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HP 7475A Plotter: Chiplotle Supershape

In this day and age, a pen plotter isn’t going to be doing anything useful, because we have better ways to draw schematics and make presentation graphics, but it can produce Algorithmic Art:

HP 7475A - Chiplotle Supershape plotting

HP 7475A – Chiplotle Supershape plotting

Well, granted, that’s a rather small value of Art, but it does show that the plotter can draw 10 k points using serial port hardware handshaking.

That’s one of an infinite variety of Supershapes produced by the Chiplotle geometry.shapes.supershape() function:

from chiplotle import *
import math
plt=instantiate_plotters()[0]
plt.set_origin_center()
plt.write(hpgl.VS(5))
ss=geometry.shapes.supershape(3900,3900,5.3,0.4,1,1,point_count=10*1000,travel=10*2*math.pi)
plt.select_pen(1)
plt.write(ss)
plt.select_pen(0)

The plotter uses absolute plotter units that range from (0,0) to (10365,7962). Telling the plotter to put its origin in the middle of the page makes perfect sense, because that automagically centers the figure.

Dialing the speed back to 5 cm/s works much better with the Sakura pens than the default 38.1 cm/s = 15.0 inch/s; hand-drawing pens just don’t have the flow rate for prolonged vigorous scribbling. HP was obviously on the edge of converting to metric engineering units in the early 1980s, with the HP 7475A designed before the transition and shipped afterward.

The supershape parameters:

  • 3900,3900 sets the maximum coordinate value along each axis. The plot may or may not exceed that value, depending on how weird the supershape turns out, but it’s generally pretty close
  • 5.3,0.4,1,1 correspond to coefficients m, n1, n2, n3
  • By default, a=1 and b=1, but you can change those as you like
  • point_count=10*1000 sets how many total points appear in the plot
  • travel=10*2*math.pi sets the number of complete cycles, in units of 2π

The function spits out a list of Cartesian XY coordinates, not the polar rΦ coordinates you might expect.

Slightly non-integer values, particularly for m, produce more interesting patterns. Other than that, there’s just no telling.

Use io.view(ss) to get an idea of what you got, it’s much faster than plotting!

Chiplotle Supershape preview

Chiplotle Supershape preview

You may find the online superformula explorers better suited to rapid prototyping, though. There’s a list at the bottom of the Wikipedia article, although some links seem defunct.

Notice that the end of the plot doesn’t quite reach the beginning over on the far right, which is a consequence of how Python produces sequences. Adding one more point does the trick:

ss=geometry.shapes.supershape(3900,3900,5.3,0.4,1,1,point_count=1+10*1000,travel=10.001*2*math.pi)
Chiplotle Supershape preview - closed

Chiplotle Supershape preview – closed

I’ll try remembering that the next time around…

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  1. #1 by jim oslislo on 2015-05-07 - 07:38

    “a pen plotter isn’t going to be doing anything useful”

    Not so. We have an antique 30″ Houston Instrument plotter to make full size layout patterns for our fitters to use while tacking together the weldments we build. In the old days we used to use sheet metal patterns but now it is easier to just plot them out when needed and throw them out when done.

    • #2 by rkward on 2015-05-07 - 07:46

      That’s one of the reasons I have kept my E-size HP plotter all these years, full-sized patterns of for anything. Plots like this take no time at all.

      • #3 by Ed on 2015-05-07 - 07:53

        I’d forgotten how much noise a pen plotter makes! A few lines of text may be fun to watch, but the plotter drives everybody else right out of the room…

        • #4 by rkward on 2015-05-07 - 08:01

          Yep, especially on lettering!

        • #5 by Red County Pete on 2015-05-07 - 16:10

          The first plotter at our IC fab was a roll-feed CalComp. It used a vacuum arrangement for the slack, and was noisy enough to warrant its own room. The HP E-sized plotter that replaced it was a major relief.

    • #6 by madbodger on 2015-05-07 - 10:03

      I kept one around for a long time to make neon bending patterns. The paper would scorch pretty quickly, but knocking out a fresh one didn’t take long.

  2. #7 by Ken Davidson on 2015-05-07 - 08:23

    How about hooking up an array of these things and plotting Mandelbrot pictures? ;o)

    • #8 by Ed on 2015-05-07 - 11:34

      Now that you mention it, a line-based fractal like a Hilbert curve would make a nice attention-getter, take basically forever to draw on a B-size sheet, and wouldn’t rattle the glassware off the shelves.

      Thanks for the reminder… [grin]

  3. #9 by solaandjin on 2015-05-07 - 11:07

    Here’s an idea: using a LiveScribe digital pen (or other means to capture pen stroke data) to write out a letter/invitation. Then use a plotter to recreate exact duplicates.

    It appears LiveScribe is not exactly open with the details of their data format, but people have put in some work to get it running on Linux (LibreScribe, libsmartpen).

    • #10 by Ed on 2015-05-07 - 11:38

      There’s a Wacom tablet taking up entirely too much of the desk in front of me, so saving-and-feeding those coordinates should work. Now, if my handwriting were legible, that would be a Good Thing.

      On the other paw, that would make a spectacular techie art installation!

      Thanks…

      • #11 by solaandjin on 2015-05-07 - 11:57

        I’ve been wanting to design and 3D print a Thomas Jefferson-style polygraph (not the lie detector) for a while.

        As an art installation, if you could do the capture/transmit in near real time, that would be even cooler.

        • #12 by Ed on 2015-05-07 - 13:19

          Thomas Jefferson-style polygraph

          That would be way more geeky than a plotter: go for it!

          So many projects, so little time…

  4. #13 by Trudi on 2015-05-07 - 15:56

    Reminds me of a spirograph toy… but ever so much more fancy

    • #14 by Ed on 2015-05-07 - 20:37

      With the major advantage that the gear wheels can’t slip after filling most of the page!

  1. HP 7475A Plotter: Exploring SuperFormula Parameters | The Smell of Molten Projects in the Morning