1. Week 7 - Monday
CS361

2. Last time What did we talk about last time? Exam 1 Before that
Specular shading
Aliasing and antialiasing

3. Questions?

4. Project 2

5. MonoGame Examples of Diffuse and Specular Shading

6. Student Lecture: Transparency

7. Transparency

8. Transparency
Partially transparent objects significantly increase the difficulty of rendering a scene
We will talk about really difficult effects like frosted glass or light bending later
Just rendering transparent objects at all is a huge pain because the Z-buffer doesn't work anymore
Workarounds:
Screen door transparency
Sorting
Depth peeling

9. Screen door transparency
We render an object with a checkerboard pattern of holes in it, leaving whatever is beneath the object showing through
Problems:
It really only works for 50% transparent
objects
Only one overlapping
transparent object really works
But it is simple and inexpensive

10. Over operator
Most transparency methods use the over operator, which combines two colors using the alpha of the one you're putting on top
c0 = αscs + (1 - αs)cd
cs is the new (source) color
cd is the old (destination) color
co is the resulting (over) color
αs is the opacity (alpha) of the object

11. Sorting The over operator is order dependent
To render correctly we can do the following:
Render all the opaque objects
Sort the centroids of the transparent objects in distance from the viewer
Render the transparent objects in back to front order
To make sure that you don't draw on top of an opaque object, you test against the Z-buffer but don't update it

12. Problems with sorting It is not always possible to sort polygons
They can interpenetrate
Hacks:
At the very least, use a Z-buffer test but not replacement
Turning off culling can help
Or render transparent polygons twice:
once for each face

13. Depth peeling
It is possible to use two depth buffers to render transparency correctly
First render all the opaque objects updating the first depth buffer
Make second depth buffer maximally close
On the second (and future) rendering passes, render those fragments that are closer than the z values in the first depth buffer but further than the value in the second depth buffer
Update the second depth buffer
Repeat the process until no pixels are updated

14. Depth peeling example
1 layer
2 layers
3 layers
4 layers

15. Other alpha effects
Alpha values can be used for antialiasing, by lowering the opacity of edges that partially cover pixels
Additive blending is an alternative to the over operator
c0 = αscs + cd
This is only useful for effects like glows where the new color never makes the original darker
Unlike transparency, it can be applied in any order

16. Gamma Correction

17. Gamma I don't want to go deeply into gamma
The trouble is that real light has a wide range of color values that we need to store in some limited range (such as 0 – 255)
Then, we have to display these values, moving back from the limited range to the "real world" range

18. Gamma correction
Physical computations should be performed in the linear (real) space
To convert that linear space into nonlinear frame buffer space, we have to raise values by a power, typically 0.45 for PCs and 0.55 for Macs
Each component of physical color (0.3, 0.5, 0.6) is raised to 0.45 giving (0.582, 0.732, 0.794) then scaled to the range, giving (148, 187, 203)

19. Gamma errors Usually, gamma correction is taken care of for you
If you are writing something where you need to do computations in the "real life" color space (such as a raytracer), you may have to worry about it
Calculations in the wrong space can have visually unrealistic effects

20. Texturing

21. Texturing We've got polygons, but they are all one color
At most, we could have different colors at each vertex
We want to "paint" a picture on the polygon
Because the surface is supposed to be colorful
To appear as if there is greater complexity than there is (a texture of bricks rather than a complex geometry of bricks)
To apply other effects to the surface such as changes in material or normal

22. Corresponder function Value transform function
Texture pipeline
We never get tired of pipelines
Go from object space to parameter space
Go from parameter space to texture space
Get the texture value
Transform the texture value
Object space
Projector function
Parameter space
Corresponder function
Texture space
Obtain value
Texture value
Value transform function
Transformed value

23. Projector function
The projector function goes from the model space (a 3D location on a surface) to a 2D (u,v) coordinate on a texture
Usually, this is based on a map from the model to the texture, made by an artist
Tools exist to help artists "unwrap" the model
Different kinds of mapping make this easier
In other scenarios, a mapping could be determined at run time

24. Corresponder function
From (u,v) coordinates we have to find a corresponding texture pixel (or texel)
Often this just maps directly from u,v  [0,1] to a pixel in the full width, height range
But matrix transformations can be applied
Also, values outside of [0,1] can be given, with different choices of interpretation

25. Texture values
Usually the texture value is just an RGB triple (or an RGBα value)
But, it could be procedurally generated
It could be a bump mapping or other surface data
It might need some transformation after retrieval

26. Mid-Semester Evaluations

27. Upcoming

28. Next time…
Image texturing techniques
Procedural texturing

29. Reminders Keep working on Project 2 Keep reading Chapter 6 Mipmapping
Anisotropic filtering