Texture mapping helped to enhance the overall appearance
of an object but failed to convey any real sense of depth.
For example, when looking at the two flat objects in Figure
9-1(a), it is clear that the three-dimensional nature of the
scene, a wall positioned perpendicular on a floor, is not being
conveyed properly. Figure 9-1(b) shows this same scene
illuminated by a properly defined light source.
This lack of depth is the result of uniform lighting, i.e. the
equal illumination of all surfaces.
Figure 9-2(a) shows a uniform lit sphere and Figure 9-2(b)
the same sphere with basic lighting enabled.
The shaded sphere is the result of graduations in the sphere’s
color based on the color of the light source.
In this case the color grey is incrementally decreased from dark grey
Light can be emitted through either self-
emission or reflection.
Light sources are categorized by their light
emitting direction and the energy emitted at
each wavelength – determining the color of
Objects can absorb or reflect light emitted from a
light source depending on the reflecting object’s
Light will thus only be ‘visible’ when illuminated
surfaces have the ability to reflect or absorb the said
Material properties are user-defined parameters
built around rules determining the amount of
scattering or reflection of incident light.
The type of light source also plays an
important role in addition to the object’s
A light type property specifies the type of
light to place in a scene.
This property simply denotes a light source as a
point light, spotlight, or directional light (also
called a parallel light).
The illumination function:
A lighting model defines light-object
interactions based on the type of light source
and the material properties of the object.
The basic graphics pipeline is constrained to
the use of just one lighting model, the fixed
function lighting model.
This lighting model is basically an extended
version of the Phong lighting model
A point light emits light uniformly in 360
Point lights have fixed color and position
values and are omnidirectional in nature.
Spotlights are specified by a color, spatial
position and some specific direction and
range in which light is emitted.
A spotlight is basically a point light with its
emitting light constrained within an angle
A parallel or directional light illuminates objects
through a series of parallel light rays.
These light sources can be considered as point lights
located a significant distance from the surface of an
Emissive light is radiated (can be considered
self-reflecting) light originating from an
This type of light blends with our other light
types, resulting in a surface smoothly colored
through the combination of all global light
1) Light travels in straight lines
2) Light travels much faster than sound
3) We see things because they reflect light into
4) Shadows are formed when light is blocked by
Reflection from a mirror:
Incident ray Reflected ray
Angle of incidence Angle of
The Law of Reflection
Angle of incidence = Angle of reflection
In other words, light gets reflected from a surface at ____ _____
angle it hits it.
The same !!!
Smooth, shiny surfaces
have a clear reflection:
Rough, dull surfaces have a diffuse reflection.
Diffuse reflection is when light is scattered in
2) A car headlight
1) A periscope
White light is not a single colour; it is made up
of a mixture of the seven colours of the
We can demonstrate this by
splitting white light with a
This is how rainbows are formed:
sunlight is “split up” by
White light can be split up to make separate colours.
These colours can be added together again.
The primary colours of light are red, blue and green:
Adding blue and red Adding blue and green
makes magenta makes cyan (light blue)
Adding red and Adding all three
green makes makes white
The colour an object appears depends on the colours of
light it reflects.
For example, a red book only reflects red light:
White Only red light is
A pair of purple trousers would reflect purple light (and red and
blue, as purple is made up of red and blue):
A white hat would reflect all seven colours:
If we look at a coloured object in coloured
light we see something different. For
example, consider a football kit:
Shirt looks red
Shorts look blue
The light reflected from the surface of an
object depends on several factors
Degree of reflectivity of the object (shiny vs. dull
Surface texture properties
Color reflectance coefficients (e.g. color of the
A surface is only visible when it has the ability to reflect or absorb light.
This ability is the result of the surface’s material properties, i.e. rules determining
the amount of scattering and/or reflection of incident light.
We can specify:
material properties for any surface, the most common types being the Phong
reflection model, ambient reflection, diffuse reflection, specular reflection, and
our own per-vertex or per-pixel reflection models via either Cg or HLSL shaders
The basic illumination model we will learn in this course includes
Ambient lighting only
Reflected light from the environment and the
nearby objects cause other objects to
Also called background light.
Ambient reflection, also called
continuous reflection, occurs whenever
light emitted from a source is reflected so
much that its origin is impossible to
Ambient light is omnidirectional in
Constant color reflected from all points on the
Tells us how bright the surface will look like
when no light source can directly reach the
When used alone, does not produce very
We can use an ambient intensity parameter Ia
that describes the level of ambient light in a
Every object in the scene will be illuminated
by this amount independent of the surface
orientation and viewer location.
But different surfaces may reflect different
amount of ambient light based on their
absorbance/reflectance properties. We can
model this by a constant factor for each
ka I a
The light that is reflected in all directions is called diffuse reflection.
The reflected light is independent of the viewing position (equally bright from all viewing
But the light position with respect to the surface orientation is important to deter
● When an object is illuminated with white light the original color of the object is what we
see as the diffuse reflection.
● If a blue object is illuminated with red light, it will appear black.
mine the light reflected from the surface.
Diffuse reflections occur when incoming light is reflected in arbitrary directions.
The main contributing factor to this form of reflection is an uneven or rough surface.
A diffuse surface appears identical to all viewers, regardless of their respective point of
This type of reflection is common for matte or uneven surfaces (such as carpets or
brushed metal) and is used for shading surfaces in such a way as to convey a sense of
For matte (non-shiny) objects
Matte paper, newsprint
Color at a point on a matte object does not
change with viewpoint.
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Incoming light is partially absorbed and partially
transmitted equally in all directions
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N 90 -
Surface 1 Surface 2
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Cp= ka (SR, SG, SB) + kd NL (SR, SG, SB)
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N = (x-cx, y-cy, z-cz)
|(x-cx, y-cy, z-cz)|
(x, y, z)
(cx, cy, cz)
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Orientation of the surface determines the amount of light incident
on the surface
Given the angle between the surface normal and the incident
light direction, we can write the diffuse reflection equation as:
I l ,diff kd I l ,incident
kd I l cos
ka I a kd I l (N L), if N L 0
I l ,diff
ka I a , if N L 0
Ambient lighting only With diffuse lighting
Light reflected from a certain spot on the
object is concentrated and appears as a lot
brighter compared to other spots. This is
due to specular reflection and is an
important property of shiny object.
Specular reflection is both dependent
on the light direction, surface
orientation, and viewer position.
Specular reflection occurs whenever light, from
a single incoming direction, is reflected at a
single outgoing direction.
Specular reflection is characterized by bright
highlights on the surface of an object reflected
in the direction of the view vector.
Let us try to picture specular reflection
● The specular reflection angle equals angle of
● The specular reflection is visible only at
directions close to R. Shiny surfaces like
mirrors have a narrow specular reflection
range (given as parameter ns, specular-
k s I l (V R) ns , if V R 0 and N L 0
I l , spec
0.0, if V R 0 and N L 0
Diffuse Reflection Diffuse and Specular
The Phong model is an illumination model that
controls the shading of individual pixels; it is
computationally efficient and leads to realistic
Phong’s goal was to create realistic looking objects
in as close to real time as possible.
The Phong reflection model basically combines
ambient, specular and diffuse lighting components
to closely approximate real world reflections.
Considering objects that can emit light and
multiple light sources in the scene
I I surfemission I ambdiff f l ,radaten f l ,angaten ( I l ,diff I l , spec )
● Also, note that, if you consider different colors
of light and surfaces, you should use similar
equations (possibly with different parameters)
and apply it to each color component (R,G,B).