如何生成osg的动画路径文件

1. 前言

2. 生成动画路径文件

2.1. 粗糙方式

2.2. 精确方式

1. 前言

在进行osg的开发中，有时需要对模型按某个路径或规则进行动画，如下：

奶牛在10秒时间段从起始的osg::Vec3d(0.0, 18, 1.0)位置匀速直线运动到osg::Vec3d(0.0, -8, 1.0)位置处。又比如下：

飞机以和Y轴成倾斜45°角绕着Z轴在地面一定高度做圆周盘旋运动。

一般地，为相机设置osgGA::AnimationPathManipulator操控器，或者为osg::PositionAttitudeTransform、osg::MatrixTransform节点通过调用setUpdateCallback函数设置动画路径回调函数osg::AnimationPathCallback类对象就可以达到动画的效果。当动画生成的时候，需要将动画路径的数据如控制点等信息，以文件形式保存下来，以供其它程序使用。如何生成这样的动画路径文件？

2. 生成动画路径文件

2.1. 粗糙方式

如果需求对动画路径要求不精确，如：没有要求具体的时间段、没有要求具体的转动角度、移动距离等, 仅仅只是要求移动或转动到估计的位置或角度就行，则可以用osg自带的在OsgViewerd.exe（release版为OsgViewer.exe）按下述步骤生成动画路径文件（以Windows为例说明）：

1. 打开cmd，输入：

E:\osg\build_osg3.6.2\bin\OsgViewerd.exe  E:\osg\OpenSceneGraph-Data\cow.osg

其中的两个路径，请根据实际更改。如下为cmd输入的命令行：

2. 按下键盘小写键z，开始记录动画路径，依照需求，在场景视图上按住鼠标移动或转动奶牛模型，直到达到预想的动画，然后按住大写"Z"保存路径文件，按住ESC键关闭场景视图窗口，此时就会在上述红色框的目录即启动cmd.exe的目录下生成saved_animation.path的动画路径文件，文件内容类似如下：

0.0018253 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.0667249 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.1337192 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.1998216 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.2653721 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.3338356 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.4002481 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.4664029 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.5332971 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.5996751 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.6665445 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.7353267 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.7997468 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.8663566 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.9328836 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
0.9993953 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.0663357 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.1383031 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.2032655 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.2654628 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.3326584 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.3995767 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.4658965 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.5320493 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.5997768 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.6665459 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.7327271 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.7986692 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.8654239 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
1.9356403 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
2.0001472 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
2.0880553 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548
2.1488845 0.776124954223633 -25.6958980560303 -0 0.707106781186548 0 0 0.707106781186548

这样动画路径文件就生成了。

2.2. 精确方式

对于2.1节提到的方法，仅仅只能用于粗略估计的平移和旋转，对于精确的，则必须通过编程及数学算出每个时刻及该时刻的控制点，然后保存到文件。以前面的奶牛需求为例子，代码实现如下：

#include<osgGA/AnimationPathManipulator>
#include<osgViewer/Viewer>
#include<osgDB/ReadFile>
#include<osgDB/WriteFile>
#include<osg/PositionAttitudeTransform>
#include<osg/AnimationPath>

osg::AnimationPath* createAnimationPath(const osg::Vec3d& startPos, const osg::Vec3d& endPos, double time)
{
    auto pAnimationPath = new osg::AnimationPath;
    pAnimationPath->setLoopMode(osg::AnimationPath::LOOP);

    auto timeDeta = 0.2;

    const auto loopCount = time / timeDeta;
    
    // 这里假定模型在x、y、z方向上运动的速度都一样且是匀速直线运动
    auto xStep = (endPos.x() - startPos.x()) / loopCount; // 每个时间段前进的距离
    auto yStep = (endPos.y() - startPos.y()) / loopCount;
    auto zStep = (endPos.z() - startPos.z()) / loopCount;

    double angle = 0.0;
    double twoPi = 2 * 3.1415926;
    osg::Vec3d lastPos = startPos;
    osg::Vec3d pos = startPos;
    for (auto timeOffset = 0.0; timeOffset <= time; timeOffset += timeDeta)
    {
        if (0.0 < timeOffset)
        {
            pos.x() = pos.x() + xStep;
            pos.y() = pos.y() + yStep;
            pos.z() = pos.z() + zStep;

            auto xOffset = pos.x() - lastPos.x();
            auto yOffset = pos.y() - lastPos.y();
            angle = atan2f(yOffset, xOffset); // 算出偏转方向
            angle = twoPi + angle;
        }

        osg::Quat roateByZAxis(0/*angle*/, osg::Vec3(0, 0, 1));
        osg::AnimationPath::ControlPoint ctrlPoint(pos, roateByZAxis); // 动画控制点
        pAnimationPath->insert(timeOffset, ctrlPoint);
        lastPos = pos;
    }

