Module 06

1
Module 06

• Advanced mapping techniques
• Environment mapping

2
Overview

• Environment mapping
• Sphere mapping
• Cube mapping

3
Environment mapping

• Environment mapping technique is used to model
  unique type of objects (like chrome, polished
  metal, glass) which reflect the environment they
  are in.
• In practice, environment mapping applies a
  special texture map that contains an image of the
  scene surrounding an object to the object itself.
  
• The result approximates the appearance of a
  reflective surface, close enough to fool the eye,
  without incurring any of the expensive
  computations involved in ray-tracing.
• The GL_SPHERE_MAP and GL_REFLECTION_MAP texture
  generation modes can be used in conjunction with
  an appropriate texture map to create realistic
  reflections.

4
Sphere mapping

• For the GL_SPHERE_MAP mode, or sphere mapping the
  texture coordinates are generated using a vector
  from the eye to the point on the surface and the
  surface normal to create a reflection vector. The
  reflection vector is used to calculate the
  texture coordinates.
• Drawbacks
• - sphere mapping is view dependent, so viewing a
  reflective object from --
• - anywhere other than the center of projection
  can produce incorrect results
• - the reflection won't pick up any objects
  moving in the world
• Advantage
• - faster than cube mapping in static
  environments

5
Cube mapping (1)

• In the GL_REFLECTION_MAP mode, or cube mapping
  the world is rendered six times (once for each
  direction) from the reflective object's
  perspective.
• The results are then stored in the six faces of a
  cube map, which is then applied to the object.
• This approach is much better than sphere mapping,
  since a reflection image can be generated
  anywhere in the world, and it can be updated
  dynamically to reflect objects in motion.
• Cube maps are also view independent and can be
  easily mapped onto any objects.
• For these reasons, sphere mapping is generally
  not used often.

6
Cube mapping (2)

• Texture coordinates used in conjunction with cube
  texture maps are typically generated at runtime.
  For instance, environment mapping can be
  performed by calculating the reflection of the
  direction to the camera and storing it in the
  s,t,r) coordinates at each vertex of a triangle
  mesh. The reflection direction calculation is
  normally implemented in hardware, so this can be
  done very efficiently.
• The s-, t-, and r-coordinates represent a
  direction vector emanating from the center of the
  cube that points toward the texel to be sampled.

7
Cube mapping (3)

• Orientation of the cube map axes relative to each
  of the six faces

8
Cube mapping (4)

• Cubemap covering the six faces of a cube

9
Final result
10
Practice

Open Lab06 /SphereMapping / SphereMapping.sln L
ab06 /CubeMapping / CubeMapping.sln