A startup script that adds interpolate() functions to RContour RSegment objects. There is also a faux RSegment.copy() function.

RContourRSegment_interpolate.py

from fontParts.base.errors import FontPartsError
from fontPens.digestPointPen import DigestPointStructurePen


def isStrictCompatible(path1,path2):

    # check interpolation compatibility 

    if type(path1).__name__.split(".")[-1] == "RSegment":
        if path1.type == path2.type:
            return True
        else:
            return False
    else:
        p1Pen = DigestPointStructurePen()
        path1.drawPoints(p1Pen)
        p1Data = p1Pen.getDigest()

        p2Pen = DigestPointStructurePen()
        path2.drawPoints(p2Pen)
        p2Data = p2Pen.getDigest()

        if p2Data == p1Data:
            return True
        else:
            return False


def interpolatePoints(a,b,v):

    # interpolate points

    x = a[0] + v * (b[0] - a[0])
    y = a[1] + v * (b[1] - a[1])

    return (x,y)


def addLastPoint(segment):

    # This adds the extra point to segment.points

    segmentPoints = []
    allSegs = [seg for seg in segment.contour.segments]
    segmentPoints.append(allSegs[segment.index-1][-1])
    for selPoint in segment.points:
        segmentPoints.append(selPoint)

    return segmentPoints


def copySegment(self):

    # This adds a faux copy function to a segment

    tempGlyph = RGlyph()
    points = addLastPoint(self)
    if len(points) == 4:
        pen = tempGlyph.getPen()
        pen.moveTo((points[0].x,points[0].y))
        if self.type == "line":
            pen.lineTo((points[1].x,points[1].y))
        else:
            pen.curveTo((points[1].x,points[1].y),(points[2].x,points[2].y),(points[3].x,points[3].y))
        pen.endPath()

    return tempGlyph.contours[-1]


def interpolateConSeg(self,factor,otherConSeg):

    # Interpolate a contour or segment
    # Segment interpolation is very hacky


    if type(self).__name__.split(".")[-1] == "RSegment":
        objType = "RSeg"
    else:
        objType = "RCon"

    glyph = self.glyph

    glyph.prepareUndo()
    if self.isCompatible(otherConSeg):
        if isStrictCompatible(self,otherConSeg):
            
            pointPen = glyph.getPointPen()
            copied = self.copy()

            if objType == "RSeg":
                conPoints = addLastPoint(self)
                otherConSegPs = addLastPoint(otherConSeg)
            else:
                conPoints = self.points
                otherConSegPs = otherConSeg.points

            for i, point in enumerate(conPoints):
                ips = interpolatePoints((point.x,point.y),(otherConSegPs[i].x,otherConSegPs[i].y),factor)
                copied.points[i].x = round(ips[0])
                copied.points[i].y = round(ips[1])


            copied.drawPoints(pointPen)

        else:
            raise FontPartsError
    glyph.performUndo()
    
    
# ------------------------------------------------------------------ #

from mojo.roboFont import RContour
from lib.fontObjects.fontPartsWrappers import RSegment


RSegment.copy = copySegment
RContour.interpolate = interpolateConSeg
RSegment.interpolate = interpolateConSeg

# ------------------------------------------------------------------ #