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Week 7 - Wednesday. CS361. Last time. What did we talk about last time? Specular shading Aliasing and antialiasing. Questions?. Project 2. Student Lecture: Transparency. Transparency. Transparency.

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Presentation Transcript
last time
Last time
  • What did we talk about last time?
  • Specular shading
  • Aliasing and antialiasing
transparency1
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
screen door transparency
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
over operator
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
sorting
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
problems with sorting
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

depth peeling
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
other alpha effects
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
gamma
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
gamma correction1
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 0-255 range, giving (148, 187, 203)
gamma errors
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
texturing1
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
texture pipeline
Texture pipeline
  • Transformed value
  • 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
projector function
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
corresponder function
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
texture values
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
next time
Next time…
  • Image texturing techniques
  • Procedural texturing
reminders
Reminders
  • Keep working on Project 2
    • Due this Friday, March 1
  • Keep reading Chapter 6
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