屏幕坐标系：左上角为(0, 0)右下角为(1, 1)
OGRE的2D坐标系：左上角为(-1, 1)右下角为(1, -1)
CEGUI坐标系：左上角为(0, 0)，单位像素
转换公式(鼠标坐标=>OGRE的2D坐标)
void setCorners(float left, float top, float right, float bottom)
{
left = left * 2 - 1;
right = right * 2 - 1;
top = 1 - top * 2;
bottom = 1 - bottom * 2;
}
对于根据鼠标位置来产生射线：
bool mousePressed(const OIS::MouseEvent &arg, OIS::MouseButtonID id)
{
...
CEGUI::Point mousePos = CEGUI::MouseCursor::getSingleton().getPosition();
Ray mouseRay = mCamera->getCameraToViewportRay(mousePos.d_x/float(arg.state.width), mousePos.d_y/float(arg.state.height));
...
}
其中函数
Ray getCameraToViewportRay(Real x, Real y) const;
// x and y are in “normalized” (0.0 to 1.0) screen coordinates
其中两个参数是对屏幕坐标系来说的，
所以
x = mousePos.d_x / float(arg.state.width)
y = mousePos.d_y / float(arg.state.height)
arg.state.width是渲染窗口的宽单位为像素
arg.state.height是渲染窗口的高单位为像素
mousePos.d_x是鼠标所在位置到渲染窗口左边界的距离单位为像素
mousePos.d_y是鼠标所在位置到渲染窗口上边界的距离单位为像素
将世界坐标转换为屏幕坐标：
bool worldCoordToScreen(Vector3 objPos, Camera* cam, Vector2 screenRect, Vector2& screenPos)
{
Matrix4 viewMatrix = cam->getViewMatrix();
Matrix4 projMatrix = cam->getProjectionMatrix();
Vector4 in = Vector4(objPos.x, objPos.y, objPos.z, 1.0);
Vector4 out = viewMatrix * in;
out = projMatrix * out;
if(out.w <= 0.0) return false; // out.w<0时，objPos 在摄像机背面
out.x /= out.w;
out.y /= out.w;
out.z /= out.w;
// Map x, y and z to range 0-1
out.x = out.x * 0.5 + 0.5;
out.y = out.y * 0.5 + 0.5;
out.z = out.z * 0.5 + 0.5;
// Map x,y to viewport
out.x = out.x * screenRect.x;
out.y = (1-out.y) * screenRect.y;
screenPos.x = out.x;
screenPos.y = out.y;
return true;
}