CS361

1. Week 6 - Friday CS361

2. Last time • What did we talk about last time? • Light • Material • Sensors • Lambertian shading

3. Questions?

4. Project 2

5. Shading

6. Shading equations • We need a mathematical equation to say what the color (radiance) at a particular pixel is • There are many equations to use and people still do research on how to make them better • Remember, these are all rule of thumb approximations and are only distantly related to physical law

7. Lambertian shading • Diffuse exitanceMdiff = cdiff ELcosθ • Lambertian (diffuse) shading assumes that outgoing radiance is (linearly) proportional to irradiance • Because diffuse radiance is assumed to be the same in all directions, we divide by π (explained later) • Final Lambertian radiance Ldiff =

8. Specular shading • Specular shading is dependent on the angles between the surface normal to the light vector and to the view vector • For the calculation, we compute h, the half vector half between v and l

9. Specular shading equation • The total specularexitance is almost exactly the same as the total diffuse exitance: • Mspec = cspec ELcosθ • What is seen by the viewer is a fraction of Mspec dependent on the half vector h • Final specular radiance • Lspec = • Where does m come from? • It's the smoothness parameter

10. Implementing the shading equation • Final lighting is: • We want to implement this in shaders • The book goes into detail about how often it is computed • Note that many terms can be precomputed, only the ones with angles in them change

11. When should it be computed? • Computing the shading equation more often gives better visual results but takes more time • Flat shading • Computes shading equation once per primitive • Gouraud shading • Computes shading equation once per vertex, linearly interpolates color for pixel values • Phong shading • Computes color per pixel

12. Aliasing

13. Aliasing • When sampling any continuous thing (image, sound, wave) into a discrete environment (like the computer), multiple samples can end up being indistinguishable from each other • This is called aliasing • We can reduce aliasing by carefully considering how sampling and reconstruction of the signal is done

14. Aliasing example • Ever seen wheels of a car spinning the wrong way? • Without enough samples, it may be impossible to tell which way it's spinning • You need a sampling frequency twice as high as the maximum frequency of the events to reconstruct the original signal • Called the Nyquist limit

15. Screen based antialiasing • Jaggies are caused by insufficient sampling • A simple method to increase sampling is full-scene antialiasing, which essentially renders to a higher resolution and then averages neighboring pixels together • The accumulation buffer method is similar, except that the rendering is done with tiny offsets and the pixel values summed together

16. FSAA schemes A variety of FSAA schemes exist with different tradeoffs between quality and computational cost

17. A-buffer • For non-interactive render speeds, the A-buffer can be used • The A-buffer generates a coverage mask for each fragment for each pixel • Fragments are thrown away if they have z-buffer values that are higher than fragments with full coverage • Final pixel color is based on fragment merging

18. Multisampleantialiasing • Supersampling techniques (like FSAA) are very expensive because the full shader has to run multiple times • Multisampleantialiasing (MSAA) attempts to sample the same pixel multiple times but only run the shader once • Expensive angle calculations can be done once while different texture colors can be averaged • Color samples are not averaged if they are off the edge of a pixel

19. Performance, speed, the future • Active research is still trying to find techniques with good visual output and good computational performance • Stochastic (random) sampling reduces the visual repetition of some artifacts • Sharing samples between pixels can reduce overall cost

20. XNA Examples

21. Quiz

22. Upcoming

23. Next time… • Alpha effects • Gamma effects

24. Reminders • Keep working on Project 2, due Friday, March 1 • Keep reading Chapter 5 • Start reading Chapter 6