Ios swift - iOS - Get CGRect around CGPoints

iOS
June 21, 2023
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2 Answers

I have array of CGPoint indicated in red. I must connect all consecutive points with CGRect as shown in the rough figure to form an array of CGRect. Consider all rect has a constant width and each point could be the mid point of the sides formed from width of the rect. How could I form this array of CGRect?

EDIT:

Here is what I am trying to do. As described in my previous question, I am trying to introduce an eraser function.

I have a set of straight lines (in green). I basically contain the start and end points of the line in model. When in eraser mode, user is allowed to draw freely and all draw points are connected by stroke (in purple).

When any green line is completely covered, the line must be identified to be erased. But, I couldn’t able to determine if the line is completed covered by the eraser’s draw points.

As in comments, I tried to follow the answer here. All I have is CGPoints and I have no CGRects. I check the intersection of both green and purple path as below and it returns false.

public override func draw(_ rect: CGRect) {
    
    let wallPath = UIBezierPath()
    wallPath.move(to: CGPoint(x: 50.0, y: 50.0))
    wallPath.addLine(to: CGPoint(x: 50.0, y: 400.0))
    wallPath.addLine(to: CGPoint(x: 300.0, y: 400.0))
    wallPath.addLine(to: CGPoint(x: 300.0, y: 50.0))
        
    let wallLayer = CAShapeLayer()
    wallLayer.path = wallPath.cgPath
    wallLayer.lineWidth = 10
    wallLayer.strokeColor = UIColor.green.cgColor
    wallLayer.fillColor = nil
    layer.addSublayer(wallLayer)
    
    let eraserPath = UIBezierPath()
    eraserPath.move(to: CGPoint(x: 40.0, y: 75.0))
    eraserPath.addLine(to: CGPoint(x: 120.0, y: 75.0))
    
    let eraserLayer = CAShapeLayer()
    eraserLayer.path = eraserPath.cgPath
    eraserLayer.lineWidth = 15
    eraserLayer.strokeColor = UIColor.purple.cgColor
    layer.addSublayer(eraserLayer)
    
    if wallPath.cgPath.intersects(eraserPath.cgPath) {
        print("Overlapping")
    } else {
        print("Not overlapping")
    }
}

To make it more clear about the requirement, when user draws in eraser mode, based on the draw points, I have to identify which green line falls completely in the purple stroke’s coverage and the identified green line must be taken for further processing. Multiple green lines could be selected at the same time.

Answers

So you have two CGPaths, one for the eraser and one for a line. You stroke those paths with a certain line width, which creates two areas. You want to check if one area completely covers another.

Let’s first not worry about "completely" – just check if the two areas intersect at any point at all. You can do this by creating new CGPaths that represent the area created when stroking with a certain line width. There is an API this – copy(strokingWithWidth:lineCap:lineJoin:miterLimit:transform:).

Then you can just use intersect on those paths.

Here is an idea of how that would look:

func eraserIntersectsLine(p1: CGPoint, p2: CGPoint, lineWidth: CGFloat, eraserPath: CGPath, eraserWidth: CGFloat) -> Bool {
    let eraserArea = eraserPath.copy(
        // you should change the line cap and line join parameters to match how your eraser works
        strokingWithWidth: eraserWidth, lineCap: .butt, lineJoin: .miter, miterLimit: 10
    )
    let line = UIBezierPath()
    line.move(to: p1)
    line.addLine(to: p2)
    let lineArea = line.cgPath.copy(
        // similarly, change these parameters to match how your "pencil" tool works
        strokingWithWidth: lineWidth, lineCap: .butt, lineJoin: .miter, miterLimit: 10
    )
    return lineArea.intersects(eraserArea)
}

Determining if these areas completely intersect is not trivial at all. And I think it would be a better UX if you allow a line to be erased just by intersection, and not completely covering.

If you really want though, here is an approximation you could do. Divide the line into small "pieces", and for each of those "piece", check if the area of that intersects with the eraser area.

