Help
This tutorial will show you how to make a Celtic knot similar to the one below.
1. Start with a square image, at least 800x800. Drag out two guides from top and left of your canvas to the exact center. I.e. for an 800x800 canvas, put them at 400, 400.
2. Using your elliptical selection tool, place your cursor on the intersection of your guides and start to drag out a selection. While still holding your mouse button, press
Drag your selection out so it is about 3/4 of the width of your image. I.e. for an 800x800 canvas, do 600x600. This doesn't have to be precise. Do Select>Save to Channel.
3. Do Select>Shrink and shrink your selection by 25 pixels, or thereabouts. The difference between the size of the first selection and the this second selection will be how thick the lines of your triskit knot will be. Save this second selection to a channel. Do Select>None.
4. Go to your channels tab. In the Gimp default configuration, this is the second tab over from the channels tab. It looks like this:
You will have two selections, one called "Selection Mask copy", and "Selection Mask copy #1". Rename the first one "outer" and the second one "inner".
5. Turn on both selections and lock both selections by clicking the two boxes to the left of each one in the Channels tab. Your canvas will look like the second image.
6. The lighter colored inner circle is your "inner" channel mask. Pick your move tool from the toolbox and place your cursor at the top of this inner section and on the vertical guide. Click your mouse and drag downwards along the guide. Watch the numbers at the bottom of the canvas. You want to keep your "X" value at zero. You will only see an outline for the "inner" mask, but you are actually dragging both layers, since you locked them together. Drag downwards until you get something like the image below. The closer you get to the horizontal guide, the smaller the inside space of your triskit knot will be.
7. Unlock the channels and turn them off. Select the "outer" channel and right click on it. Click on "Channel to Selection."
8. Now right click on the "inner" channel and do "Subtract from Selection". You should get a ring shaped selection like below. Save it to a channel. Name the channel "bottom ring". Do Select>None.
9. Duplicate the channel you just made twice. Click the box to the left of each channel (the one with the little eye) so that each one is turned on.
10. Select the first duplicate. Now do Layer>Transform>Arbitrary Rotation. Rotate the channel by 120 degrees. Name the channel "ring 120" You will see this:
11. Now select the second duplicate. Rotate it by -120 (minus 120). Name it "ring -120".
12. Now we need to do some trimming. This is going to seem a bit complicated, since we are going to be using each of the channels we just created to modify the other two and cut off the excess part of our knot.
Turn off the "ring 120" channel and the "bottom ring" channel. Select the "bottom ring" channel, right click and do "Channel to Selection".
13. Do Select>Invert. Turn on Quickmask by clicking the small icon in the lower left corner of your canvas. You want your image to look like the one below. If it does, then turn off Quickmask.
14. With the selection active, click on the "ring 120" channel. With your brush tool and a fairly large brush, fill in the part of the ring as shown below with black. The selection will keep you from painting in the wrong place.
15. Now turn on your "ring -120" channel and do the same thing. I think you can see what we are doing, we are trimming each ring so it only has the part we want. You will need to go through this procedure with each channel to trim the other two. This is kind of hard to explain, just look at the following screenshots and you will see what is going on. We are using a selection made from one channel as a mask so that we can paint on another channel without painting the parts we want to keep. Each channel will be used to "trim" the excess off the curves in the other two channels. In the end, with all the channels turned on, you will get the final image in the following series.
16. Make sure you have only the "bottom ring" channel and the "ring 120" channel turned on. We are going to cut the weave into our channels. Select the "bottom ring" channel and do "Channel to Selection". Do Select>Grow and grow the selection by 10 pixels (you may want to adjust this number to your own preferences).
17. Turn on Quickmask. See how the selection crosses the "ring 120" channel in a couple of places? Once in the middle and once at the tip. What we want to do is to paint over where the selection cuts the "ring 120" channel at the tip. The following pictures illustrate this and what sort of a selection we get when we turn off Quickmask.
18. Now we have a selection that will cut the "ring 120" channel where we want. With the "ring 120 channel selected, bucket fill the selection with black. Now we have made it so that the "bottom ring" selection goes "over" the "ring 120" selection.
19. Use the same method, only this time use "ring 120" to cut "ring -120". Then we will use "ring -120" to cut "bottom ring".
Clear as mud?
If you didn't get lost and did everything right, your final result will look like this:
20. Now we will combine all three "ring channels" into a final finished channel that will have the entire knot as a single selection. Starting with the "bottom ring" channel, do "Channel to Selection".
21. Work your way up the channel stack and do "Add to Selection." Turn off the eyes on your channels. You should have a selection that looks like the one below. Save it to a channel and name it "knot".
22. Go back to your Layers tab. Create a new transparent layer and bucket fill your selection with black. Or fill it with plasma. Or use the selection to cut out something else. Or use it as a bump map source. You have all sorts of choices from this point on. You can also use elliptical rather than round selections. You could also offset the inner circle from the outer so they don't share the same center. This will make part of your knot thicker and part thinner. I am also thinking of trying to do some more angular shapes by converting the selection to a path and playing with the nodes. Also, if you are planning to use this to make a bump map source, I would suggest increasing the size of your cuts in the section about weaving things together. I.e. if you are planning to blur the bump map source, increase the width from 10 to probably at least 15 pixels, maybe more.
Finally, here is a link to the .xcf file (zipped) that was created when I made this tutorial. I am including it as an example...I would hope you would make the effort and learn how to make yer own.
http://www.webfileho...load&id=2646599
