OpenGL ES圆内放大、缩小、向某一点拉伸(曲线形变)算法

1.圆内放大

//圆内放大
vec2 enlargeFun(vec2 curCoord,vec2 circleCenter,float radius,float intensity,float curve)
{
    float currentDistance = distance(curCoord,circleCenter);

    {
        float weight = currentDistance/radius;

        weight = 1.0-intensity*(1.0-pow(weight,curve));//默认curve 为2 ,当 curve 越大时, 会放大得越大的,
        weight = clamp(weight,0.0,1.0);
        curCoord = circleCenter+(curCoord-circleCenter)*weight;
    }
    return curCoord;
}

这是圆内放大算法, 
输入:坐标,放大中心坐标,放大半径,放大比例系数,放大算法参数. 
返回:放大之后应该取的像素的位置.

通常 intensity = 1.0,curve = 2.0

2.圆内缩小

vec2 narrowFun(vec2 curCoord,vec2 circleCenter,float radius,float intensity,float curve)
{
    float currentDistance = distance(curCoord,circleCenter);

    {
        float weight = currentDistance/radius;
        weight = 1.0-intensity*(1.0-pow(weight,curve));//默认curve 为2 ,当curve 越大时, 会缩小得越小的,
        weight = clamp(weight,0.0001,1.0);
        curCoord = circleCenter+(curCoord-circleCenter)/weight;
    }
    return curCoord;
}

参数含义 
输入:坐标,放大中心坐标,放大半径,放大比例系数,放大算法参数. 
返回:放大之后应该取的像素的位置.
通常 intensity = 1.0,curve = 2.0

3.向某一点拉伸

// 拉伸
vec2 stretchFun(vec2 textureCoord, vec2 originPosition, vec2 targetPosition, float radius,float curve)
{
    vec2 offset = vec2(0.0);
    vec2 result = vec2(0.0);

    vec2 direction = targetPosition - originPosition;


    float infect = distance(textureCoord, originPosition)/radius;

    infect = pow(infect,curve);// 默认 curve 为1,这个值越大,拉伸到指定点越圆润,越小越尖
    infect = 1.0-infect;
    infect = clamp(infect,0.0,1.0);
    offset = direction * infect;
    result = textureCoord - offset;

    return result;
}

参数含义
输入:坐标,拉伸中心坐标,拉伸目标坐标,拉伸半径,拉伸算法参数. 
返回:放大之后应该取的像素的位置.
通常 curve = 2

具体原理分析和参数对效果的影响请参考 大神 zfgrinm
的博客 https://blog.csdn.net/zfgrinm/article/details/79291693

用途，圆内放大可以放大眼睛，圆内缩小可以缩小嘴巴，向某一点拉伸的曲线形变可以瘦脸、削脸、瘦下巴、换脸等等

形变应用请参考大神 cain_huang
https://www.jianshu.com/p/3334a3af331f