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POLY2KVX

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• Alexey at 2005-03-18 19:00
  Ken,
  
  You wrote that POLY2KVX uses hashing to store 1024x1024x256 color data.
  It runs the 28-bit (x,y,z) coordinate through some equation which results in a 16-bit
  integer, which is then used as an index to a list of (x,y,z) coordinates and
  colors. If a matching (x,y,z) pair in the list is found, then it has valid
  data, else it is added to the list.
  
  And the trick is to find the equation so that the lists are balanced, meaning they
  have close to the same number of entries.
  
  Question:
  What equation / hashing function / hashing method do you use in POLY2KVX?
  
  Stupid question: Why the (x,y,z) coordinate is a 28-bit size?
  
  Weird question:
  Does POLY2KVX converts meshes that arent "closed"?
  I mean, I tried to convert the famous chrmface.3ds to chrmface.kvx but it doesnt seem to work :(
  Have I done something wrong?
  
  Thank you in advance.
• Awesoken at 2005-03-19 05:40
  I extracted the relevant parts of my POLY2KVX code and cleaned it up for you. This should give you an idea of how my hash code works:
  
  

  #define MAXDIM 1024
  
  #define MAYDIM 1024
  
  #define MAZDIM 512
  
  
  
  #define LHASHHEAD 20
  
  long hashhead[1<<LHASHHEAD];
  
  typedef struct { long prev, ind, val; } hash_t;
  
  hash_t *hash;
  
  
  
  long getval (long ind)
  
  {
  
     long i = ((ind>>8) & ((1<<LHASHHEAD)-1));
  
     for(i=hashhead[i];i>=0;i=hash[i].prev)
  
        if (hash[i].ind == ind) return(hash[i].val);
  
     return(-1);
  
  }
  
  
  
        //Example call: (setval function not shown)
  
     long value_at_xyz = getval((x*MAYDIM + y)*MAZDIM + z);

  
  
  I chose a very large hash head size (1048576 indices) so it would be fast enough for large objects.

  Why the (x,y,z) coordinate is a 28-bit size?

  You need exactly 28 bits to specify an index into a Voxlap-sized array. Voxlap boards are always: 1024x1024x256. (log2(1024)+log2(1024)+log2(256) = 28).

  Does POLY2KVX converts meshes that arent "closed"?

  Yes, they work, but their interior of the object will be hollow. My normal estimation code isn't designed for hollow objects with thin surfaces, so that's why the colors look terrible when you load it in SLAB6. If you press 'R' to disable lighting, it will look a little better.