15

I am experimenting with creating high-performance, good-looking pencil tools using SVG paths.

I am logging the mouse coordinates to draw a path. To get a high-fidelity path (accurate to the user's movements) I need to log a point for every pixel movement.

Keeping each and every point in the path creates a huge amount of points which is not ideal for collaborative features later-on (sending huge amount of points back and forth is not efficient), plus parsing huge paths every time I need to manipulate them is a bottleneck

On linear areas of the path, redundant points are removed keeping only the points necessary to represent the segment - I do this using the Ramer-Douglas-Peucker algorithm.

But simplifying a path turns it into a low-fidelity polygon

At this point the paths are effectively just connected lines - therefore the paths look jagged.

A possible solution is to connect the path points with Cubic Bezier's - however this doesn't work nice on simplified paths. The distance between each point is too large for the Cubic Bezier's to "sit" nice so the smoothed path no longer accurately represents the intended path of the user.

Another solution is to simply use a "post-processing" algorithm such as Schneider's Algorithm on the original path - This algorithm won't practically work in real-time though since it's a performance hog

An ideal solution

A solution that(I think) could work is to use a Centripetal Catmull-Rom interpolation.

Centripetal Catmull Rom vs rest of Catmull-Rom variants

Out of all the algorithms I researched, this seems to be the most promising since:

  1. It doesn't create self-intersections on tight corners
  2. It fits more snug on the points thus it more accurately represents the original path.

Is Catmull-Rom an algorithm that interpolates a series of regular x/y points or does the original path need to be comprised of curves?

4

3 回答 3

15

直接回答您的问题:

  1. 是的。Catmull-Rom spline 是一种插入一系列 (x, y, z) 点的算法。它将在每两个连续点之间生成一条三次多项式曲线。
  2. 您不能直接将 Catmull Rom 样条用于 SVG 路径。您需要先将其转换为三次贝塞尔曲线。

对于由点 P0、P1、P2 和 P3 以及节点序列 t0、t1、t2、t3 定义的曲线段,可以通过https中提供的递归公式计算向心 Catmull-Rom 样条(定义在点 P1 和 P2 之间) : //en.wikipedia.org/wiki/Centripetal_Catmull%E2%80%93Rom_spline。因此,我在此不再赘述。

要将其转换为三次贝塞尔曲线,您需要计算 P1 和 P2 处的一阶导数为

M1 = (t2-t1)*(c1*(P1-P0)/(t1-t0) + c2*(P2-P1)/(t2-t1))
M2 = (t2-t1)*(d1*(P2-P1)/(t2-t1) + d2*(P3-P2)/(t3-t2))

在哪里

 c1 = (t2-t1)/(t2-t0),
 c2 = (t1-t0)/(t2-t0),
 d1 = (t3-t2)/(t3-t1),
 d2 = (t2-t1)/(t3-t1)

然后您可以将其转换为具有 4 个控制点的三次贝塞尔曲线:Q0、Q1、Q2 和 Q3:

Q0 = P1
Q1 = P1 + M1/3
Q2 = P2 - M2/3
Q3 = P2
于 2015-06-14T06:04:56.547 回答
3

我为此使用插值三次

  1. 采样点

    • 鼠标在任意轴方向变化
    • 如果鼠标移动了一些距离以获得更高的精度可以添加:如果方向角度变化太大(我不使用这个)
    • 如果鼠标停止(如果您也使用鼠标向上/向下,这可以忽略)
  2. 使用采样点作为插值三次的控制点

    插值三次与贝塞尔三次,你还发现插值和贝塞尔三次之间的转换

  3. 如果这还不够,你可以加入相同方向的线

    这里以这种方式的手绘示例:

    跟随鼠标

    和 SVG:

    <svg width="512" height="512" viewBox="-131.734968 -63.890725 383.802249 203.65153" >
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    </svg>
    

    如果您使用阈值游戏,它大约为 20KB,您可以显着降低点数。这是调整以满足我的需要。

    这里是绘图过程中鼠标跟随感觉的示例(在绘图过程中实时捕获)

    spektre 手绘

于 2015-06-10T08:07:52.470 回答
2

关于 Catmull-Rom 曲线的参数化 参见公式[2]

在线演示

卡特莫尔和贝塞尔曲线 贝塞尔点

于 2021-10-27T03:41:54.693 回答