Here is an idea of how that would look:

func eraserCompletelyCoversLine(p1: CGPoint, p2: CGPoint, lineWidth: CGFloat, eraserPath: CGPath, eraserWidth: CGFloat) -> Bool {
    let angle = atan2(p2.y - p1.y, p2.x - p1.x)
    let distance = hypot(p2.x - p1.x, p2.y - p1.y)
    // split the whole line into pieces of 0.1 (some arbitrarily chosen small length)
    for start in stride(from: 0, to: distance, by: 0.1) {
        // check if each "piece" intersects with the eraser path
        let startX = p1.x + (start * cos(angle))
        let startY = p1.y + (start * sin(angle))
        let endX = p1.x + ((start + 0.1) * cos(angle))
        let endY = p1.y + ((start + 0.1) * sin(angle))
        guard eraserIntersectsLine(
            p1: .init(x: startX, y: startY),
            p2: .init(x: endX, y: endY),
            lineWidth: lineWidth,
            eraserPath: eraserPath,
            eraserWidth: eraserWidth
        ) else { return false }
    }
    return true
}

Note that I’m calling eraserIntersectsLine every time here for convenience, which creates the same path for the eraser area every time. It should be trivial to optimise this so that it only creates one copy of the eraser area if you need to.

For a more accurate approximation, you could consider the width of the line too. Split the line width into small pieces too.

CGPath has an .lineIntersection(_:using:) method that can come in really handy here…

Let’s start with a single line path:

We create an "eraser" path – using strokingWithWidth instead of .lineWidth so we get a "filled" outline path – and cross the path:

When we call:

let iPth = segPth.lineIntersection(eraserPth)

That returns a new path, consisting of .move(to: pt1) and .addLine(to: pt2) – if we draw that iPth in cyan we see:

We can then easily compare the resulting path to the original path and determine that the original is not completely encompassed.

If we continue defining our eraser path:

and call segPth.lineIntersection(eraserPth) again, it will return a path of move to + line to + move to + line to:

and again, we can easily determine that it is not the same path as the original line segment.

If we keep adding points to the eraser path until we get this:

segPth.lineIntersection(eraserPth) will now return a path:

That matches the original.

So… a couple notes first…

Define your "walls" path as a series of "line segments" rather than a single path:

struct LineSegment: Equatable {
    var pt1: CGPoint = .zero
    var pt2: CGPoint = .zero
}

That makes it easy to:

loop through the segments
compare them to the returned path from .lineIntersection()
remove individual wall-segments
and… create non-contiguous walls

Next, because points use floating-point values, and UIKit likes whole numbers, we can save ourselves some headaches by rounding everything.

For example, if we have a line from 20, 10 to 80, 10, and we have an eraser path that encompasses the entire line path, we might get a returned path from .lineIntersection() with points like 20.00001, 10.0 to 79.99997, 10.000001.

Here is a complete example to play with…

The LineSegment struct:

struct LineSegment: Equatable {
    var pt1: CGPoint = .zero
    var pt2: CGPoint = .zero
}

extension to round the x,y values of a point:

extension CGPoint {
    var rounded: CGPoint { .init(x: self.x.rounded(), y: self.y.rounded()) }
}

This extension to return the points of a path (found here: How to get the CGPoint(s) of a CGPath):

extension CGPath {
    
    /// this is a computed property, it will hold the points we want to extract
    var points: [CGPoint] {
        
        /// this is a local transient container where we will store our CGPoints
        var arrPoints: [CGPoint] = []
        
        // applyWithBlock lets us examine each element of the CGPath, and decide what to do
        self.applyWithBlock { element in
            
            switch element.pointee.type
            {
            case .moveToPoint, .addLineToPoint:
                arrPoints.append(element.pointee.points.pointee)
                
            case .addQuadCurveToPoint:
                arrPoints.append(element.pointee.points.pointee)
                arrPoints.append(element.pointee.points.advanced(by: 1).pointee)
                
            case .addCurveToPoint:
                arrPoints.append(element.pointee.points.pointee)
                arrPoints.append(element.pointee.points.advanced(by: 1).pointee)
                arrPoints.append(element.pointee.points.advanced(by: 2).pointee)
                
            default:
                break
            }
        }
        
        // We are now done collecting our CGPoints and so we can return the result
        return arrPoints
        
