作者：numenzq 文章来源：http://www.j2medev.com/Article/ShowArticle.asp?ArticleID=1091
JSR-184与Mascot Capsule v3主要的不同就是关于照相机的实现。JSR-184支持照相机结合矩阵堆栈处理，例如，我们经常使用transform对象移动照相机。而Mascot Capsule v3依靠”look-at”方法，这是在某些3D API里的通用方法。look-at方法从一个position，一个 look-at direction和一个up vector创建一个照相机转换矩阵，为了方便Mascot Capsule v3与JSR-184之间的转换，Digital Chocolate采纳了Mascot Capsule v3照相机设计方法，并写了一个支持JSR-184的包。
在JSR-184 API规范里，NodeTransform类指定了一系列方法。这些方法有利于在JSR-184实现look-at方法。然而，它被认为与JSR-184执行说明规范有所不同，有时甚至会忽略这一系列方法。
自己实现look-at方法其实并不复杂。下面的代码例子是Digital Chocolate公司如何处理照相机的封装设计。请注意Digital Chocolate公司在Mascot Capsule v3中使用整数来处理，而在设计更高层的游戏类设计中使用浮点数来处理。
/**
* Wrapper method for setting look at camera.
*
* The method requires that look and up vectors normalized.
*/
public static final void setLookAt(float a_posX, float a_posY, float a_posZ,
float a_lookX, float a_lookY, float a_lookZ,
float a_upX, float a_upY, float a_upZ)
{
// JSR-184 version
if (USE_M3G)
{
// Cross product to get side vector
float sideX = (a_lookY * a_upZ) - (a_lookZ * a_upY);
float sideY = (a_lookZ * a_upX) - (a_lookX * a_upZ);
float sideZ = (a_lookX * a_upY) - (a_lookY * a_upX);
float inv_len = 1.0f /
(float) java.lang.Math.sqrt(sideX * sideX
+ sideY * sideY
+ sideZ * sideZ);
sideX *= inv_len;
sideY *= inv_len;
sideZ *= inv_len;
// make up vector perpendicular
a_upX = (sideY * a_lookZ) - (sideZ * a_lookY);
a_upY = (sideZ * a_lookX) - (sideX * a_lookZ);
a_upZ = (sideX * a_lookY) - (sideY * a_lookX);
// footnote: up is unit size because side and look are perpendicular
sm_mtx[0] = sideX;
sm_mtx[1] = a_upX;
sm_mtx[2] = -a_lookX;
sm_mtx[3] = a_posX;
sm_mtx[4] = sideY;
sm_mtx[5] = a_upY;
sm_mtx[6] = -a_lookY;
sm_mtx[7] = a_posY;
sm_mtx[8] = sideZ;
sm_mtx[9] = a_upZ;
sm_mtx[10] = -a_lookZ;
sm_mtx[11] = a_posZ;
sm_mtx[12] = 0.0f;
sm_mtx[13] = 0.0f;
sm_mtx[14] = 0.0f;
sm_mtx[15] = 1.0f;
sm_m3gTransform.set(sm_mtx);
}
// Mascot version
if (USE_MASCOT)
{
sm_mascotTmpVectorA.set((int)a_posX, (int)a_posY, (int)a_posZ);
sm_mascotTmpVectorB.set((int)(a_lookX * MASCOT_ONE),
(int)(a_lookY * MASCOT_ONE),
(int)(a_lookZ * MASCOT_ONE));
sm_mascotTmpVectorC.set(0, DajmGraphics.MASCOT_ONE, 0);
sm_mascotAffineTrans.lookAt(sm_mascotTmpVectorA,
sm_mascotTmpVectorB,
sm_mascotTmpVectorC);
}
}
