SceneKit – Drawing on 3D Object

Question

I created sample application to draw on 3d objects, the output drawing is not smooth(as in figure). I spent nearly a day to figure out what is the issue still not able to solve. What might be the issue?

I am creating custom drawing geometry like below

//I am creating self.drawingNode touches begin and adding to the rootnode
//points are nothing but 2d points from touches

func createGeometryForPoints(points:[CGPoint]) -> SCNGeometry {

    var all_3d_points:[SCNVector3] = []

    for point in points {

        let result = self.get3dPoint(point)

        if result.1 == true {
            all_3d_points.append(result.0)
        } else {
            print("INVALID POINT")
        }
    }

    var indices: [Int32] = []
    var index:Int32 = 0

    var previousIndex:Int32 = -1

    for _ in all_3d_points {

        if(previousIndex != -1) {

            indices.append(previousIndex)
        }
        indices.append(index)
        index = index + 1
        previousIndex = index
    }

    if indices.count > 0 {
        indices.removeLast();
    }

    let indexData = NSData(bytes: indices, length: sizeof(Int32) * indices.count)

    let source = SCNGeometrySource.init(vertices: all_3d_points, count: all_3d_points.count)
    let element = SCNGeometryElement.init(data: indexData, primitiveType: .Line, primitiveCount: indices.count/2, bytesPerIndex: sizeof(Int32))

    let line = SCNGeometry(sources: [source], elements: [element])
    line.materials.first?.diffuse.contents = UIColor.redColor()
    line.materials.first?.doubleSided = true

    return line
}

func get3dPoint(point:CGPoint) -> (SCNVector3,Bool) {

    var canAdd = false
    let scnView = self.view as! SCNView
    var unprojectedStartPoint = SCNVector3.init(x:Float(point.x), y:Float(point.y), z:0)

    let hitResults = scnView.hitTest(point, options: nil)
    if hitResults.count > 0 {

        let result: AnyObject! = hitResults[0]
        if hitResults.count > 1 {
            print("SOME PROBLEM")
        }
        unprojectedStartPoint = result.localCoordinates
        unprojectedStartPoint = result.node.convertPosition(unprojectedStartPoint, toNode: self.drawingNode!)

        canAdd = true
    }

    return (unprojectedStartPoint,canAdd)
}

EDIT

I am thinking of finding the offset along with the camera direction or normal, since I am new to scenekit i am not getting how to add this offset to drawing node, looking of help here in this direction

func createMarkup() -> Void {   
    let drawingNode = SCNNode()
    drawingNode.renderingOrder = 1000
    self.parentNode!.addChildNode(drawingNode)
    let geometry = self.createGeometryForPoints(allPoints,node: drawingNode)
    drawingNode.geometry = geometry
    drawingNode.geometry?.materials.first?.doubleSided = true
}

EDIT: 2

Solved the issue by adding minor offset along the normal of the hit-test

let offsetToAvoidFlickering: Float = 0.05
unprojectedStartPoint = unprojectedStartPoint + result.localNormal.normalized() * offsetToAvoidFlickering

Answer 1

I suspect you have depth precision issues; it looks like parts of your line are being clipped because they are intersecting parts of the sphere. Try drawing the line with depth testing disabled.

Answer 2

the SceneKit State of the Union Demo sample code shows a similar effect, where you can paint on a torus. It's more powerful because you can draw anything (not just line, but splashes from texture etc.) and much more efficient because it doesn't rely on creating new geometry.