    }
}

A UIView subclass, with shape layers, path logic, and touch handling:

class StrokedView: UIView {
    
    let sampleWallSegments: [LineSegment] = [
        // an "outline" box
        .init(pt1: .init(x: 40.0, y: 40.0), pt2: .init(x: 260.0, y: 40.0)),
        .init(pt1: .init(x: 260.0, y: 40.0), pt2: .init(x: 260.0, y: 120.0)),
        .init(pt1: .init(x: 260.0, y: 120.0), pt2: .init(x: 120.0, y: 120.0)),
        .init(pt1: .init(x: 120.0, y: 120.0), pt2: .init(x: 120.0, y: 80.0)),
        .init(pt1: .init(x: 120.0, y: 80.0), pt2: .init(x: 60.0, y: 80.0)),
        .init(pt1: .init(x: 60.0, y: 80.0), pt2: .init(x: 60.0, y: 120.0)),
        .init(pt1: .init(x: 60.0, y: 120.0), pt2: .init(x: 40.0, y: 120.0)),
        .init(pt1: .init(x: 40.0, y: 120.0), pt2: .init(x: 40.0, y: 40.0)),
        
        // couple criss-crossed lines
        .init(pt1: .init(x: 180.0, y: 50.0), pt2: .init(x: 220.0, y: 70.0)),
        .init(pt1: .init(x: 220.0, y: 50.0), pt2: .init(x: 180.0, y: 70.0)),
        
        // some short vertical lines
        .init(pt1: .init(x: 150.0, y: 90.0), pt2: .init(x: 150.0, y: 110.0)),
        .init(pt1: .init(x: 180.0, y: 90.0), pt2: .init(x: 180.0, y: 100.0)),
        .init(pt1: .init(x: 210.0, y: 90.0), pt2: .init(x: 210.0, y: 100.0)),
        .init(pt1: .init(x: 240.0, y: 90.0), pt2: .init(x: 240.0, y: 110.0)),
    ]

    // this holds the "wall" line segments
    //  will initially be set to asmpleWallSegments
    //  segments may be removed
    var wallSegments: [LineSegment] = []

    // holds the points as the user touches/drags
    var eraserPoints: [CGPoint] = []
    
    // will hold the indexes of the wallSegments that are completely
    //  encompassed by the eraser line
    var encompassedSegments: [Int] = []
    
    let wallLayer = CAShapeLayer()
    let eraserLayer = CAShapeLayer()
    let highlightLayer = CAShapeLayer()

    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    required init?(coder aDecoder: NSCoder) {
        super.init(coder:aDecoder)
        commonInit()
    }
    
    private func commonInit() {
        
        // this layer will hold our line segments path
        wallLayer.fillColor = UIColor.clear.cgColor
        wallLayer.strokeColor = UIColor.systemGreen.cgColor
        wallLayer.lineWidth = 1.0

        // instead of using a path with a line-width of 10,
        //  eraser layer will be filled with no line/stroke,
        //  because we will set its .path to the "stroked" path
        eraserLayer.fillColor = UIColor.systemBlue.withAlphaComponent(0.25).cgColor
        eraserLayer.lineJoin = .round
        eraserLayer.lineCap = .round

        // this layer will "highlight" the fully encompassed segments
        highlightLayer.fillColor = UIColor.clear.cgColor
        highlightLayer.strokeColor = UIColor.red.withAlphaComponent(0.9).cgColor
        highlightLayer.lineWidth = 2.0
        
        [wallLayer, eraserLayer, highlightLayer].forEach { lay in
            layer.addSublayer(lay)
        }

        reset()
    }
    
    func reset() {
        eraserPoints = []
        wallSegments = sampleWallSegments
        setNeedsLayout()
    }
    func removeSegments() {
        encompassedSegments.reversed().forEach { i in
            wallSegments.remove(at: i)
        }
        eraserPoints = []
        setNeedsLayout()
    }
    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        guard let t = touches.first else { return }
        let pt = t.location(in: self).rounded
        
        // append a new point
        eraserPoints.append(pt)
        
        setNeedsLayout()
    }
    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
        guard let t = touches.first else { return }
        let pt = t.location(in: self).rounded
        
        // always append a new point, or
        eraserPoints.append(pt)
        
