Week 11 - Monday CS361

Last time Radiosity Ray tracing Precomputed lighting Precomputed occlusion

Questions?

Project 3

Student Lecture: Billboarding and Particle Systems

Image Based Effects

Rendering spectrum 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

Skyboxes 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 (our) skyboxes look crappy because there isn't enough resolution Minimum texture resolution (per cube face) =

Lightfields If you are 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 orientation High realism output! Remember the video with the robot and the omnidirectional camera

Sprites and layers 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

Even old "3D" games used sprites

Billboarding 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

Screen-aligned billboard 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

World-oriented billboard 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

Unlimited options 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 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

Particle Systems

Particle systems 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

MonoGame Particle System

Impostors An impostor is a billboard created on the fly by rendering a complex object to a texture Then, the impostor can be rendered more cheaply This technique should be used to speed up the rendering of far away objects The resolution of the texture should be at least:

Displacement techniques An impostor that also has a depth map is called a depth sprite This depth information can be used to make a billboard that intersects with real world objects realistically The depth in the image can be compared against the z-buffer Another technique is to use the depth information to procedurally deform the billboard

Duck Video

Upcoming

Next time… Image processing Tone mapping HDR lighting Lens flare Bloom

Reminders Keep working on Project 3 Keep reading Chapter 10 Due Friday before midnight Keep reading Chapter 10 Exam 2 is next Monday in class Review chapters 5 – 10