1. Week 14 - Wednesday
CS361

2. Last time What did we talk about last time? Collision handling
Collision detection
Collision determination
Collision response
BSPs and hierarchies
Multiple object CD
Broad phase then exact phase

3. Questions?

4. Assignment 5

5. Project 4

6. Finish up MonoGame collision example

7. Student Lecture: Non-Photorealistic Rendering

8. Non-Photorealistic Rendering

9. Non-photorealistic rendering
Most of the work we've focused on all semester is doing rendering that in some way mirrors the natural world
However, a wide area of rendering is non-photorealistic rendering (NPR)
Goals:
Simplified technical drawings
Simulating artistic styles

10. Toon shading
The most common form of NPR in video games is toon shading
Also called cel shading
The goal is to render 3D models as if they were cartoons
Shading is often done with either a single color or a two tone (color and shading) approach
Then a thick black silhouette is added around edges

11. Jet Grind Radio

12. Okami

13. Prince of Persia

14. Coloring
The color is often determined by the dot product n · l (surface normal dot light vector)
If negative, the surface should be darkened
Otherwise, it's some flat color
Or a threshold other than 0 can be used
A more complex system uses a one dimensional texture indexed into with the dot product
Highlight
Normal
Shadow

15. Silhouette edge rendering
The more complicated problem is properly rendering edges with a thick dark line
A number of different edges are of interest
Boundary or border edges are edges where one polygon is not adjacent to any other
Not found in 3D solid objects
A crease, hard, or feature edge is an edge between two polygons that is sharper than some threshold angle
A material edge is an edge between two polygons with different materials
A silhouette edge is when two neighboring polygons face different directions, relative to the eye

16. Procedural geometry silhouetting
Cel shading focuses on rendering silhouette edges
Many of these techniques rely on manipulating back facing polygons
The crossover between front facing and back facing polygons is the silhouette
It's easy to determine which is which
After some manipulation of the backfaces, they are rendered in black

17. z-biased backfaces
If the backfaces are rendered in black without any change, they will be hidden
Before rendering, all backfaces can be translated to be closer to the viewer
Translation can be by
A fixed amount
An amount that takes into account non-linear z-depths
An amount based on the angle of the polygon normal
None of these techniques give uniform thickness lines

18. Triangle fattening
Another approach is to fatten each backface triangle
The slope of the triangle and the distance from the viewer determine the expansion of each edge
It doesn't work well for thin traingles

19. Expanding shell technique
A similar approach is to expand backface vertices along their normals
The expansion amount is proportional to their z-distance
This technique fails for situations like a cube, in which faces with very different normals share vertices

20. Silhouetting by image processing
Rather than using geometry, there is an image processing approach
Render all the normal values or depth values to a buffer and use edge detection algorithms to draw lines where the values change abruptly
Using normal values can find crease edges
This technique works for many cases that failed before
Even GPU generated surfaces are not a problem

21. Other styles NPR techniques are very broad
Many approaches try to recreate hand-drawn or hand-painted styles
Silhouette lines can be drawn as paintbrush strokes of varying thickness
Tonal art maps (TAMs) use palettes of hand-drawn textures to do black and white crosshatch-style shading

22. Line rendering
For CAD programs and other 3D tools, we may want to highlight certain lines or all polygon edges
Rendering polygon edges on top of existing polygons can be a pain
Z-buffer algorithms might hide the lines
In practice, a small bias is usually added to the polygons

23. Hidden lines
For drawing simplified models, we may want to include or exclude the hidden lines
Wireframe is the easiest, since it is just the lines
Hidden-line uses the z-buffer directly
Obscured-line renders twice, rendering z-fail lines in a lighter color
Haloed lines also use the z-buffer, drawing thick white lines first and then black lines on top
Problems happen when lines get too close

24. Cel shading in MonoGame

25. Quiz

26. Upcoming

27. Next time… The future of graphics Directions in games
Directions in hardware
Open research topics

28. Reminders Finish Assignment 5 Keep working on Project 4 Due on Friday
Due next Friday