Terrain Rendering

1
Terrain Rendering
2
New Concepts

• Loading geometry from file
• Camera animation
• Advanced lighting and materials
• Texture mapping
• Display lists

3
Loading geometry from file

• Motivations
• Built-in geometry is limited (spheres, teapots,
  cubes, etc)
• Artists want to experiment with new visual
  designs without recompiling program
• Fancy editors can be used to work with data if it
  is in a common file format

4
Loading geometry from a file

• For this assignment
• 2D height-field stored as image
• Each pixel represents the height of the terrain
  at that point (only 1 channel used)
• Stored in ppm format
• Lines, triangles, quads must be generated from
  this before rendering (as well as the normals if
  lighting is required)

5
Loading geometry from a file

• Interpreting a pixel as a 3D point
• Let s units per index
• Let h units at max height
• For each pixel at i,j in image
• (x,y,z) (si, sj, hintensity(i,j))

6
Loading geometry from file

• Generating wire-frame geometry
• For each pixel not on top or left border
• Emit a line from current pixel to upper neighbor
• Emit a line from current pixel to left neighbor

7
Loading geometry from file

• Generating surfaces (GL_QUADS)
• For each pixel not on top or left border
• A current point
• B upper neighbor
• C upper-left neighbor
• D left neighbor
• Emit quad of A,B,C,D

8
Loading geometry from file

• Generating surface with normals for proper
  lighting (overview)
• Build 3D point representations
• Calculate normals per triangle
• Average all normals sharing a vertex
• Normalize averages

9
Loading geometry from file

• Calculate a triangles normal
• Let the triangle be the points A,B,C
• Let u (B-A) -- vector subtraction
• Let v (C-A)
• Normal normalize(u cross v)

10
Loading geometry from file

• Shortcut math for height-fields
• All points are evenly spaced in the x-y plane.
• They only differ in their z value
• For triangle A,B,C in question (a point and its
  upper and left neighbors)
• Let a A.z
• Let b B.z
• Let c C.z
• Let u b-a
• Let v c-a
• N Normalize(u,v,1)
• This is just for triangle normals, must still
  average and renormalize to get vertex normals

11
Loading geometry from file

• To use your normals call glNormal3f before the
  glVertex3f corresponding vertex
• If modelview matrix never has any scaling and
  your normals are always unit length
  glEnable(GL_NORMALIZE) is unnecessary (better
  performance)

12
Camera animation

• Motivation
• Motion is a change over time that can help convey
  an extra dimension of detail
• 3D geometric details more visible in motion
• Let user decide what view they want

13
Camera animation

• Helicopter camera
• gluLookAt
• Eye point is helicopter (on some fixed path)
• LookAt point is target (user controlled?)
• Up vector is always up (for most worlds)
• First-person-shooter camera
• glTranslate
• using x,y,z of player location
• glRotate
• Rotate about z (up vector) by player heading
• Rotate about y (horizontal axis) by player tilt
• Keyboard/mouse events
• Update player parameters

14
Advanced lighting/materials

• Scene may have several lights
• Each light has ambient,diffuse,specular emmision
  coefficients
• Scene may have many objects
• Each object has ambient,diffuse,specular
  reflection coefficients

15
Advanced Lighting

• Relevant OpenGL functions
• glLight
• glMaterial
• glLightModel

16
Texture Mapping

• Motivation
• Real objects are not all one color
• We can make geometry look more detailed if we
  glue a picture onto it
• Geometry can be re-used with different textures
• Real world image data can be sampled for use in
  3D to gain more realism (photographs)

17
Texture Mapping

• A texture is an image with a different color at
  each pixel (RGB/RGBA)
• No matter the size, the texture is indexed using
  u,v in the range 0-1
• These are texture coodinates
• Geometry can be augmented with texture
  coordinates to describe how image should be
  stretched over the surface

18
Texture Mapping

• An artist can pick the u,v for each vertex
  (manual mapping)
• A formula can compute u,v for each vertex
  (automatic mapping)
• (u,v) (x,y) -- planar mapping
• (u,v) (theta,phi) -- spherical mapping
• (u,v) (something, t) -- temporal mapping

19
Texture Mapping

• Loading a texture for use
• glEnable GL_TEXTURE_2D -- turn on text
• glGenTextures -- request an unused texture id
• glBindTexture -- make a certain texture id the
  active one
• gluBuildMaps
• Target GL_TEXTURE_2D
• Internal format GL_RGB
• Width canvas width
• Height canvas height
• Format GL_RGBA (ppmLoadCanvas make 4-byte
  pixels)
• Type GL_UNSIGNED_BYTE (one byte per channel,
  0-255)
• Data cavas.pixels (the raw image data)
• glTexCoord2f
• Specify a u,v before calling the corresponding
  glVertex3f

20
Display lists

• Motivation
• Usually we draw the same scene over and over with
  only minimal changes
• GPU is idle much of the time waiting for the CPU
  to feed it the scene again
• By submitting the non-changing parts of the scene
  once excess processing can be avoid and
  parallelism can be improved

21
Display lists

• Using a display list
• // before main loop
• list glGenList(1) -- get an unused list id
• glNewList(list,GL_COMPILE)
• Lots of (almost any) GL commands and expensive
  geometry and texture coordinate generation code
  go here
• Resulting geometry will be saved on device (if
  possible)
• glEndList(list)
• // at rendering time
• glCallList(list) // returns almost instantly

22
Requirements

• Load a map.ppm and render it as a height field
  (given on command line)
• Load a skin.ppm and use it to texture the height
  field (given on command line)
• Light height field realistically (at least one
  directional light and correct normals)
• Use a display list to cache hieght field geometry
• Animate viewpoint using either helicopter or FPS
  style camera

23
Impressive extensions

• Shoreline
• render another height field with a fixed height
  and different material properties, possibly use
  difference between heights to decide a color
• Distant Haze
• use glFog and friends to lighten distant terrain
• Dune Buggy
• allow user to move an object around the height
  field with arcade-style driving controls and
  track it with the animated camera
• Lazy Camera
• soften camera movements
• lookAt lookAt_last k(target-lookAt_last) --
  k in 0-1
• Creative maps
• edit map and skin to design your own terrain
• Accurate maps
• find height field and satellite picture for a
  real-world location

24
Resources

• OpenGL Programming Guide (The Red
  Book)http//www.glprogramming.com/red/
• Viewinghttp//www.glprogramming.com/red/chapter03
  .html
• Lightinghttp//www.glprogramming.com/red/chapter0
  6.html
• PyOpenGL Man Pages (Great OpenGL API
  doc)http//pyopengl.sourceforge.net/documentation
  /manual/index.xml
• NeHe OpenGL Tutorialshttp//nehe.gamedev.net/