Illumination

Overview of Lighting
 Different types of lighting (diffuse, specular, ambient,
etc.)
 In almost all cases, we need to know about how light
bounces off of a surface
 This means knowing about the normal at the intersection
point
Ray Tracing
 In 3D computer graphics, ray tracing is a technique for
modeling light transport for use in a wide variety
of rendering algorithms for generating digital images.
 Ray tracing is capable of simulating a variety
of optical effects, such as reflection, refraction, soft
shadows, scattering, depth of field, motion
blur, caustics, ambient occlusion and dispersion
phenomena (such as chromatic aberration).
Ray Tracing
 Ray Tracing

The ray-tracing
algorithm builds an
image by extending
rays into a scene and
bouncing them off
surfaces and towards
sources of light to
approximate the color
value of pixels.
Ray Tracing
 Ray Tracing

Ray tracing can

create
photorealistic
images.
 Ray Tracing
In addition to the high degree of realism, ray tracing can simulate the effects of a
camera due to depth of field and aperture shape
What is Light?
 Can be thought of as small packets of photons
 When an electron drops from a higher level orbit to a lower orbit,
energy is emitted as photons
 Different energy levels of electrons allow for different wavelengths of
light to be reflected
 Metals have “loose” electrons, which can move relatively freely and occupy
many different levels, which is why they are more reflective
 Insulators have more constrained electrons which cannot change energy levels
as easily, so they convert more absorbed light into heat instead of reflecting it
What is Light?

 Another way to think

of light is to treat it as a
continuous wave (as
opposed to discrete
photons)

 The wavelength (𝜆) of a wave is measured as the distance from one

peak of wave to the next
 Different colors correspond to different wavelengths
 Visible light has a wavelength between 400nm – 700nm
 Different ways of thinking about light (photons vs. waves)
can result in different rendering styles
Illumination
 Problem:
 In ray casting, we have generated a ray (from the eye through a
pixel), found the point in which the ray intersects with an
object
 Now we want to determine the color and brightness of that
point so that we can color in the pixel
 Solution: we model the interactions between light
sources and the surface
Illumination Models

 An “illumination model” describes inputs,

assumptions, and outputs used to calculate
illumination (color / brightness) of surface
elements
 Usually includes
 Light attributes (intensity, color, position, direction, shape)
 Object surface properties (color, reflectivity, transparency, etc.)
 Interaction between lights and objects
Illumination Models

 Physically-based Illumination Models

 Some models of illumination are based on real physics
 Require accurate input data and make few assumptions
 Rarely have enough information to specify all inputs
 Takes a long time to compute
 Non-Physically-Based Illumination Models
 Just need a model that looks good enough for a specific
application
 Emphasis on speed and efficiency (use of resources like
memory, CPU, etc.)
Light Attenuation: Inverse Square Law
 The amount that a light illuminates an object decreases with
the square of the distance between them. This is known as
Light Attenuation.
 Inverse square law: for a given 3D angle (solid angle), the area it
covers grows with the square of the distance
 Intensity of light per unit area falls off with the inverse square
 Conversely, for a fixed area, the angle it covers decreases with the square
of the distance
Basic Light Sources

 As part of illumination models, one can specify

whether the light intensity varies with the distance
between the object and the light source
Illumination Models
 Local Illumination
 Takes only direct lighting information
 Is an approximation of global illumination
 Usually involves the use of an “ambient” term to simulate
indirect lighting
 Global Illumination
 Most light striking a surface element comes directly from a
light emitting source (direct illumination)
 Sometimes light from a source is blocked by another object,
resulting in shadows
 However, objects in the shadow can still receive light from light
bouncing off other objects (indirect illumination)
Example 1: Local Illumination

 Only considers the light, the observer position, and

the object’s material properties
 OpenGL does this.
Example 2: Global Illumination

 Takes into account of the interaction of light from

all the surfaces in the scene
 Recursive ray tracing is an example. It models light
rays bouncing between objects
Example 3: Global Illumination, Radiosity

 Models energy moving

from emitters (light
sources) into the scene
as patches
 Is view independent
 Examples of Global Illumination