    // 保存动画路径到文件
    osgDB::ofstream out("cow_animationpath.txt");
    pAnimationPath->write(out);

    return pAnimationPath;
}

int main(int argc, char *argv[])
{
    auto pCow = osgDB::readRefNodeFile(R"(E:\osg\OpenSceneGraph-Data\cow.osg)");
    if (nullptr == pCow)
    {
        OSG_WARN << "cow node is nullpr!" << std::endl;
        return 1;
    }

    osgViewer::Viewer viewer;

    auto pRoot = new osg::Group();

    auto pPosAttTransform = new osg::PositionAttitudeTransform;
    auto initPos = osg::Vec3d(0.0, 18, 1.0);
    pPosAttTransform->setPosition(initPos);
    pPosAttTransform->addChild(pCow);
    auto pAnimationCallback = new osg::AnimationPathCallback(createAnimationPath(initPos, osg::Vec3d(0.0, -8, 1.0), 10.0));
    pPosAttTransform->setUpdateCallback(pAnimationCallback);
    
    pRoot->addChild(pPosAttTransform);

    viewer.setSceneData(pRoot);
   
    return viewer.run();
}

运行该程序，则在本工程目录下，会生成cow_animationpath.txt文件，内容如下：

0 0 18 1 0 0 0 1
0.2 0 17.48 1 0 0 0 1
0.4 0 16.96 1 0 0 0 1
0.6 0 16.44 1 0 0 0 1
0.8 0 15.92 1 0 0 0 1
1 0 15.4 1 0 0 0 1
1.2 0 14.88 1 0 0 0 1
1.4 0 14.36 1 0 0 0 1
1.6 0 13.84 1 0 0 0 1
1.8 0 13.32 1 0 0 0 1
2 0 12.8 1 0 0 0 1
2.2 0 12.28 1 0 0 0 1
2.4 0 11.76 1 0 0 0 1
2.6 0 11.24 1 0 0 0 1
2.8 0 10.72 1 0 0 0 1
3 0 10.2 1 0 0 0 1
3.2 0 9.68000000000001 1 0 0 0 1
3.4 0 9.16000000000001 1 0 0 0 1
3.6 0 8.64000000000001 1 0 0 0 1
3.8 0 8.12000000000001 1 0 0 0 1
4 0 7.60000000000001 1 0 0 0 1
4.2 0 7.08000000000001 1 0 0 0 1
4.4 0 6.56000000000001 1 0 0 0 1
4.6 0 6.04000000000001 1 0 0 0 1
4.8 0 5.52000000000001 1 0 0 0 1
5 0 5.00000000000001 1 0 0 0 1
5.2 0 4.48000000000001 1 0 0 0 1
5.4 0 3.96000000000001 1 0 0 0 1
5.6 0 3.44000000000001 1 0 0 0 1
5.8 0 2.92000000000001 1 0 0 0 1
6 0 2.40000000000001 1 0 0 0 1
6.2 0 1.88000000000001 1 0 0 0 1
6.4 0 1.36000000000001 1 0 0 0 1
6.6 0 0.840000000000011 1 0 0 0 1
6.8 0 0.320000000000011 1 0 0 0 1
7 0 -0.199999999999989 1 0 0 0 1
7.2 0 -0.719999999999989 1 0 0 0 1
7.4 0 -1.23999999999999 1 0 0 0 1
7.6 0 -1.75999999999999 1 0 0 0 1
7.8 0 -2.27999999999999 1 0 0 0 1
8 0 -2.79999999999999 1 0 0 0 1
8.2 0 -3.31999999999999 1 0 0 0 1
8.4 0 -3.83999999999999 1 0 0 0 1
8.6 0 -4.35999999999999 1 0 0 0 1
8.8 0 -4.87999999999999 1 0 0 0 1
9 0 -5.39999999999999 1 0 0 0 1
9.2 0 -5.91999999999999 1 0 0 0 1
9.4 0 -6.43999999999999 1 0 0 0 1
9.6 0 -6.95999999999999 1 0 0 0 1
9.8 0 -7.47999999999999 1 0 0 0 1
10 0 -7.99999999999999 1 0 0 0 1