Why does this offset fail?

[raydeleu]

I am still experimenting with a curved shape, trying to apply an offset. However, the offset on this shape fails. I thought it could be caused by the sharp edge. A small test shape of a house with a pointed roof however worked fine. What do I miss?

I added the code below:

const main = ({ Sketcher, 
                makeOffset }) => 

{
// create a house with a pointed roof
let houseSketch = new Sketcher("XZ")
        .vLine(50)
        .line(10,25)
        .line(10,-25)
        .vLine(-50)
        .close()

let house = houseSketch.extrude(40)

// offset this shape
house = makeOffset(house,3).translate([0,60,0])

// create a shape with a wave-like curve
let p0 = [0,0]
let p1 = [50,100]
let p2 = [60,-95]
let p3 = [80,30]
let p4 = [100,25]
let points = [p1,p2,p3,p4]

let testBezier = new Sketcher("XZ")
        .movePointerTo(p0)
        .bezierCurveTo(p4,points)
        .vLine(-30)
        .hLine(-100)
        .close()
testBezier = testBezier.extrude(30)

// offset this shape
// testBezier = makeOffset(testBezier,3)

house = house.fuse(testBezier)

return house
}

[sgenoud]

I am not sure how what happens here. The makeOffset function is a very small wrapper around opencascade's BRepOffsetAPI_MakeOffsetShape.

export const makeOffset = (
  face: Face,
  offset: number,
  tolerance = 1e-6
): Shape3D => {
  const oc = getOC();
  const offsetBuilder = new oc.BRepOffsetAPI_MakeOffsetShape_2(
    face.wrapped,
    offset,
    tolerance,
    oc.BRepOffset_Mode.BRepOffset_Skin as any,
    false,
    false,
    oc.GeomAbs_JoinType.GeomAbs_Arc as any,
    false
  );

  const newShape = cast(downcast(offsetBuilder.Shape()));
  offsetBuilder.delete();

  if (!isShape3D(newShape)) throw new Error("Could not offset to a 3d shape");
  return newShape;
};

You may want to play with the options that I do not expose.

The problem might be that:

All the faces of the shape S should be based on the surfaces with continuity at least C1.

Normally the Bezier should be C1 though...

I don't have much time this week, but I will be happy to modify the function if you find what is going on!

[raydeleu]

I tried the same shape in CascadeStudio and there it is possible to apply an offset.

I tried to copy the function Offset from CascadeStudio into my source code, but I quickly ran into the issue that this code requires other functions as well, leading me down the rabbitshole. Even after some experiments I was not able to get it running.

Then I tried to copy the function you supplied above and tinker with the values and the calls. But also there I stumbled upon things I simply do not understand. Pasting the function into the source code and renaming it did not work, probably because the code is written to be used from a library. I did not succeed to rewrite it as a simple function. Furthermore I tried to analyse the parameters in your code by reading the documentation at https://ocjs.org. At the page https://ocjs.org/reference-docs/classes/BRepOffsetAPI_MakeOffset_2 I found a different declaration of the function, requiring only 3 parameters instead of 8. The function BRepOffsetAPI_MakeOffsetShape_2 did not appear on https://ocjs.org (notice the difference between MakeOffset_2 and MakeOffsetShape_2). After these experiments I gave up, I have to learn a lot about programming in Javascript it seems.

[raydeleu]

I had a small success by replacing the bezierCurveTo by a cubicBezierCurveTo. It is really amazing that this works , especially considering the almost overlapping bulges at the end of this random shape.
With the quadraticBezierCurveTo it also works fine. Can we conclude that the error lies in the bezierCurveTo function?