9. Illumination and Shading

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1 9. Illumination and Shading Approaches for visual realism: - Remove hidden surfaces - Shade visible surfaces and reproduce shadows - Reproduce surface properties Texture Degree of transparency Roughness, etc 1 CS Cept, UK

2 9.1 How to shade a visible surface? depends on: position orientation characteristics of the surfaces light sources 2 CS Cept, UK

3 9.1 How to shade a visible surface? Light sources: Light-emitting sources -- light bulb, sun,... Light-reflecting sources illuminated surfaces of other objects 3 CS Cept, UK

4 9.1 How to shade a visible surface? 4 CS Cept, UK

5 9.1 How to shade a visible surface? Ambient light (background light) - results of multiple reflections from nearby objects - can be considered to be of uniform intensity I in all a directions - use a general level of brightness to model it to produce a uniform illumination - reflected in all directions 5 CS Cept, UK

6 9.1 How to shade a visible surface? Shade at a point of a surface = intensity of light reflection from that point of the surface = diffuse reflection of ambient light + diffuse and specular reflection of one or more point light sources + diffuse and specular reflection of transparent effect 6 CS Cept, UK

7 Diffuse Reflection 7 CS Cept, UK

8 Diffuse Reflection 8 CS Cept, UK

9 Phong Shading n: Phong exponent 9 CS Cept, UK

10 Phong Shading 10 CS Cept, UK

11 Phong Shading Alternative approach: Instead of R, compute H, unit vector halfway between L and V: H ( V L) / V then use instead of In Phong model L Specular Reflection = I p W( ) cos n 11 CS Cept, UK

12 9.1 How to shade a visible surface? Hence, shade at point P (viewpoint at infinity) I I a k a I p n [ k cos W( )cos ] d or (viewpoint not at infinity) I I a k a I p [ k cos W n ( )cos ]/( r r d 0) r: distance from viewpoint to P; : constant 12 CS Cept, UK r 0

13 9.1 How to shade a visible surface? 13 CS Cept, Why? UK

14 9.1 How to shade a visible surface? Let R=aL+bN, From we get From a, b to be determined. cos R N acos b b ( 1 a)cos 1 acos(2 ) bcos we get a 1 Hence, b 2cos 2( L N) 14 CS Cept, UK

15 9.1 How to shade a visible surface? Ambient + Diffuse + Specular = Result 15 CS Cept, UK

16 Effect of Transparency: If the surface is translucent then contribution from light sources behind the object has to be included as well. 16 CS Cept, UK

17 Effect of Transparency: Shade at point P I(1 k ) k = pt pt pt I I: intensity computed on page CS Cept, UK

18 Effect of Transparency: Shade at point P I(1 k ) k = pt pt pt I Simplified model: refraction and object thickness are neglected 18 CS Cept, UK

19 9.2 Polygon-Mesh Rendering Methods Rendering of graphics objects approximated by Polygonal meshes Shading Models: Flat Shading Gouraud shading (color interpolation shading ) Phong shading (normal interpolation shading ) 19 CS Cept, UK

20 9.2 Polygon-Mesh Rendering Methods Flat Shading (Constant-Intensity Shading) a single intensity is computed for each polygon assumption: light source and viewer are both sufficiently far from the object works only if the polygons are small 20 CS Cept, UK

21 9.2 Polygon-Mesh Rendering Methods Gouraud Shading (Intensity- Interpolation Shading) Eliminate intensity discontinuities Basic idea: 1. compute polygon (unit) normals 2. compute the average unit normal vector at each polygon vertex 3. compute vertex intensities 4. shade polygon by linear interpolation of vertex intensities along each edge and then between edges along each scan line 21 CS Cept, UK

22 9.2 Polygon-Mesh Rendering Methods May cause Mach band effect (bright or dark intensity streaks, occurs when the light intensity on a surface changes abruptly) 22 CS Cept, UK

