resume lookat what to change in furture

inc/hgl/math/Matrix.h

@@ -75,6 +75,13 @@ namespace hgl
                             float aspect_ratio,                                
                             float znear,
                             float zfar);
+    /**
+     * 产生一个lookat变换矩阵
+     * @param eye 眼睛位置
+     * @param target 目标位置
+     * @param up 向上向量
+     */
+    Matrix4f lookat(const Vector3f &eye,const Vector3f &target,const Vector3f &up);
 
     inline Matrix4f translate(const Vector3f &v)
     {

src/Math/Matrix4f.cpp

@@ -102,4 +102,33 @@ namespace hgl
           0.0f
         );
     }
+
+    Matrix4f lookat(const Vector3f &eye,const Vector3f &target,const Vector3f &up)
+    {
+        Vector3f forward=normalize(target-eye);
+        Vector3f right=normalize(cross(forward,up));
+
+        Vector3f nup=cross(right,forward);
+
+        return Matrix4f(   right.x,
+                             nup.x,
+                        -forward.x,
+                              0.0f,
+
+                           right.y,
+                             nup.y,
+                        -forward.y,
+                              0.0f,
+
+                           right.z,
+                             nup.z,
+                   -forward.z/2.0f,
+                              0.0f,
+
+                 -dot(eye,right  ),
+                 -dot(eye,nup    ),
+                  dot(eye,forward),
+                              1.0f
+        );
+    }
 }//namespace hgl