1. Week 11 - Monday
CS361

2. Last time Radiosity Ray tracing Precomputed lighting
Precomputed occlusion

3. Questions?

4. Project 3

5. Student Lecture: Billboarding and Particle Systems

6. Image Based Effects

7. 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

8. 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) =

9. 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

10. 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

11. Even old "3D" games used sprites

12. 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

13. 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

14. 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

15. 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

16. 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

17. Particle Systems

18. 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

19. MonoGame Particle System

20. 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:

21. 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

22. Duck Video

23. Upcoming

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

25. 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