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Expand a Polygon along one dimension only with SKILL

markmalin

I have a polygon with a given bounding box.  I want to take that polygon and create a new one, but have the X dimension 10% wider.

I'm somewhat new to SKILL, so bear with me.  I understand how I can get access to the given polygon object from the database, and I understand that there is a "bbox" list defining the bounding box.  If I calculate the width in X to be some number of user units, I want to add them like so:

xDistance = sqrt(caar(myPolygon->bBox) - cadr(myPolygon->bBox))
xDistance = expt(xDistance 2)

xExpandDist = xDistance * (0.1)

Now I want to expand the myPolygon->bBox "xExpandDist/2" to the left (i.e. on the X axis), and "xExpandDist/2" to the right (i.e. on the X axis).

I understand that a polygon has a bounding box, I can get the X,Y coordinates for the bbox, I know how much I want to expand it, but can I simply assign a new value to it?  In other words, in pseudo code:

myPolygon's bbox lower left x value = myPolygon's bbox lower left x value - xExpandDist 

myPolygon's bbox upper right x value = myPolygon's bbox upper right x value + xExpandDist 

How do I do this?  How do I change the X coordinates of the bounding box?

Thanks
Mark

  • DavidJHutchins

    No, the bounding box is calculated from the min/max values of the segments

    To modify the X dimensions you will need to recalculate the X coordinates of the min & max locations, then rebuild the shape

  • eDave

    You need to take the vertices from the polygon and add your delta x to those.

    This code should help you:

    defun( DE_polyVertsToPath (poly @key (expandx 0), (expandy 0) (width 0))
    let((dx, dy, midPoly, verts, pt1, pt2, rpath, radius, cw)
    dx = expandx / 2.0
    dy = expandy / 2.0
    midPoly = axlMidPointLine(poly ->bBox)
    verts = poly ->vertices
    pt1 = DE_polyVertsToPath_expandPt(caar(verts), dx, dy, midPoly)
    rpath = axlPathStart(list(pt1), width)
    foreach(vert, car(tconc(lconc(nil, cdr(verts)), car(verts)))
    pt2 = DE_polyVertsToPath_expandPt(car(vert), dx, dy, midPoly)
    unless(axlGeoPointsEqual(pt1, pt2)
    radius = cadr(vert)
    if(zerop(radius)
    then axlPathLine(rpath, width, pt2)
    else cw = plusp((cadr(pt2) - cadr(pt1)) * radius), axlPathArcRadius(rpath, width, pt2, cw, nil, abs(radius))
    )
    )
    pt1 = pt2
    ); foreach vert
    rpath
    ))

    defun( DE_polyVertsToPath_expandPt (pt, dx, dy, midPt)
    list(
    if(car(pt) < car(midPt) then car(pt) - dx else car(pt) + dx)
    if(cadr(pt) < cadr(midPt) then cadr(pt) - dy else cadr(pt) + dy)
    )
    )

  • markmalin
    in reply to eDave

    Thanks.  This is helpful.  It wasn't clear from the documentation how to either modify and existing shape or how to re-create/create one.  I'll look closely at the example you posted.  Thanks!  Mark

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