Week 13 - Monday Radiosity Ray tracing Precomputed lighting - - PowerPoint PPT Presentation

Week 13 - Monday

 Radiosity  Ray tracing  Precomputed lighting  Precomputed occlusion

 We can imagine all the different rendering techniques as

sitting on a spectrum reaching from purely appearance based to purely physically based

Sprites Layers Billboards Triangles Appearance Based Lightfields Physically Based Global illumination

 When objects are close to the viewer, small changes in

viewing location can have big effects

 When objects are far away, the effect is much smaller  As you know by now, a skybox is a large mesh containing the

entire scene

 Some skyboxes look crappy because there isn't enough

resolution

• Minimum texture resolution (per cube face) =

tan(fov/2) resolution screen

 If you're trying to recreate a complex scene from reality, you

can take millions of pictures of it from many possible angles

 Then, you can use interpolation and warping techniques to

stitch them together

• Huge data storage requirements
• Each photograph must be catalogued based on location and
• rientation

 High realism output!  Remember the video with the robot and the omnidirectional

camera

 A sprite is an image that moves

around the screen

 Sprites were the basis of most old

2D video games (back when those existed, before the advent of Flash)

 By putting sprites in layers, it is

possible to make a compelling scene

 Sequencing sprites can achieve

animation

 Applying sprites to 3D gives billboarding  Billboarding is orienting a textured polygon based on view

direction

 Billboarding can be effective for objects without solid surfaces

• Vegetation
• Smoke
• Fire

 Each polygon (thought of as a quadrilateral, even if often two

triangles in practice) needs a surface normal n and an up vector u

 A billboard also has an anchor location as a point of reference

 A screen-aligned billboard is one

that sits on the screen

 The u vector comes from the

camera

 The n vector is the negation of the

camera's view vector

 MonoGame handles all of this for

you in the SpriteBatch class, of course

 If the object is supposed to exist in the world, it needs to change as the world

changes

 The world has some implied up vector that can be used to derive an appropriate

up vector (and thereby rotation matrix) for the sprites

 For small sprites (such as particles) the billboard's surface normal can be the

negation of the view plane normal

 Larger sprites should have different normals that point the billboard directly at

the viewpoint

 Many (sometimes hundreds) of billboards can be put together to make smoke or fire

effects

 A small set of billboards can be drawn many times with different scaling and rotation

factors and overlapped

 Issues can happen if these billboards intersect with objects  Soft particles is a technique for lowering the opacity of billboards when they are close to

"real" objects

 Axial billboards are another common technique  In axial billboards, a polygon rotates around some world space axis and

tries to face the viewer as much as is allowed

 This technique is useful for trees viewed from a distance

• Like cross trees, the illusion is ruined if the viewer moves too high
• Some implementations may switch between the impostor billboard and a real

model if the viewer gets close enough

 It works for laser beams too



In a particle system, many small, separate objects are controlled using some algorithm



Applications:

• Fire
• Smoke
• Explosions
• Water



Particle systems refer more to the animation than to the rendering



Particles can be points or lines or billboards



Modern GPUs can generate and render particles in hardware

 Image processing  Tone mapping  HDR lighting  Lens flare  Bloom

 Keep working on Assignment 4

• Due Friday by midnight

 Keep working on Project 3  Keep reading Chapter 10