Rendering involves several steps: identifying visible surfaces, projecting surfaces onto the viewing plane, shading surfaces appropriately, and rasterizing. Rendering can be real-time, as in games, or non-real-time, as in movies. Real-time rendering requires tradeoffs between photorealism and speed, while non-real-time rendering can spend more time per frame. Lighting is an important part of rendering, as the interaction of light with surfaces through illumination, reflection, shading, and shadows affects realism.

1. RENDERING
Michael Heron

2. INTRODUCTION
 As part of the process of creating a 3D
scene, we are building up a model of
objects.
The geometry of those objects
The relationship of those objects to each other
The relationship of the objects to the camera.
 However, in order to turn this scene into a
useable image, we must go through a
separate process.
Rendering

3. RENDERING
 The aim of rendering is to view objects as
solids.
Not polymeshes, wireframes, or any other of
the representations we have discussed.
 Requires several steps
Identify which surfaces are visible
Project surfaces onto the 2D viewing plane
Shade surfaces appropriately
Rasterise
 Convert from mathematical representation into
bitmaps.

4. RENDERING – TWO FLAVOURS
 Depending on the intended application,
rendering can have one of two flavours.
 Real-time
 Games, simulations
 Non Real-Time
 Movies, static scenes
 In both cases, the end goal is the same.
 Turn the mathematical representations of a 3D scene
into something pretty.

5. REAL-TIME RENDERING
 For real-time scenarios, trade-offs are
required.
Scenes must be rendered quickly enough for
the human eye to be fooled
 Around 30 frames per second
Trade off is between photorealism and speed of
processing.
 Can use tricks to ‘fool’ viewers.
 As time goes by, this trade-off becomes
less important.
Computers get faster, our eyes don’t.

6. NON REAL-TIME RENDERING
 For non-interactive media, time is not important.
 We can spent many more seconds rendering an image
because we have no throughput to reach.
 Each frame may take minutes/hours to render.
 Played back at 25-30 frames per second.
 Like old fashioned cartoon animations.

7. RENDERING - LIGHT
 Simply assigning a colour to a surface
gives an unrealistic appearance.
The interaction of light with objects is one of
the primary mechanisms for achieving
‘realism’
 Light interaction is complex
Illumination
Reflection
Shading
Shadows
Texture of surfaces
Transparency/Translucency

8. RENDERING - LIGHT
 The appearance of an object depends on:
 The type of light source
 Distance to the light source
 Orientation of the object relative to the light source
 Surface properties of the object
 Determines the nature of light interaction
 Repeat for all light sources.
 Lots of processing.

9. RENDERING - LIGHT
 In non real-time situations, we can precisely
model the physics of light interaction.
 Such as with ray-tracing
 Or radiosity
 For real-time situations, an approximation will
suffice.
 As long as it looks good.

10. LIGHT SOURCES
 There are several kinds of light sources
that are used in 3D modelling.
Point light sources
Ambient light sources
Directional light sources
Spotlight sources
 They are all used to give differing
‘textures’ of light and shadow.
Choosing the right kind and number of light
sources an important part of good scene
output.

11. LIGHT SOURCES - AMBIENT
 Ambient light is the general background
illumination.
 All objects reflect light to some degree
 Light can reach parts of a room/scene that are in no
direct line of light.
 Illumination is constant on all surfaces
 Regardless of position or orientation
 Light is uniform across a scene.

12. LIGHT SOURCES - DIRECTIONAL
 Directional light sources emit light
strongly in a particular direction.
Like the sun
 Objects in the line of light more brightly
illuminated than other objects.
 Light is uniformly provided in a fixed
direction.
 Light source is assumed to be very far
away.
No need to deal with light distance
calculations.
 Objects at the back of a scene lit to the
same degree as objects at the front
Provided they are directly in the path of light.

13. LIGHT SOURCES - POINT
 Point light sources have
 Location
 Colour
 They emit light equally in all directions.
 Like a lightbulb.
 Light strength attenuates
 Gets weaker with distance
 Gradients of illumination across a surface
 Can be moved around with a scene.

14. LIGHT SOURCES - SPOTLIGHT
 Very similar to a point light source
 Difference is that spotlight source does not
radiate in all directions.
 Direction of light can be varied within a scene.
 This distinguishes it from directional light.
 Light attenuates with distance.
 This also distinguishes it from directional light.

15. SURFACES
 The surface of an object represents the next
implication for light interaction.
 Surfaces can:
 Absorb light
 Reflect light
 Refract light
 Emit light
 They can glow in the dark

16. ABSORPTION
 The colour of the surface determines the level of
absorption
 Black absorbs all visible spectrums
 White reflects all visible spectrums.
 Coloured surfaces vary depending on colour.
 Reflect some wavelengths
 Absorbs others
 Wavelength interaction with colours a matter of
physics.

17. REFLECTION
 Reflection can be either
 Specular
 Reflects like a mirror
 Light gets reflected at angles
 Angle of reflection
 Ever tried shooting a laser pointer at a mirror?
 It’s like that
 Diffuse
 Reflects at many angles at once
 Diffuse reflection is the primary way in which we can see
objects.

18. REFLECTION
http://www.curriki.org/xwiki/bin/view/Coll_Athabasca/Unit3-
Lesson2TheMovementofLight

19. REFLECTION
 Reflection is dependant upon
 The arrangements of facets on a surface
 The roughness of the surface
 The orientation of the surface
 The absorbption of the surface.
 Light can also be refracted
 As light moves from one medium to another, the path
of the light bends.
 Mechanics of this are not trivial

20. REFRACTION
http://www.curriki.org/xwiki/bin/view/Coll_Athabasca/Unit3-
Lesson2TheMovementofLight

21. TRANSMISSION
 Sometimes, objects transmit light through
themselves.
 Such objects can be transparent (like glass) or
translucent
 Transparent objects refract light passing through.
 White light through a prism
 Translucent objects diffuse the light that passes
through
 Frosted glass

22. LIGHT RENDERING
 Two approaches used to render light
 Local reflection models
 Everything floats in ‘dark space’
 Only one reflection of light considered
 Global reflection models
 Light reflects off of objects which reflect onto other objects
 Indirect light
 Very costly to render
 Not yet perfected

23. SUMMARY
 One aspect of rendering is the interaction
of light upon surfaces.
This can be
 Real-time
 Non Real-Time
 Issue complicated by the physics of light
Reflection, Refraction
Different kinds of lights
 Lighting is an important part of rendering
believable 3D scenes.