In two related posts over the next week I want to look at ways of controlling how flat 2D objects appear to overlap in vector graphics.

Below is an example of a Celtic Knot design – notice how the various bands ‘thread’ through eachother. The circle motif in the centre passes infront of and behand each arm of the second triangular motif. Clearly, this would be impossible to achieve with 2 separate, flat, graphics as layer hierarchy only allows for an object to be either infront of or behind another object – there is no option to ‘thread’ back and forth.

In a second post I will look at achieving a similar effect with Live Paint but for now let’s see how we can use the Shape Builder Tool. I’ve included a quick step-by-step guide to create the graphic as you see it here but the important take-away is the principle of how the tools are used.

creating a celtic know with the shape builder in illustrator

Let’s start with the Ellipse Tool and instead of click-dragging out a shape simply click in the middle of the document – this brings up a dialgue box so we can enter numeric values. Set width and height to 70mm.

create a precise elipse in illustrator

Now repeat the process to draw a second circle. This time we need to position the cursor over the bottom node of the first circle and hold down Alt/Opt when we click – this will insert the circle with the cursor postion at its centre-point rather than the top-left. Set the width/height to 90mm.
I’ve set the fills to none so we can see the outline of both circles. The centre-point of the larger circle should be aligned to the bottom-centre of the smaller circle.

blog pic 260

The upper part of the larger circle will form one part of the triangular knot shape – I found it easier to draw circles and then use the intersecting arcs to create the final shape. We need two more circles evenly spaced around the central (smaller) circle.

With only the bottom circle selected choose the Rotate Tool and Alt/Opt+Click in the centre of the smaller circle – this sets the centre of rotation. Set the Angle to 120° and click copy.

set reference point for rotation in illustrator

You should have a second circle above and to the right of the first. Now hit Ctrl/Cmd+D or go to Object>Transform>Transform Again to repeat the last transform function – this duplicates our new circle using the same rotation function. You should now have all the necessary parts in place (as below).

duplicate shape with transform again in illustrator

Now change the Stroke Weight to 18pt to thicken up the lines. In order to use the Shape Builder Tool we need to work with paths rather than strokes – go to Object>Paths>Outline Stroke to convert our strokes to paths.

object path outline stroke to convert strokes to paths

Hit the “D” key to set the default White Fill/Black Stroke – now change the Stroke Weight to 6pt.

set default white fill and black stroke in illustrator

Now, with the whole shape selected, choose the Shape Builder Tool. When you mouse-over the various intersections you will see cross-hatching appear.

illustrator shape builder tool

Click on any section to add this as an individual shape.
Alt/Opt+click will delete a section.
Draw through two or more sections to join them together.

Start by deleting the outer arc of each of the larger circles then work through the shape joining together the various sections as needed. Pay attention to where each piece ‘threads’ through the other and whether it sits in front or behind.

overlapping shapes with the shape builder tool in illustrator

The Shape Builder Tool is ideal for creating this type of graphic – by overlapping basic forms and using Shape Builder to add, join or delete you should be able to create very intricate designs. If Celtic Knots are your thing you’ve now got everything you need – so get knotting!

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