23 9.2 Polygon-Mesh Rendering Methods Mach band effect (bright or dark intensity streaks, occurs when the light intensity on a surface changes abruptly) 23 CS Cept, UK

24 9.2 Polygon-Mesh Rendering Methods Phong Shading (Normal-Vector-Interpolation Shading) Interpolates the normal vector N rather than intensity Basic idea: 1. compute polygon normals 2. compute vertex normals 3. compute normals of start and end points of a visible span using linear interpolation 4. compute normal at each pixel along a scan line, then compute intensity using a desired illumination model 24 CS Cept, UK

25 Comparison of shading models 25 CS Cept, UK

26 Comparison of shading models 26 CS Cept, UK

27 Comparison of shading models 27 CS Cept, UK

28 Gouraud Shading can be integrated with the scan-line method 28 CS Cept, UK

29 Phong Shading can also be integrated with the scan-line method Highlights are more faithfully reproduced Reduces Mach band problems 29 CS Cept, UK

30 y 2 R 2 y y 1 R R min Rmin R 1 R min R min R current scan line when we move toa new scan line R ( R2 R1 ) /( y2 y1) 30 CS Cept, UK

31 9.3 Shadows Determines which faces or parts of faces are visible when viewed from the light source(s) Surfaces visible from both the viewpoint and the light source are not in shadow Surfaces visible from the viewpoint but not from the light source are in shadow 31 CS Cept, UK

32 Shadow Volume Method basic idea silhouette unshadowed light Side view front-facing Shadow polygon light back-facing shadow polygon shadowed eye CS Cept, UK

33 Shadow Volume Method light Implementation: shadow polygon silhouette shadow volume 1.Each edge of the object that is part of the object s silhouette when viewed from the light source is extended to form a shadow polygon 33 CS Cept, UK

34 Shadow Volume Method Shadow polygon silhouette light If several light sources are being simulated then shadow polygons generated by different light sources will be tagged differently Shadow polygons are passed to the scan conversion process along with polygons in the scene 34 CS Cept, UK

35 Shadow Volume Method 2. As the scan conversion process proceeds, if an even number of shadow polygons with the same tags is encountered, the face is not shadowed by the object and the light source given in the tags. A set of tags encountered an odd times indicates that the face is shadowed by the light source in the tags 35 CS Cept, UK

36 Shadow Volume Method How to determine if an edge is part of the silhouette when viewed from the light source? Draw a plane through e that is parallel to the DOP If all the other vertices of the object are on the same side of this plane then e is an edge of the silhouette 36 CS Cept, UK

37 Shadow Map Method Apply Z-buffer method twice Once with respect to the viewpoint (needs Depth array, D1, and Intensity array, I ) Once with respect to the light source (needs a depth array, D2, only) 37 CS Cept, UK

38 Shadow Map Method Depth array D2 visible to the eye but not to the light light Depth array D1 viewpoint 38 CS Cept, UK

39 Shadow Map Method For each (x, y) in D1, if (x, y, z) is the visible point for this pixel, compute the corresponding (a, b, c) in the light source coordinate system If the depth of (a, b) in D2 is smaller than c, then (x, y, z) is in shadow 39 CS Cept, UK

40 Potential problem of Shadow Map Method Depth array D2 A B Depth array D1 light Theoretically B should not be in shadow. But because of rounding error, sometime we might conclude that B is in shadow (blocked by A) viewpoint 40 CS Cept, UK

41 9.4 Light-Transmitting Surfaces specular transmittance: refracted & unrefracted diffuse transmittance: not easy to model 41 CS Cept, UK

42 9.4 Light-Transmitting Surfaces Using scan-line algorithm (ignoring refraction) 1. Within a span, sort the segments by depth 2. Find the opaque polygon closest to the viewer and determine its shade 3. This shade is then modified by any transparent segments that lie between the observer and the opaque surface opaque I I1 ( 1 k) I2k k : degree of transmittance of P1 transparent 42 CS Cept, UK

43 End of CS Cept, UK

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