Today we will start to look at illumination models in computer graphics


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1 1 llumination Today we will start to look at illumination models in computer graphics Why do we need illumination models? Different kinds lights Different kinds reflections Basic lighting model
2 2 Why Lighting? f we don t have lighting effects nothing looks three dimensional!
3 3 Why Lighting? (cont )
4 4 Point Light Sources A point source is the simplest model we can use for a light source We simply define: The position the light The RGB values for the colour the light Light is emitted in all directions Useful for small light sources
5 5 Radial ntensity Attenuation As light moves from a light source its intensity diminished At any distance d l away from the light source the intensity diminishes by a factor 2 However, using the factor d l does not produce very good results so we use something different d l
6 6 Radial ntensity Attenuation (cont ) We use instead in inverse quadratic function the form: 1 fradatten( dl ) 2 a a d a d 0 where the coefficients a 0, a 1, and a 2 can be varied to produce optimal results 1 l 2 l
7 7 nfinitely Distant Light Sources mages taken from Hearn & Baker, Computer Graphics with OpenGL (2004) A large light source, like the sun, can be modelled as a point light source However, it will have very little directional effect Radial intensity attenuation is not used
8 8 Directional Light Sources & Spotlights mages taken from Hearn & Baker, Computer Graphics with OpenGL (2004) To turn a point light source into a spotlight we simply add a vector direction and an angular limit θ l
9 mages taken from Hearn & Baker, Computer Graphics with OpenGL (2004) 9 Directional Light Sources & Spotlights We can denote V light as the unit vector in the direction the light and V obj as the unit vector from the light source to an object The dotproduct these two vectors gives us the angle between them V obj V cos light (cont ) f this angle is inside the light s angular limit then the object is within the spotlight
10 10 Angular ntensity Attenuation As well as light intensity decreasing as we move away from a light source, it also decreases angularly A commonly used function for calculating angular attenuation is: fangatten al ( ) cos 0 where the attenuation exponent a l is assigned some positive value and angle measured from the cone axis is
11 11 Reflected Light The colours that we perceive are determined by the nature the light reflected from an object For example, if white light is shone onto a green object most wavelengths are absorbed, while green light is reflected from the object White Light Green Light Colours Absorbed
12 12 Surface Lighting Effects The amount incident light reflected by a surface depends on the type material Shiny materials reflect more the incident light and dull surfaces absorb more the incident light For transparent surfaces some the light is also transmitted through the material
13 13 Diffuse Reflection Surfaces that are rough or grainy tend to reflect light in all directions This scattered light is called diffuse reflection
14 14 Specular Reflection Additionally to diffuse reflection some the reflected light is concentrated into a highlight or bright spot This is called specular reflection
15 15 Ambient Light A surface that is not exposed to direct light may still be lit up by reflections from other nearby objects ambient light The total reflected light from a surface is the sum the contributions from light sources and reflected light
16 16 Example
17 17 Example Ambient Diffuse Specular Final mage
18 18 Nate Robin s Tutorial Nate Robin s OpenGL Tutorials available at:
19 19 Basic llumination Model We will consider a basic illumination model which gives reasonably good results and is used in most graphics systems The important components are: Ambient light Diffuse reflection Specular reflection For the most part we will consider only monochromatic light
20 20 Ambient Light To incorporate background light we simply set a general brightness level for a scene This approximates the global diffuse reflections from various surfaces within the scene We will denote this value as a
21 21 Diffuse Reflection First we assume that surfaces reflect incident light with equal intensity in all directions Such surfaces are referred to as ideal diffuse reflectors or Lambertian reflectors
22 22 Diffuse Reflection (cont ) A parameter k d is set for each surface that determines the fraction incident light that is to be scattered as diffuse reflections from that surface This parameter is known as the diffusereflection coefficient or the diffuse reflectivity k d is assigned a value between 0.0 and : dull surface that absorbs almost all light 1.0: shiny surface that reflects almost all light
23 23 Diffuse Reflection Ambient Light For background lighting effects we can assume that every surface is fully illuminated by the scene s ambient light a Therefore the ambient contribution to the diffuse reflection is given as: k ambdiff Ambient light alone is very uninteresting so we need some other lights in a scene as well d a
24 24 Diffuse Reflection (cont ) When a surface is illuminated by a light source, the amount incident light depends on the orientation the surface relative to the light source direction
