SubdivideAndDrawQuad 函数解析

void SubdivideAndDrawQuad( int uSteps, int vSteps,
                           float s0, float t0, float x0, float y0, float z0,
                           float s1, float t1, float x1, float y1, float z1,
                           float s2, float t2, float x2, float y2, float z2,
                           float s3, float t3, float x3, float y3, float z3 )
{
    float tc0[3] = { s0, t0, 0.0 };  float v0[3] = { x0, y0, z0 };
    float tc1[3] = { s1, t1, 0.0 };  float v1[3] = { x1, y1, z1 };
    float tc2[3] = { s2, t2, 0.0 };  float v2[3] = { x2, y2, z2 };
    float tc3[3] = { s3, t3, 0.0 };  float v3[3] = { x3, y3, z3 };

    glBegin( GL_QUADS );

    for ( int u = 0; u < uSteps; u++ )
    {
        float uu = (float) u / uSteps;
        float uu1 = (float) (u + 1) / uSteps;
        float Atc[3], Btc[3], Ctc[3], Dtc[3];
        float Av[3], Bv[3], Cv[3], Dv[3];

        for ( int i = 0; i < 3; i++ )
        {
            Atc[i] = tc0[i] + uu  * ( tc1[i] - tc0[i] );
            Btc[i] = tc3[i] + uu  * ( tc2[i] - tc3[i] );
            Ctc[i] = tc0[i] + uu1 * ( tc1[i] - tc0[i] );
            Dtc[i] = tc3[i] + uu1 * ( tc2[i] - tc3[i] );
            Av[i] = v0[i] + uu  * ( v1[i] - v0[i] );
            Bv[i] = v3[i] + uu  * ( v2[i] - v3[i] );
            Cv[i] = v0[i] + uu1 * ( v1[i] - v0[i] );
            Dv[i] = v3[i] + uu1 * ( v2[i] - v3[i] );
        }

        for ( int v = 0; v < vSteps; v++ )
        {
            float vv = (float) v / vSteps;
            float vv1 = (float) (v + 1) / vSteps;
            float Etc[3], Ftc[3], Gtc[3], Htc[3];
            float Ev[3], Fv[3], Gv[3], Hv[3];

            for ( int i = 0; i < 3; i++ )
            {
                Etc[i] = Atc[i] + vv  * ( Btc[i] - Atc[i] );
                Ftc[i] = Ctc[i] + vv  * ( Dtc[i] - Ctc[i] );
                Gtc[i] = Atc[i] + vv1 * ( Btc[i] - Atc[i] );
                Htc[i] = Ctc[i] + vv1 * ( Dtc[i] - Ctc[i] );
                Ev[i] = Av[i] + vv  * ( Bv[i] - Av[i] );
                Fv[i] = Cv[i] + vv  * ( Dv[i] - Cv[i] );
                Gv[i] = Av[i] + vv1 * ( Bv[i] - Av[i] );
                Hv[i] = Cv[i] + vv1 * ( Dv[i] - Cv[i] );
            }

            glTexCoord2fv( Etc );  glVertex3fv( Ev );
            glTexCoord2fv( Ftc );  glVertex3fv( Fv );
            glTexCoord2fv( Htc );  glVertex3fv( Hv );
            glTexCoord2fv( Gtc );  glVertex3fv( Gv );
        }
    }

    glEnd();
}

主要需要理解的点：

1 ustep和vstep是沿着x，y两个方向的增量；

2 uu = u/ustep,float类型参数，假如ustep=24，u=1，则uu=1/24，因为如果直接放1和24的话，结果也会是int类型，这样就得不到精确结果；

3 同时需要uu，uu1，是因为uu和uu1使之能够话出小多边形，uu1是uu的下一点；

4 同时使用 t0,t1 与 t2,t3 来计算上下边在u方向上的变化（t0t1是下面的边，t2t3是上面的边）是因为当这个正方形是斜着的，就是边不与坐标轴垂直或平行时，x，y，z的坐标都不同，都得重新计算，当然，当垂直或平行时，可以只计算一边，另一边只需相应的改变y就行。