The first tutorial (and a half) was pretty fun, and appears to have been pretty popular, so since I’m on a roll I thought I’d have fun with another tutorial on making Celtic knots in Inkscape. This time we’re gonna make a triquetra, specifically the Trinity knot variation.
That looks pretty close to the example image on Wikipedia, but it’s actually the finished product of this tutorial (at 200% zoom). The goal this time around will be that cool outlined look.
Step 1: Setup the document. As before, 100 by 100 px, rectangular grid, grid lines every 1 px in both directions and major grid lines every 5. The first shape we’ll draw is a circle, so choose the circle tool and drag from 15,15 to 85,85. At 400% zoom each grid line you see should represent 5 pixels, so this is 3 squares in from the corner. Another way to draw this is to start at the center and hold Ctrl+Shift while dragging out towards 15,15. Open up Fill and Stroke and choose no fill, black stroke paint, 5 px stroke width. Also while we’re at it, the join type should be the Miter join (the one that produces pointed corners); we’ll need it later.
Step 2: Right below the circle tool is the tool for creating stars and polygons. Click that, and then take a look at the toolbar at the top. Make sure the polygon option is chosen, not the star option. Change the number of corners to 3. Now click on the center of the page, and drag upward to 50,100—the top center. It’s a perfect triangle. But wait, we’re not done! Again starting in the center, drag down to 50,10, to create a smaller triangle going the other direction. I’m calling this our “guide” triangle. Change the stroke paint to lower the alpha value, so it’s translucent. (Why is it this size? Trial and error proved this was a good size.)
Step 3: Select all three objects, and in the Path menu choose Object to Path. This will change them from simple built-in shape types to paths with nodes we can edit. Select the big triangle, and switch to the node tool. Hit Ctrl+A to select all three nodes, and then look for an icon on the node editing toolbar that says “Make selected segments curves”. You will now see the curve handles for each segment, which are shaped like circles. Dragging these around will alter the curve. Try it a bit, then when you’re all set hit Undo to get back to where the node handles first appeared.
Step 4: At the top of the screen, the very top toolbar should have an icon near the right side that says “Snap to grids”; it looks like a grid, and should be selected already. Click it to deselect. If “Snap to guides” next to it is selected too, deselect that as well. To the left, there’s another section of icons, and the one at the far left of that section says “Snap nodes or handles”. Make sure it’s selected. Then also select another icon in that group: “Snap to cusp nodes”. Now that that’s set, grab the curve handles for one side of the big triangle, and drag them to the two corners of the guide triangle that they’re “facing”.
Step 5: Repeat this for the other two sides. It should now look like the Trinity knot, without so much the knot part yet. Change the grid snapping back to the way it was, and delete the guide triangle. Oh, and these shapes are going to be our templates now, so use the Select tool and hit Ctrl+A, then hit Shift+Down 5 times to drag them off the page.
Step 6: We’re going to start stenciling and cutting. The first piece I want to make is the top part of the knot, and the interior crossing will come along as a bonus. So first, select just the triquetra part, and hit Ctrl+D to duplicate. Change the stroke color to green so we can see it. This will be the subsection. Duplicate it again and change the color to dark red, and change the stroke width to 7.5 px; this will be the stencil. And now select the circle and duplicate that; change the color to orange and the stroke width again to 7.5 px.
Step 6b: Starting from the top corner and going clockwise, the first crossing goes under the circle. So we want to keep the northeast quadrant of the circle stencil, but ditch the northwest one. Select the northwest line segment on the circle stencil and choose “Delete line segment between two non-endpoint nodes”.
Step 6c: The red triquetra stencil needs to be cut next; all we really need is the curve that goes between the lower two corners of the triangle. Select any two other points on this stencil and snip away their line segment. Then delete the node at the top corner.
Step 6d: Finally, the subsection needs some trimming too. The line segment that the stencil covers isn’t need at all, so select the lower two nodes (just drag a selection rectangle across both) and snip it away. The one that goes from the top to the southwest corner is only partially needed, so select that line segment and choose “Insert new nodes into selected line segments” to subdivide it. Delete the southwest node.
Step 7: As in the last tutorial, go to the Select tool and use clicking and Shift+click to select all three of these shapes: both stencils, and the subsection. Choose Stroke to Path in the Path menu. Now select both stencils, go back to the Path menu, and pick Union to combine them into one stencil. Shift+click to also select the subsection now, and choose Difference. You should now have the two pieces we want—the top part of the knot and the interior crossing—along with three other short pieces that we don’t. Open up the node tool and select the nodes around the pieces we don’t want, and delete them.
Step 8: In the Select tool, select the subsection and hit Shift+Up 5 times to move it back onto the page. Click it again to make the rotation handles appear, and drag the center of rotation to the center of the page. Let’s change its fill color back to black. Now, hit Ctrl+D to duplicate it. Go to the Object menu and pick Transform. This opens up the transformation panel, where you can go to the Rotate tab and enter 120 degrees. Click Apply. You could duplicate and do this again, but since I’m a stickler for accurate calculations I prefer to duplicate the first shape again and rotate the new duplicate by -120°. Either way works. When all three pieces have been created, select them all and choose Combine in the Path menu. Now we should have everything but the circle.
Step 9: Time to cut the circle. Duplicate its template to make the new subsection, and make this green as usual. Now duplicate the triquetra template, and make the duplicate dark red with a 7.5 px stroke width, since it’s our new stencil. We want only the counter-clockwise half of each corner of the knot. In the node tool select the stencil and hit Ctrl+A to select all three nodes. Subdivide using “Insert new nodes into selected segments”. Click the line segment on the right side of the top corner, and snip that line segment away. Do the same for the other two corners.
Step 9b: No surprises here. Select the stencil and the subsection, choose Stroke to Path, then Difference. Go to the Select tool and hit Shift+Up 5 times to move the cut circle into place, and change its fill color to black. Select the other piece as well and Combine them both so you have just a single path. But don’t delete the templates yet, because I’m not stopping here.
Step 10: At this point we have the classic Trinity knot. But I want this to only be outlines, nothing else. Select both pieces of the template and Combine them. Change the stroke width to 7.5 px, and then choose Stroke to Path. Also in the Path menu, choose Reverse. Now select it again and Shift+Up 5 times to move it into place. Finally, hit Ctrl+A to select all, and then Combine. Because this path’s nodes run the opposite direction from the other one, the knot we made before will become enclosed whitespace.
Step 11: There’s nothing left to do now but open up Align and Distribute, and vertically center the knot. All finished!
The width of the outline is 1.25 px, thanks to the stroke widths that were chosen; this is the same as the knot gaps in the first tutorial. I think the ratios are pretty aesthetically pleasing, but do feel free to experiment.
If you wanted to keep a fill color but also have the outlines, then step 10 is a bit different. First you’d want to choose your fill for the knot produced in the earlier steps. Then you’d follow step 10 right up until the Reverse. Instead of using Reverse and Combine, you’d simply hit Page Down; this would move the outline down to a lower layer, so the filled section showed on top.