Scenes, Cameras & Lighting

Outline  Constructing a scene  Using hierarchy  Camera models  Light models

Introduction  We can model and surface our objects  We now need to bring the objects together in a scene  We’ll then be able to render and/or animate them  This is a relatively simple process but requires some thought to produce effective scenes

Constructing a scene  Simply a case of loading and positioning our objects.  We need to consider the animation, lighting camera requirements.  We may reuse elements, motions, etc. from other scenes just as you reuse scenery and props.

Positioning objects  Objects can be positioned, scaled and rotated interactively or numerically  Scene designs (just like object designs) should be carried out on paper first with reference to the storyboard

Using hierarchy  To help us easily position objects we can create a hierarchy  This should match the structure of the object  We can use this later when we animate the object

Hierarchical models  Not many real objects can be described by a simple, single entity: most objects are made up of many parts  Each part will have its own centre (or pivot point)  This can cause problems when we try and transform them...

Example: a simple table XX X XX X XX X XX X Rotate Scale

Why use a hierarchy?  To solve the previous problem, you could combine all the parts into a single object BUT  It would be difficult to alter the parts separately (e.g. to make the table top thicker without affecting the legs)

Creating a hierarchy  Create a hierarchical model: the table is the entire model  The top and legs form parts of the hierarchy  We can then either operate on the whole model or on its parts  Each level of the hierarchy has its own local coordinate system

Table hierarchy TABLE TOPLEG1LEG2LEG3LEG4  Schematic representation of hierarchy Child node Parent node

A better example: a car  Allows entire car (body and wheels) to translated  Wheels can rotate about their own local pivot points BODY WHEEL1WHEEL2WHEEL3WHEEL4

Textual representation of car { translate 22,0,0 rotate 0,45,0 scale 2,2,2 { scale 0.7,2.1,1.8 } { rotate 0,0,342 } … }

Cameras  The ‘virtual’ camera  Basic camera defined by a number of parameters:  Camera (or eye) location (or point)  Centre of interest (or ‘look at point’)  Up direction (e.g. OpenGL) OR  Position (x,y,z)  Orientation (Heading, Pitch, Banking) OR …… y z x “Look at” point “Eye” position “Up” direction

Camera motion  Pan  Angular change of direction maintaining position and up direction  Tumble (or orbit)  Maintain centre of interest and move camera around it  Track  Move both centre of interest and camera linearly  Dolly  Move towards or away from centre of interest  Tilt  Rotate the ‘up’ direction  Zoom (not actually a camera motion)  Magnify area and exaggerate perspective

Additional camera parameters  Because your pupil is round, your eyes have a cone of vision within which objects are visible  Because the computer uses rectangular images, virtual cameras have a pyramid of vision  The vertical angle that defines the size of this pyramid is the field of view (fov)  The aspect ratio may be specified or a second angle defined that gives us the final pyramid

Back clipping plane The viewing pyramid  All objects outside the pyramid are ‘clipped’, i.e. they are not processed by the rendering stage because they are not visible to the virtual camera  We also use near and far clipping planes to prevent rendering of objects too close to or too far from the camera  The truncated pyramid is called the viewing frustum y z x Front clipping plane

A better camera model  Focus point/depth of field  Effective lens focal length  Motion blur/shutter speed

Lights  Need lights to be able to see anything!  Often packages provide a default  Basic lights defined by a number of parameters:  Location  Intensity (within some limits, e.g. 0-10, although in Maya you can have negative intensity which will take light away from a scene)  Colour (usually RGB components, may be combined with intensity)

Ambient  Background light level  No specific location: defined by intensity & colour  On its own, very unrealistic but good with other lights  Produces no shadows  In Maya, default lighting has this set to 1, which is a bit high

Directional  Directional (or ‘distant’)  As if light is infinite distance away  No location but direction  Useful for sunlight etc.  Default Maya lighting has one distant light

Point  Like a perfect light bulb hanging in space  Radiates equally in all directions from location  Often have an additional parameter, fall-off (or decay) which simulates the limited area illuminated by the light

Spotlight  Has properties of point light, plus additional features  Throws light in a cone shaped beam, size of which is defined by the spread or cone angle  Light emitted in a direction  Also may have parameter to define how light fades towards the edge of the cone (drop-off or fall-off)  May also have a value that defines when the intensity reaches zero after the edge of the cone angle (the penumbra angle) Decay Dropoff

Spotlights (cont)  Spotlights can also use shadow maps  This is a faster alternative to ray tracing to generate shadows  It calculates the area behind an object that would be in shadow and shades it accordingly  Doesn’t work for transparent objects  Spotlights can be used to project images  To simulate a slide/film projector  To create special lighting effects

Area light  Light is emitted from a (usually) rectangular area (like an array of point lights)  Simulates banks of fluorescent lights, TV screens, etc.  Gives soft shadows

Setting up the lighting for a scene  Same issues are important as in video work  Example: Classic three-point lighting  Key light  Fill light  Backlight

Summary  Constructing a scene  Camera parameters  Light types and settings