Lighting and Shading. Slides: Tamar Shinar, Victor Zordon

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1 Lighting and Shading Slides: Tamar Shinar, Victor Zordon

2 Why we need shading Suppose we build a model of a sphere using many polygons and color each the same color. We get something like But we want 2

3 Shading Why does the image of a real sphere look like Light-material interactions cause each point to have a different color or shade Need to consider Light sources Material properties Location of viewer Surface orientation (normal) 3

4 General rendering The most general approach is based on physics - using principles such as conservation of energy a surface emits light (e.g., light bulb) or reflects light from other illumination sources, or both [Angel and Shreiner] light interaction with materials is recursive the rendering equation is an integral equation describing the limit of this recursive process

5 Fast local shading models the rendering equation can t be solved analytically numerical methods aren t fast enough for real-time for our fast graphics rendering pipeline, we ll use a local model where shade at a point is independent of other surfaces use Phong reflection model shading based on local light-material interactions

6 Local shading model direct light [Angel and Shreiner] reflected light

7 Global Effects shadow [Angel and Shreiner] translucent surface multiple reflection 7

8 Light-material interactions at a surface, light is absorbed, reflected, or transmitted specular diffuse translucent [Angel and Shreiner]

9 Lights

10 General light source Illumination function: [Angel and Shreiner] integrate contributions from all sources to shade the point

11 Idealized light sources Ambient light Point light Spotlight distant (directional) light [Angel and Shreiner] luminance:

12 Ambient light source achieve a uniform light level no black shadows ambient light intensity at each point in the scene

13 Point light source illumination intensity at p: [Angel and Shreiner]

14 Point light source Most real-world scenes have large light sources Point light sources alone aren t too realistic - add ambient light to mitigate high contrast [Angel and Shreiner] penumbra umbra

15 Point light source Most real-world scenes have large light sources Point light sources alone aren t too realistic - drop off intensity more slowly [Angel and Shreiner] (We will do the right thing for the raytracing project)

16 Spotlights [Angel and Shreiner]

17 Spotlights [Angel and Shreiner]

18 Distant light source characterized by direction [Angel and Shreiner]

19 Shading models

20 Lambertian Reflection Model

21 Lambertian Reflection Model Lambert s cosine law direct: maximum light intensity indirect: reduced light intensity color intensity

22 Lambertian Reflection Model Lambert s cosine law direct: maximum light intensity indirect: reduced light intensity color intensity

23 Lambertian Reflection Model Lambert s cosine law direct: maximum light intensity indirect: reduced light intensity color intensity reflectance

24 Lambertian Reflection Model Lambert s cosine law illumination direct: maximum light intensity indirect: reduced light intensity color intensity reflectance

25 Lambertian Reflection Model

26 Ambient Reflection Surfaces facing away from the light will be totally black

27 Ambient Reflection All surfaces get same amount of ambient light

28 Phong Reflection Model

29 Ambient reflection different ambient coefficients for different colors ambient reflection coefficient

30 Diffuse reflection

31 Diffuse reflection Lambert s cosine law diffuse reflection coefficient direct: maximum light intensity indirect: reduced light intensity

32 Specular reflection Ideal reflector angle of incidence angle of reflection r is the mirror reflection direction

33 Specular reflection Specular surface specular highlight specular reflection is strongest in mirror reflection direction

34 Specular reflection specular reflection coefficient Phong exponent specular reflection drops off with increasing angle

35 Specular reflection plastic Phong exponent metal

36 Phong Reflection Model [Brad Smith, Wikimedia Commons] Ambient Diffuse Specular

37 Phong Reflection Model [Brad Smith, Wikimedia Commons] color intensity reflectance illumination

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