25 25 Diffuse Reflection mages taken from Hearn & Baker, Computer Graphics with OpenGL (2004) The angle between the incoming light direction and a surface normal is referred to as the angle incidence given as θ
26 26 Diffuse Reflection (cont ) So the amount incident light on a surface is given as: l, incident l cos So we can model the diffuse reflections as: l, diff k k d d l, incident l cos
27 27 Diffuse Reflection (cont ) mages taken from Hearn & Baker, Computer Graphics with OpenGL (2004) Assuming we denote the normal for a surface as N and the unit direction vector to the light source as L then: So: l, diff N L cos k d l ( N 0 L ) if if N N L L 0 0
28 28 Combining Ambient And ncident Diffuse To combine the diffuse reflections arising from ambient and incident light most graphics packages use two separate diffusereflection coefficients: k a for ambient light k d for incident light Reflections The total diffuse reflection equation for a single point source can then be given as: diff k a a k k d a l a ( N L ) if if N N L L 0 0
29 29 Examples
30 30 Specular Reflection mages taken from Hearn & Baker, Computer Graphics with OpenGL (2004) The bright spot that we see on a shiny surface is the result near total the incident light in a concentrated region around the specular reflection angle The specular reflection angle equals the angle the incident light
31 31 Specular Reflection (cont ) mages taken from Hearn & Baker, Computer Graphics with OpenGL (2004) A perfect mirror reflects light only in the specularreflection direction Other objects exhibit specular reflections over a finite range viewing positions around vector R
32 32 The Phong Specular Reflection Model mages taken from Hearn & Baker, Computer Graphics with OpenGL (2004) The Phong specular reflection model or Phong model is an empirical model for calculating specular reflection range developed in 1973 by Phong Bui Tuong The Phong model sets the intensity specular reflection as proportional to the angle between the viewing vector and the specular reflection vector
33 33 The Phong Specular Reflection Model So, the specular reflection intensity is proportional to cos n s (cont ) The angle Φ can be varied between 0 and 90 so that cosφ varies from 1.0 to 0.0 The specularreflection exponent, n s is determined by the type surface we want to display Shiny surfaces have a very large value (>100) Rough surfaces would have a value near 1
34 mages taken from Hearn & Baker, Computer Graphics with OpenGL (2004) 34 The Phong Specular Reflection Model (cont ) The graphs below show the effect n s on the angular range in which we can expect to see specular reflections
35 35 The Phong Specular Reflection Model (cont ) For some materials the amount specular reflection depends heavily on the angle the incident light Fresnel s Laws Reflection describe in great detail how specular reflections behave However, we don t need to worry about this and instead approximate the specular effects with a constant specular reflection coefficient k s For an explanation Fresnel s laws try here
36 36 The Phong Specular Reflection Model (cont ) So the specular reflection intensity is given as: n k cos s l, spec s l Remembering that V R cos we can say: l, spec k s l ( V 0.0 R ) n s if if V V R R 0 and 0 or N N L L 0 0
37 37 Example
38 38 Combining Diffuse & Specular Reflections For a single light source we can combine the effects diffuse and specular reflections simply as follows: diff spec k a a k d l ( N L ) k ( V R ) s l n s
39 Common Exam Question 39 Diffuse & Specular Reflections From Multiple Light Sources We can place any number light sources in a scene We compute the diffuse and specular reflections as sums the contributions from the various sources k ambdiff a a n l1 n l1 l k l, diff d l, spec N L k V R s n s
40 40 Adding ntensity Attenuation To incorporate radial and angular intensity attenuation into our model we simply adjust our equation to take these into account So, light intensity is now given as: ambdiff n f l, radatten fl, angatten l, diff l, spec l1 where f radatten and f angatten are as discussed previously
41 41 RGB Colour Considerations For an RGB colour description each intensity specification is a three element vector So, for each light source: l,, lr Similarly all parameters are given as vectors: a k, k k k k, k, k k, ar ag ab lg d lb dr dg db s k k, k, k sr sg sb
42 42 RGB Colour Considerations (cont ) Each component the surface colour is then calculated with a separate expression For example: lr lg lb, diff kdrlr ( N L ), diff kdglg ( N L ), diff kdblb ( N L )
43 43 Summary To create realistic (or even semirealistic) looking scenes we must model light correctly To successfully model lighting effects we need to consider: Ambient light Diffuse reflections Specular reflections
44 44 nteresting llumination Demo There s a very nice Java illumination model demo which may help you understand the effects different kinds reflections available at: n/materials/hypergraph/illumin/v rml/pellucid.html Try playing with the various parameters and see if you can predict what the sphere will look like
45 45
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