path_bezier2 - 2D quadratic bezier curves

`local bezier2 = require'path_bezier2'`

Math for 2D quadratic bezier curves defined as

x1, y1, x2, y2, x3, y3

where (x1, y1) and (x3, y3) are the end points and (x2, y2) is the control point.

`bezier2.bounding_box(x1, y1, x2, y2, x3, y3) -> left, top, width, height`

Compute the bounding box using derivative root finding (fast, no dynamic allocations).

`bezier2.to_bezier3(x1, y1, x2, y2, x3, y3) -> x1, y1, x2, y2, x3, y3, x4, y4`

Return the cubic bezier curve that best approximates the quadratic curve, using degree elevation.

`bezier2._3point_control_point(x1, y1, x0, y0, x3, y3) -> x2, y2`

Return a fair candidate based on chord lengths for the control point of a quad bezier given its end points (x1, y1) and (x3, y3) and a point (x0, y0) that lies on the curve.

`bezier2.point(t, x1, y1, x2, y2, x3, y3) -> x, y`

Evaluate a quadratic bezier at parameter t using De Casteljau linear interpolation.

`bezier2.length(t, x1, y1, x2, y2, x3, y3) -> length`

Return the length of the curve at parameter t. The approximation is done by way of Gauss quadrature and is thus very fast and accurate and does no dynamic allocations. The algorithm is explained in detail here.

`bezier2.split(t, x1, y1, x2, y2, x3, y3) -> ax1, ay1, ax2, ay2, ax3, ay3, bx1, by1, bx2, by2, bx3, by3`

Split a quadratic bezier at parameter t into two quadratic curves using De Casteljau linear interpolation.

`bezier2.hit(x0, y0, x1, y1, x2, y2, x3, y3) -> d2, x, y, t`

Find the nearest point on a quadratic bezier curve by way of solving a 3rd degree equation.

Return the shortest distance squared from point (x0, y0) to a quadratic bezier curve, plus the touch point, and the parametric value t on the curve where the touch point splits the curve.

The algorithm is from [http://blog.gludion.com/2009/08/distance-to-quadratic-bezier-curve.html].

The Lua implementation is closed form and makes no dynamic allocations.

`bezier2.interpolate(write, x1, y1, x2, y2, x3, y3[, m_approximation_scale[, [m_angle_tolerance]])`

Approximate a quadratic bezier curve with line segments using recursive subdivision. The segments are outputted by calling the provided write function as write('line', x2, y2). Only the second point of each segment is thus outputted.

m_approximation_scale must be adjusted to the overall scale of the world-to-screen transformation.
m_angle_tolerance should only be enabled when the width of the scaled stroke is greater than 1.

The algorithm is from the AGG library and it's described in detail here.

The Lua implementation makes no dynamic allocations and the recursion is depth limited.