《Focus On 3D Terrain Programming》中一段代码的注释二

取自《Focus On 3D Terrain Programming》中的一段:

 1 bool CTERRAIN::MakeTerrainFault( int iSize, int iIterations, int iMinDelta, int iMaxDelta, float fFilter )
 2 {
 3   ......
 4   .............
 5   for( iCurrentIteration=0; iCurrentIteration<iIterations; iCurrentIteration++ )
 6   {
 7     //calculate the height range (linear interpolation from iMaxDelta to iMinDelta) for this fault-pass
 8     iHeight= iMaxDelta - ( ( iMaxDelta-iMinDelta )*iCurrentIteration )/iIterations;
 9
10     //pick two points at random from the entire height map
11     iRandX1= rand( )%m_iSize;
12     iRandZ1= rand( )%m_iSize;
13
14     //check to make sure that the points are not the same
15     do
16     {
17       iRandX2= rand( )%m_iSize;
18       iRandZ2= rand( )%m_iSize;
19     } while ( iRandX2==iRandX1 && iRandZ2==iRandZ1 );
20
21
22     //iDirX1, iDirZ1 is a vector going the same direction as the line
23     iDirX1= iRandX2-iRandX1;
24     iDirZ1= iRandZ2-iRandZ1;
25
26     for( z=0; z<m_iSize; z++ )
27     {
28       for( x=0; x<m_iSize; x++ )
29       {
30         //iDirX2, iDirZ2 is a vector from iRandX1, iRandZ1 to the current point (in the loop)
31         iDirX2= x-iRandX1;
32         iDirZ2= z-iRandZ1;
33
34         //if the result of ( iDirX2*iDirZ1 - iDirX1*iDirZ2 ) is "up" (above 0),
35         //then raise this point by iHeight
36         if( ( iDirX2*iDirZ1 - iDirX1*iDirZ2 )>0 )
37           fTempBuffer[( z*m_iSize )+x]+= ( float )iHeight;
38       }
39     }
40     //erode terrain
41     FilterHeightField( fTempBuffer, fFilter );
42   }
43   .......
44   ..............
45   return true;
46 }

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代码的核心内容就是实现一个fault formation算法，算法是一个简单的数学模型，所以只要按照在纸上画个图就明白了。随笔的重点只是一行代码：

iDirX2*iDirZ1 - iDirX1*iDirZ2>0

这行代码参考的是二维向量共线定理：若设a=（x1，y1），b=（x2，y2），则有x1y2=x2y1。即与平行概念相同x1y2 - x2y1=0

另外，iMinDelta和iMaxDelta确实不符合我们的习惯，我们一般都习惯iMinValue和iMaxValue这样