Texture Mapping

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Texture Mapping
CAP4730 Computational Structures in Computer
Graphics
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Outline

• What is texture mapping?
• Math behind texture mapping
• OpenGL texture mapping
• Advanced Topics in texture mapping

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Marching towards Visual Realism
http//www.3drender.com/jbirn/productions.html
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How do we do this?
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Another Example
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Using only Triangles

• Using only triangles to model everything is hard
• Think about a label on a soup can
• Instead of interpolating colors, map a pattern
• Run Demo
• How do you create a pattern?

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Texture

• A color array that holds an image
• Similar to a framebuffer
• gluint bufferxy3
• 255 0 255

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Procedural Texture

• Calculate pixel values via functions

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Image Textures
Load pixel values from a file
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Lets look at some games
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Texture Space

• How would you store this in your program?
• How much memory?

s
s 01 t 01
t
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Object Space
(u1,v1)
(u0,v0)
s
Texture -gt Object Transformation
s 01 t 01 u 01 v 01
(u3,v3)
t
(u2,v2)
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Going from pixels to textures
Object -gt Texture Transformation
Image -gt Object Transformation
(x,y)
(s,t)
(u,v)
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What is the question?

• A pixel (x,y) corresponds to what texture element
  (s,t)?
• Q How do we convert (s,t) into texture element
  coordinates?
• TEXture Elements (TEXELS)
• A Linearly Interpolate texture coordinates!
• http//graphics.lcs.mit.edu/classes/6.837/F98/Lect
  ure21/Slide05.html

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Tiling

• Lets look at the texture coordinates

(u1,v1)
(u0,v0)
s
(u3,v3)
t
(u2,v2)
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Textures built to Tile

• Think Windows Background
• Why would you want this? Discuss the Pros and
  Cons of tiling.
• Look at code

0,0
1,0
0,1
1,1
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Filtering

• Why does this happen?

(u0,v0)
(u1,v1)
(u2,v2)
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Filtering

• Why does this happen?
• What are some alternatives?

(ui,vi)
(uj,vj)
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Combining Lighting Texturing

• If you notice there is no lighting involved with
  texture mapping!
• They are independent operations, which MAY (you
  decide) be combined
• It all depends on how you apply the texture to
  the underlying triangle

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Combining Lighting Texturing

• CT Texture Color
• CC Base Triangle
• Replace, CF CT
• Blend, CF CT CC
• OpenGL
• glTexEnv(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE,
  GL_BLEND)
• http//trant.sgi.com/opengl/docs/man_pages/hardcop
  y/GL/html/gl/texenv.html

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Screen space vs. world space
P0(x,y,z)
V0(x,y)
V1(x,y)
P1(x,y,z)
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Interpolation in Screen Space
P0(x,y,z)
V0(x,y)
V1(x,y)
P1(x,y,z)
z1
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Interpolation in World Space
P0(x,y,z)
V0(x,y)
V1(x,y)
P1(x,y,z)
z1
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Comparing the two

• What do we need to interpolate in screen space
  now?
• What happens if we dont have Z? What have we
  discussed that doesnt have Z?
• Perspectively Correct Z
• http//graphics.lcs.mit.edu/classes/6.837/F98/Lect
  ure21/Slide14.html

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Other solutions

• Cost of screen space interpolation now?
• Increase of triangles

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Linear Interpolation is wrong?

• Gouraud is wrong, but why does it look okay?
• Q Why does Texture mapping not look right even
  though we are doing the same thing?
• A All about the slope of the interpolation and
  the gradualness or abruptness of the change

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Transparencies

• We will revisit this later, but we can use the
  alpha channel to stencil out textures.
• Thus per pixel, set alpha 1 or alpha 0.
  Where alpha 0,

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Lets Look at some games

• Pick a favorite game from the class