joint.all.js
function(end, selectorOrPoint, referenceSelectorOrPoint) {

        var spot;

        if (this._isPoint(selectorOrPoint)) {

            // If the source is a point, we don't need a reference point to find the sticky point of connection.
            spot = g.point(selectorOrPoint);

        } else {

            // If the source is an element, we need to find a point on the element boundary that is closest
            // to the reference point (or reference element).
            // Get the bounding box of the spot relative to the paper viewport. This is necessary
            // in order to follow paper viewport transformations (scale/rotate).
            // `_sourceBbox` (`_targetBbox`) comes from `_sourceBboxUpdate` (`_sourceBboxUpdate`)
            // method, it exists since first render and are automatically updated
            var spotBbox = end === 'source' ? this._sourceBbox : this._targetBbox;
            
            var reference;
            
            if (this._isPoint(referenceSelectorOrPoint)) {

                // Reference was passed as a point, therefore, we're ready to find the sticky point of connection on the source element.
                reference = g.point(referenceSelectorOrPoint);

            } else {

                // Reference was passed as an element, therefore we need to find a point on the reference
                // element boundary closest to the source element.
                // Get the bounding box of the spot relative to the paper viewport. This is necessary
                // in order to follow paper viewport transformations (scale/rotate).
                var referenceBbox = end === 'source' ? this._targetBbox : this._sourceBbox;

                reference = g.rect(referenceBbox).intersectionWithLineFromCenterToPoint(g.rect(spotBbox).center());
                reference = reference || g.rect(referenceBbox).center();
            }

            // If `perpendicularLinks` flag is set on the paper and there are vertices
            // on the link, then try to find a connection point that makes the link perpendicular
            // even though the link won't point to the center of the targeted object.
            if (this.paper.options.perpendicularLinks) {

                var horizontalLineRect = g.rect(0, reference.y, this.paper.options.width, 1);
                var verticalLineRect = g.rect(reference.x, 0, 1, this.paper.options.height);
                var nearestSide;

                if (horizontalLineRect.intersect(g.rect(spotBbox))) {

                    nearestSide = g.rect(spotBbox).sideNearestToPoint(reference);
                    switch (nearestSide) {
                      case 'left':
                        spot = g.point(spotBbox.x, reference.y);
                        break;
                      case 'right':
                        spot = g.point(spotBbox.x + spotBbox.width, reference.y);
                        break;
                    default:
                        spot = g.rect(spotBbox).center();
                        break;
                    }
                    
                } else if (verticalLineRect.intersect(g.rect(spotBbox))) {

                    nearestSide = g.rect(spotBbox).sideNearestToPoint(reference);
                    switch (nearestSide) {
                      case 'top':
                        spot = g.point(reference.x, spotBbox.y);
                        break;
                      case 'bottom':
                        spot = g.point(reference.x, spotBbox.y + spotBbox.height);
                        break;
                    default:
                        spot = g.rect(spotBbox).center();
                        break;
                    }
                    
                } else {

                    // If there is no intersection horizontally or vertically with the object bounding box,
                    // then we fall back to the regular situation finding straight line (not perpendicular)
                    // between the object and the reference point.

                    spot = g.rect(spotBbox).intersectionWithLineFromCenterToPoint(reference);
                    spot = spot || g.rect(spotBbox).center();
                }
                
            } else {
            
                spot = g.rect(spotBbox).intersectionWithLineFromCenterToPoint(reference);
                spot = spot || g.rect(spotBbox).center();
            }
        }

        return spot;
    }
function(end, selectorOrPoint, referenceSelectorOrPoint) {

        var spot;

        if (this._isPoint(selectorOrPoint)) {

            // If the source is a point, we don't need a reference point to find the sticky point of connection.
            spot = g.point(selectorOrPoint);

        } else {

            // If the source is an element, we need to find a point on the element boundary that is closest
            // to the reference point (or reference element).
            // Get the bounding box of the spot relative to the paper viewport. This is necessary
            // in order to follow paper viewport transformations (scale/rotate).
            // `_sourceBbox` (`_targetBbox`) comes from `_sourceBboxUpdate` (`_sourceBboxUpdate`)
            // method, it exists since first render and are automatically updated
            var spotBbox = end === 'source' ? this._sourceBbox : this._targetBbox;
            
            var reference;
            
            if (this._isPoint(referenceSelectorOrPoint)) {

                // Reference was passed as a point, therefore, we're ready to find the sticky point of connection on the source element.
                reference = g.point(referenceSelectorOrPoint);

            } else {

                // Reference was passed as an element, therefore we need to find a point on the reference
                // element boundary closest to the source element.
                // Get the bounding box of the spot relative to the paper viewport. This is necessary
                // in order to follow paper viewport transformations (scale/rotate).
                var referenceBbox = end === 'source' ? this._targetBbox : this._sourceBbox;

                reference = g.rect(referenceBbox).intersectionWithLineFromCenterToPoint(g.rect(spotBbox).center());
                reference = reference || g.rect(referenceBbox).center();
            }

            // If `perpendicularLinks` flag is set on the paper and there are vertices
            // on the link, then try to find a connection point that makes the link perpendicular
            // even though the link won't point to the center of the targeted object.
            if (this.paper.options.perpendicularLinks) {

                var horizontalLineRect = g.rect(0, reference.y, this.paper.options.width, 1);
                var verticalLineRect = g.rect(reference.x, 0, 1, this.paper.options.height);
                var nearestSide;

                if (horizontalLineRect.intersect(g.rect(spotBbox))) {

                    nearestSide = g.rect(spotBbox).sideNearestToPoint(reference);
                    switch (nearestSide) {
                      case 'left':
                        spot = g.point(spotBbox.x, reference.y);
                        break;
                      case 'right':
                        spot = g.point(spotBbox.x + spotBbox.width, reference.y);
                        break;
                    default:
                        spot = g.rect(spotBbox).center();
                        break;
                    }
                    
                } else if (verticalLineRect.intersect(g.rect(spotBbox))) {

                    nearestSide = g.rect(spotBbox).sideNearestToPoint(reference);
                    switch (nearestSide) {
                      case 'top':
                        spot = g.point(reference.x, spotBbox.y);
                        break;
                      case 'bottom':
                        spot = g.point(reference.x, spotBbox.y + spotBbox.height);
                        break;
                    default:
                        spot = g.rect(spotBbox).center();
                        break;
                    }
                    
                } else {

                    // If there is no intersection horizontally or vertically with the object bounding box,
                    // then we fall back to the regular situation finding straight line (not perpendicular)
                    // between the object and the reference point.

                    spot = g.rect(spotBbox).intersectionWithLineFromCenterToPoint(reference);
                    spot = spot || g.rect(spotBbox).center();
                }
                
            } else {
            
                spot = g.rect(spotBbox).intersectionWithLineFromCenterToPoint(reference);
                spot = spot || g.rect(spotBbox).center();
            }
        }

        return spot;
    }
Find a point that is the start of the connection. If `selectorOrPoint` is a point, then we're done and that point is the start of the connection. If the `selectorOrPoint` is an element however, we need to know a reference point (or element) that the link leads to in order to determine the start of the connection on the original element.