v2 + γ v3 1 1

10 Procedure - continued When rasterizing a pixel in the triangle, compute the original normal vector n and the its uv coordinates by barycentric coordinates Perturb the normal vector by where (Fu, Fv) are the partial derivatives of the bump map, and Pu, Pv are the partial derivative of the geometry with respect to the uv Do the lighting computation using n

11 Computing Fu, Fv Simply compute the finite difference of the height map at the corresponding uv v u

12 Computing Pu, Pv How P changes according to u, v in the texture space v u

13 Or compute Pu, Pv in screen space using chain rule y x

14 Or compute Pu, Pv in screen space using chain rule y x These can be computed in the screen space by finite difference

15 Or compute Pu, Pv in screen space using chain rule y where x

16 Preparing Bump Maps Can be produced from photos or designed by the user If it is from an image, convert the image to greyscale Adjust the contrast, such the min is 0 and max is 1 White part bumps out and black color bump inwards Need to make sure there is no specular highlights in the image

17 Some more examples

18 Some more examples

19 Some more examples

20 What's Missing? There are no actual bumps on the silhouette of a bump-mapped object

21 Displacement Mapping - Use the texture to actually move the surface point - The texture is a 3D mesh defined in the uv space

22 How it works Tessellate (subdivide) the base polygon mesh such that its resolution is same as the displacement map 2. Move each vertex in the normal direction of the base polygon mesh as much as the height in the displacement map

23 Displacement Mapping: Discussions The cost increases as the polygon numbers is high Done in the geometry stage and not in the rasterization stage Can adaptively tessellate the base polygon mesh for the region the spatial frequency is high Methods to do it in the image space are getting popular

24 Today - Bump mapping - Displacement mapping - Global Illumination Radiosity

25 Background Rendering methods can be classified into Local Illumination techniques Global Illumination techniques

26 Local Illumination methods Considers light sources and surface properties only. Phong Illumination, Phong shading, Gouraud Shading Very fast Used for real-time applications such as 3D computer games

27 Local Illumination : problems Images synthesized appear artificial No inter-surface reflections Ambient light is a very simplified model Requires the user to add the effects like shadows and mirrors one by one Shadow maps, shadow volume, shadow texture for producing shadows Mirroring by reflecting the world or environment mapping

28 Global Illumination In the real-world, light comes from all directions (ambient light) This is due to inter-reflections Global illumination methods handle such effects

29 Global Illumination Methods Radiosity Monte-Carlo ray tracing, Photon Mapping Requires more computation and is slow

30 The Radiosity Method (85 -) Based on a method developed by researchers in heat transfer in 1950s Applied to computer graphics in the mid 1980s by Michael Cohen Tomoyuki Nishita

31 The Radiosity Method (85 -) Can simulate inter-surface reflection Can produce nice ambient effects Can simulate effects such as Soft shadows, color bleeding Can only handle diffuse color need to be combined with raytracing to handle specular light

32 Color bleeding

33 Color Bleeding

34 The Radiosity Model At each surface in a model the amount of energy that is given off (Radiosity) is comprised of Bj the energy that the surface emits internally (E), plus the amount of energy that is reflected off the surface (ρh) Eq. 1

35 The Radiosity Model (2) The amount of incident light hitting the surface can be found by summing for all other surfaces the amount of energy that they contribute to this surface Eq. 2 Form factor

36 Form Factor (Fij) The fraction of energy that leaves surface i and lands on surface j Between differential areas, it is The overall form factor between i and j is

37 Form Factor (2) Also need to take into account occlusions The form factor for those faces which are hidden from each other must be zero

38 The Radiosity Matrix The radiosity equation now looks like this: The derived radiosity equations form a set of N linear equations in N unknowns. This leads nicely to a matrix solution: Solve for Bi Use methods like Gauss-Seidal

39 Radiosity Steps: 1. Generate Model (set up the scene) 2. Compute Form Factors and set the Radiosity Matrix 3. Solve the linear system 4. Render the scene

40 Radiosity Steps: 1. Generate Model (set up the scene) 2. Compute Form Factors and set the Radiosity Matrix 3. Solve the linear system 4. Render the scene

41 Radiosity Steps: 1. Generate Model (set up the scene) 2. Compute Form Factors and set the Radiosity Matrix 3. Solve the linear system 4. Render the scene

42 Radiosity Steps: 1. Generate Model (set up the scene) 2. Compute Form Factors and set the Radiosity Matrix 3. Solve the linear system 4. Render the scene

43 Radiosity Steps: 1. Generate Model 2. Compute Form Factors and set the Radiosity Matrix 3. Solve the linear system 4. Render the scene Where do we resume from if objects are moved? Where do we resume from if the lighting is changed? Where do we resume from if the reflectance parameters of the scene are modified? Where do we resume from if the view point changes?

44 Radiosity Features: Very costly The faces must be subdivided into small patches to reduce the artefacts The computational cost for calculating the form factors is expensive Solving for Bi is also very costly Cubic to the number of patches Cannot handle specular light Quadratic to the number of patches

45 Hemicube: by Michael Cohen 85 Accelerates the computation of the form factor The form factor for the right four faces with respect to a small patch in the bottom is the same Then, we can project the patches onto a hemicube

46 Hemicube (2) Prepare a hemicube around the patch i Project those polygons you want to compute the form factor with patch i onto the hemicube Then, compute the form factor between them

47 Hemicube (3) This can be done by perspective projection We can use the Z-buffer algorithm to find the closest polygon Handling the occlusion Setting the form factor of the pairs that occlude each other to zero

48 Hemicube (4) The form factor between each pixel of the hemicube and the patch at the origin can be pre-computed and saved in a table Only 1/8 of all are needed, thanks to symmetry

49 Summary Bump maps can be used to increase the reality without increasing the resolution of the meshes Global illumination methods simulate inter-reflectance Radiosity can simulate diffuse interreflectance The form factor computation can be accelerated by hemi-cube.

50 Readings Blinn, "Simulation of Wrinkled Surfaces", Computer Graphics, (Proc. Siggraph), Vol. 12, No. 3, August 1978, pp Real-time Rendering, Chapter 5, dk/projects/downloads/tangent_matrix_deriv ation.php Cohen et al., The hemi-cube: a radiosity solution for complex environments, SIGGRAPH 85

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