Simulating a loom. UI vs creativity.

I bought another weaving design program last week at the Midwest Weavers' Conference. Both Tina and I use our"old" program regularly to design cloth. Why would I pay $100 for a new program when I have a perfectly good program that obviously works? The answer says something about the impact of user interface design on creativity.

Since I can't assume that everyone who reads this understands how a loom works, I have to digress. I'm going to describe the design issues faced by a weaver using a jack loom. There are many other types of looms that have their own design issues, but jack looms are very common among handweavers so it's a good place to start.

The purpose of a loom is to interleave two sets of threads that run at right angles to each other -- thereby creating cloth. (All you people with triangular looms, hush -- I'm trying to keep this simple.) One set of threads, the warp, is installed on the loom before the actual weaving begins in a process known as dressing the loom. The second set, the weft, is added to the cloth one thread at a time by running a shuttle containing a bobbin full of weft thread between the warp threads in a preplanned pattern. (and I'm not even going to *mention* how many details and variations I just omitted.)

When you ask a weaver to describe his or her loom, you can be sure that one of the first things they mention is how many shafts the loom has (unless, of course, they've been weaving for a long time in which case they'll tell you how many harnesses the loom has. I'm sure there's a really good reason for the terminology change -- other than to confuse the innocent.) That's because the number of harnesses (oops, I mean shafts) has a strong influence on the complexity of the cloth that can be produced. So what's a shaft?

Part of dressing the loom is threading. Each of several hundred threads in the warp goes through through the eye of a heddle (Imagine a large (12" long) needle with the eye in the middle rather than near one end.) The heddle is attached to a shaft, so that when you lift the shaft, all of the heddles attached to that shaft, and therefore all the threads in the eyes of those heddles are lifted. The the remaining warp threads -- the ones that are attached to shafts that do not get lifted -- remain down and a triangular space is opened up between the two sets of threads. This space, called a shed, is where the shuttle is thrown -- trailing its warp thread behind it.

Once the shuttle is through the shed, the shed is closed, the warp thread is pressed into place at the edge of the newly woven cloth using part of the loom called the beater, and a different shed is opened for the next warp thread. Thus each warp goes under the set of lifted warp threads, and over the remaining ones and cloth happens.

Since each warp thread is associated with a single shaft, the warp is divided into independantly controllable sets of threads. The number of shafts on the loom defines an upper limit on the number of sets of warp threads. More than one shaft can be lifted to produce any particular shed so the number of potential sheds (aka lifts) goes up dramatically as the number of shafts increases. In fact, a loom that has n shafts can produce 2**n - 2 meaningful lifts. (The -2 is there because it doesn't make sense to lift 0, or n shafts.) Some of the common cases are:
2 shafts -> 2 lifts
4 shafts -> 14 lifts
8 shafts -> 254 lifts.
16 shafts -> 65534 lifts.

Thus motivating a common malady among weavers: shaft envy [ no questionable jokes allowed here.] and it's converse: shaft pride [note 1].

There is another limitation, however, that shows up when I describe a previously unmentioned part of the loom -- the treadles. In order to lift the shafts, the weaver presses down a foot treadle. Each treadle is tied to one or more shafts so that the shafts are lifted as the treadle is pressed. The number of treadles imposes an additional upper bound on the number of lifts. For example, most 8 shaft looms have 10 treadles, so part of the design process is select which of the 254 possible lifts will be used during the weaving process. Of course it is possible to press more than one treadle at the same time (two feet can produce 100 possible lifts on a 10 treadle loom), and of course the tie up between treadles and shafts can be changed during the weaving process, but that's a slow and awkward proposition. Most handweavers using treadle-operated looms end up restricting the number of distinct lifts to the number of treadles.

...unless....

Unless the loom has a dobby instead of treadles and a tie-up. For computer history buffs, dobbies are the thing that Joseph Jacquard invented that led via Herman Hollerith to punched cards (which no one under 30 remembers, anyway.) A mechanical dobby uses holes punched in a wooden board, or more commonly nowdays pegs screwed into a wooden board to indicate which shafts should be lifted to form a shed. These cards are chained together so when the weaver is ready to move to the next weft thred, the chain is advanced to the card containing next lift pattern.[note 2]

A dobby provides two benefits. First the number of possible sheds is no longer limited by the number of treadles -- the weaver can design to the full capability of the loom, and second the weaver no longer has to remember the treadling sequence. No longer is the complexity of the pattern limited by the capacity of the weaver's memory, or the speed of weaving limited by the need to carefully follow a treadling sequence.

An electronic dobby takes this one step further. Rather than pegs in a wooden card to select a lift pattern, an electronic dobby uses solenoids to select the shafts to be lifted. These solenoids can be computer activated, so the chain of dobby cards can be relplaced with a lift plan stored in the computer. This removes yet another limitation in that the length of a woven pattern is no longer limited by the number of dobby cards in a chain. Instead it is limited only by the capacity of the computer and the ability of the weaver to design the pattern. Suddenly those 65 thousand possible lift patterns are accessable -- if only the weaver can figure out how to actually use them.

Which brings us back, finally, to the issues of user interface design for the computer assisted design programs used by weavers -- a topic for the next entry since this has gotten way to long.

[note 1] Tina and I have looms with 4, 8, 16, and 24 shafts. The 16 and 24 shaft looms are computer controlled.

[note 2] A dobby controlled jack loom described here is not the same as a modern Jaquard loom. A Jaquard loom provides individual control of each thread. It could be (but isn't) described as a loom with several hundred shafts. Jaquard looms typically cost 10 to 100 times as much as dobby controlled jack looms, and wouldn't fit in a handweaver's studio anyway.