Direct illumination + specular reflection + soft shadows and caustics + diffuse reflection (color bleeding)
Ray trace Ray trace + caustic photon map Ray trace + caustic and diffuse photon maps
Simple Local Illumination (Phong)
 The model used by OpenGL
 Reduce the light model to 3 simple components:
 Ambient
 Diffuse
 Specular
 Illumination (color intensity) of a point is equal to:
 Intensity = ambient + diffuse + specular
 Materials reflect each component differently
 Specified as material reflection coefficients: 𝐾𝑎 , 𝐾𝑑 , 𝐾𝑠
Ambient Light Contribution

 Ambient light = background light

 As noted before, it is used to simulate indirect lighting

 Ambient component
 Independent of
 Object’s position
 Viewer’s position
 Light source’s position
 Dependent of
 A constant factor (in each of the R, G, B channels)
Diffuse Light Contribution

 Diffuse light = illumination that a surface receives

from a light source that reflects equally in all
directions

 Independent of:
 Viewer’s position

 Dependent of:
 Light’s position
 Surface property (normal, reflectance property)
Diffuse Light Calculation

 Need to know how much light a point on the object

receives from the light source
 Solution based on Lambert’s Law

Point receives more light Point receives less light

Diffuse Light Calculation
 Lambert’s Law: The radiant energy D that a small surface
patch (or a point) receives from a light source is:
 Diffuse = 𝐾𝑑 𝑥 𝐼 𝑥 cos(𝜃)
 𝐾𝑑 = diffuse reflection constant
 I = light intensity
 𝜃 = angle between the light vector and the surface normal
 How do we compute cos(𝜃)?
 𝑁 ∙ 𝐿 (dot product between N and L)

As the angle between the light The hemisphere represents

and the normal increases, the equal magnitude of the reflected
light’s energy spreads across a intensity for any outgoing
larger area vector.
Diffuse Light Examples
 Diffuse = 𝐾𝑑 𝑥 𝐼 𝑥 cos(𝜃)
 For I = 1.0
Specular Light Contribution

 Specular light = light reflection from shiny surfaces

 Color depends on material and how it scatters light
 Shiny surfaces (metal, mirror, etc.) reflect more light
 Specular light depends on both light source position
and view position
Specular Light Calculation

 𝜙 is the angle between the view direction and the

reflective vector
 When 𝜙 is small, the viewer sees more specular
light
Specular Light Calculation
 The Phong lighting model (not the Phong shading model)
 Specular = 𝐾𝑠 𝑥 𝐼 𝑥 cos 𝑓 (𝜙)
 𝐾𝑠 = specular reflection constant
 𝐼 = light intensity
 𝜙 = angle between reflective ray and view vector
 𝑓 = surface property for specular highlight
Specular Light Calculation
 Specular = 𝐾𝑠 𝑥 𝐼 𝑥 cos𝑓 (𝜙)

Diffuse hemisphere with specular Shape of the Gaussian

Gaussian surface
Specular Light Examples
 Specular = 𝐾𝑠 𝑥 𝐼 𝑥 cos𝑓 (𝜙)
 For I = 1.0
Variation in Diffuse and Specular

 Putting Diffuse and Specular together in our local

illumination model
Lighting Equation

 Direct Illumination = ambient + diffuse + specular

 color =𝐾𝑎 𝑥 𝐼 + 𝐾𝑑 𝑥 𝐼 𝑥 (𝑁 ∙ 𝐿) + 𝐾𝑠 𝑥 𝐼 𝑥 𝑅 ∙ 𝑉 𝑓

 If there are m lights, we sum over each light

 color =𝐾𝑎 𝑥 𝐼 + σ𝑚(𝐾𝑑 𝑥 𝐼𝑚 𝑥 (𝑁 ∙ 𝐿𝑚 ) + 𝐾𝑠 𝑥 𝐼𝑚 𝑥 𝑅𝑚 ∙ 𝑉 𝑓 )
 color =𝐾𝑎 𝑥 𝐼 + σ𝑚 𝐼𝑚 𝑥 (𝐾𝑑 𝑥 (𝑁 ∙ 𝐿𝑚 ) + 𝐾𝑠 𝑥 𝑅𝑚 ∙ 𝑉 𝑓 )
Cukup Sekian