Off Files.

• This is a file format which is used to store 3d data.
• There does not seem to be an "official" standard but
  • There are at least two public collections of off files.
  • off files in Geomview
  • Princeton Shap Benchmark.
  • Ryan Holmes mentions that there are other formats, and extensions.
  • But these will do for now.
• They all agree that the basic format is
  • The word OFF
  • A line containing
    • v the number of vertices, an integer
    • f the number of faces, an integer
    • e the number of edges, an iteger, both standards say this can be ignored
  • Followed by vlines containing at least three floats
    • The first three floats represent the coordinates of a vertex.
    • There may be four more floats representing the color of the vertex. (Geomview, not Princeton)
    • Vertices are numbered 0 to v-1
  • This is followed by f lines containing
    • A polygon description.
    • An integer, n, representing the number of points in the polygon (at least three)
    • n integers representing the vertex corrsponding to this point.
    • There is a possiblity that there are r,g,b,a for each face as well.
  • A comment begins with #
• An example from Geomview is

  OFF
  #
  #  cube.off
  #  A cube.
  #  There is extra RGBA color information specified for the faces.
  #
  8 6 12
    1.632993   0.000000   1.154701
    0.000000   1.632993   1.154701
   -1.632993   0.000000   1.154701
    0.000000  -1.632993   1.154701
    1.632993   0.000000  -1.154701
    0.000000   1.632993  -1.154701
   -1.632993   0.000000  -1.154701
    0.000000  -1.632993  -1.154701
    4  0 1 2 3  1.000 0.000 0.000 0.75
    4  7 4 0 3  0.300 0.400 0.000 0.75
    4  4 5 1 0  0.200 0.500 0.100 0.75
    4  5 6 2 1  0.100 0.600 0.200 0.75
    4  3 2 6 7  0.000 0.700 0.300 0.75
    4  6 5 4 7  0.000 1.000 0.000 0.75
  

• We will use Off files in this format for our 3d explorations.
  • The format is simple and easy to parse.
  • There are LOTS of files in this format.
• As a side note, I have some data in a derived format
  • The files are here.
  • For some reason I converted the format but that was silly.
  • I will eventually convert them back.
• Working with off files.
  • You should begin to create an object which can load and draw objects in off format.
  • You need to be able to convert polygons to triangles, but for now, just use a trianlge fan approch.
    • When you encounter more than three points in an object
      • Add P₀, P₁, P₂
      • Then add P₀, P₂, P₃
      • ...
      • finally add P₀, P_n-2, P_n-1
  • I would compute the min and max of the points (more on this later) for scaling the object, I eventually scale everything to the unit cube.
  • Store vertices in a structure, which you can expand as you need to
    • we will eventually add normal data for each vertex.
  • Start with the code from Demo6 and modify it as you go.
• Sources
  • The Princeton Shape Benchmark page.
  • Files from Ryan Holmes