        // if we want to "rubber-band" the highlight, use this instead
        //points[points.count - 1] = pt
        
        setNeedsLayout()
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        if #available(iOS 16.0, *) {
            // clear the layer paths
            [wallLayer, eraserLayer, highlightLayer].forEach { lay in
                lay.path = nil
            }
            // clear the encompassed segments array
            encompassedSegments = []
            
            // create the "walls" path
            let wallPth = CGMutablePath()
            
            wallSegments.forEach { seg in
                wallPth.move(to: seg.pt1)
                wallPth.addLine(to: seg.pt2)
            }
            
            // set "walls" layer path
            wallLayer.path = wallPth
            
            // return if we have no eraser points yet
            guard eraserPoints.count > 0 else { return }

            // create eraser path
            let eraserPth = CGMutablePath()
            
            // create highlight path (will "highlight" the encompassed segments in red)
            let highlightPath = CGMutablePath()

            // add lines to the eraser path
            eraserPth.move(to: eraserPoints[0])
            if eraserPoints.count == 1 {
                eraserPth.addLine(to: .init(x: eraserPoints[0].x + 1.0, y: eraserPoints[0].y))
            }
            for i in 1..<eraserPoints.count {
                eraserPth.addLine(to: eraserPoints[i])
            }
            
            // get a "stroked" path from the eraser path
            let strokedPth = eraserPth.copy(strokingWithWidth: 10.0, lineCap: .round, lineJoin: .round, miterLimit: 1.0)

            // normalize it
            let normedPth = strokedPth.normalized()

            // set eraser layer path
            eraserLayer.path = normedPth
            
            // for each wall segment
            for (i, thisSeg) in wallSegments.enumerated() {
                // create a new two-point path for the segment
                let segPth = CGMutablePath()
                segPth.move(to: thisSeg.pt1)
                segPth.addLine(to: thisSeg.pt2)
                // get the intersection with the normalized path
                let iPth = segPth.lineIntersection(normedPth)
                // get the points from that intersecting path
                let iPoints = iPth.points
                // if we have Zero or any number of points other than Two,
                //  the segment will not be completely encompassed
                if iPoints.count == 2 {
                    // create a LineSegment with rounded points from the intersecting path
                    let thatSeg = LineSegment(pt1: iPoints[0].rounded, pt2: iPoints[1].rounded)
                    // if that segment is equal to this segment, add a line to the hightlight path
                    //  and append the index of this segment to encompassed array (so we can remove them on demand)
                    if thatSeg == thisSeg {
                        highlightPath.move(to: thisSeg.pt1)
                        highlightPath.addLine(to: thisSeg.pt2)
                        encompassedSegments.append(i)
                    }
                }
                // set the highlight layer path
                highlightLayer.path = highlightPath
            }
        }
        
    }
    
}

and a simple view controller to show it in use:

class StrokedVC: UIViewController {

    let testView = StrokedView()

    override func viewDidLoad() {
        super.viewDidLoad()
        
        var cfg = UIButton.Configuration.filled()

        cfg.title = "Remove Highlighted"
        let btnA = UIButton(configuration: cfg, primaryAction: UIAction() { _ in
            self.testView.removeSegments()
        })
        
        cfg = UIButton.Configuration.filled()
        
        cfg.title = "Reset"
        let btnB = UIButton(configuration: cfg, primaryAction: UIAction() { _ in
            self.testView.reset()
        })

        [btnA, btnB, testView].forEach { v in
            v.translatesAutoresizingMaskIntoConstraints = false
            view.addSubview(v)
        }

        let g = view.safeAreaLayoutGuide
        NSLayoutConstraint.activate([

            btnA.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
            btnA.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 40.0),
            btnA.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -40.0),

            testView.topAnchor.constraint(equalTo: btnA.bottomAnchor, constant: 20.0),
            testView.widthAnchor.constraint(equalToConstant: 300.0),
            testView.heightAnchor.constraint(equalToConstant: 160.0),
            testView.centerXAnchor.constraint(equalTo: g.centerXAnchor),

            btnB.topAnchor.constraint(equalTo: testView.bottomAnchor, constant: 20.0),
            btnB.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 40.0),
            btnB.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -40.0),
            
        ])
        
        testView.backgroundColor = UIColor(white: 0.95, alpha: 1.0)
    }
}

Here’s what it looks like when running:

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Ios swift – iOS – Get CGRect around CGPoints

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