Lecture 6 (Illumination Models) Flashcards
What are the different surface types?
- Self-luminious
- Refractive
- Translucent
- Reflective (diffuse)
- Reflective (specular)
Give an example of a self-luminious surface type.
The sun
- Any object that emits its own light
WTF is a ‘Refractive’ surface type? And give an example
Changes direction of light as it passes through (due to change in transmission medium)
e.g. Water or glass
WTF is a ‘Translucent’ surface type?
Scatters light that passes through it
Break down Reflective surface types.
Diffuse and Specular
WTF is a ‘Reflective (Diffuse)’ surface type?
Reflects light in all directions (scattered)
e.g. Carpets, Rugged surfaces
WTF is a ‘Reflective (Specular)’ surface type?
Reflects light at the same angle as the incidident ray
e.g. Glossy surfaces, polished steel
WTF is the difference between ‘Diffuse’ and ‘Specular’ surface types?
Diffuse reflects light in all directions (scattered).
Specular reflects light in one direction.
What factors must be considered when choosing an illumination model?
Realism and computational cost.
Must balance between the two.
What are the components of light?
- Ambient
- Lambertian (diffuse)
- Specular
WTF is ‘Ambient’ component in lighting?
Light rays coming UNIFORMLY from all directions hitting the object
WTF is ‘Specular’ component in lighting?
Highlights and glossiness
WTF is ‘Phong Illumination/reflection model’?
Combines all three components of light
- Ambient Illumination
- Lambertian (diffuse) Reflection
- Specular Reflection
WTF is an ‘Isotropic Surface’?
The relationship between the incoming and outgoing direction of light is the same over the whole surface (otherwise anisotropic)
- Illumination models generally only consider isotropic sufrace
What is simple illumination models?
Do not consider shadows, reflections or photon-based effects
What is full ray tracing
Considers all rays of light -> very computationally complex
What factors affect/influence Lambertian (diffuse)?
Distance of the surface from the light source and angle of light source???
What factors affect/influence Specular?
Surface orientation & distance of both viewer and light source
In real scenes, what are the variations in shading over objects caused by?
- Surface material
- Orientation of surfaces
- Nature and direction of light sources
- View direction
- Shadows
Why do certain materials and certain rock or stone faces look different depending on the viewing angle?
This is the result of asymmetric texture
What are the illumination model umbrellas?
- Simple illumination models
- Full-ray tracing
What is the radiated light intensity formula (Ambient Component)?
Light Intensity = (intensity of the ambient light) * (percentage of the light reflected by the object)
WTF is Lambertian (Diffuse) Component?
Light is re-radiated uniformly in all directions
- This is why a projector image is the same no matter where you are viewing it from
With regards to Lambertian, what does brightness depend on?
Depends on the angle between the direction of the light source and the surface normal
Explain why Surface Orientation does not affect the intensity of Lambertian reflection (with respect to the viewer)?
YOU SEE MORE LIGHT WHEN SURFACE IS FACING FLAT ON YOU
- For a given surface patch, the amount of light radiated towards the viewer is greatest when the surface normal is pointing straight at the viewer (falls off according to a cosine law as the surface slants away from the viewer)
YOU SEE MORE OF THE SURFACE AS IT SLANTS AWAY FROM YOU
- For a given visual angle subtended at the viewer, more of the surface is seen from that angle as the surface slants away from the viewer (according to a cosine law)
These two effects CANCEL OUT (more light when surface is flat facing you, more surface when slanted) so therefore the intensity of Lambertian is not affected by surface orientation (w/ respect to the viewer)
What is the Lambertian (diffuse) illumination equation?
I = (intensity of light) * (coefficient of diffuse reflection) * cos(theta)
Cosine can be expressed as scalar product, therefore,
I = (intensity of light) * (coefficient of diffuse reflection) * (N . L)
where N = direction surface normal and L = direction light source
Explain why Distance of Viewer From Surface does not affect the intensity of Lambertian reflection (with respect to the viewer)?
LIGHT INTENSITY IS LOWER AS YOU MOVE BACK
- As the surface moves further away from the viewer, the light intensity falls off as an inverse-square law in distance
YOU SEE MORE LIGHT RAYS AS YOU MOVE BACK
- For a given angle subtended at the viewer, the amount of surface included grows in proportion to the square of the distance
These two effects cancel out (you get less intensity per light ray, but you get more light rays).
So intensity of Lambertian reflection is not affected by the distance of the surface from the viewer
Is there a difference in the intensity of Lambertian reflection of the projector hitting the wall for those sitting on the front row and for those sitting on the back row?
No, it is the same.
Explain why Distance of Light Source From Surface does affect the intensity of Lambertian reflection (light)?
The intensity of the incoming light (and therefore of the reflected light Lambertian) depends on the distance of the surface from the light source.
- If we moved the projector forward it would have increased intensity of Lambertian reflection
- If we moved it backwards, it would have decreased intensity
How to model the spread of reflection?
Spread of reflection is modelled by angle a
- a is the angle between the direction of perfect reflection (R) and the viewer direction (V), and modify the reflected intensity by the factor (cos a)^n
i. e. a determines the size of the specular component
n = specular reflection exponent, this controls the degree of spread
- High values of n (100 or 200) lead to rapid fall-off and sharp highlights = this is very glossy surface
- Low values (1 or 2) lead to slow fall-off and spread-out, more diffuse highlights = more matte surface
What is the spread of reflection in simpler terms?
Size of the specular component
Describe how you can achieve a more glossy surface and a more matte surface appearance.
Glossy = High values of specular reflection exponent (n)
- Rapid fall-off and sharp highlights
Matte = Low values of specular reflection exponent (n)
- Slow fall-off and spread-out, more diffuse highlights
Why is Phong Reflection Model (Phong Illumination Model) so popular?
Efficient use of computation time per rendered pixel
