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Plotter as digitizer

Discussion in 'Electronic Design' started by Don Y, Oct 8, 2011.

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  1. Don Y

    Don Y Guest


    [I've asked this before -- always with disappointing
    results -- but will try again before embarking on
    a home-grown solution]

    (Many? All?) HP pen plotters have the capability of
    being used as oversized digitizers. The pen holder is
    replaced with a "sight" (essentially, an optical pipe)
    and the joystick used to slew the paper/sight to each
    location of interest, etc.

    The location of the paper/sight is then reported to
    the external device.

    I have many oversized (D&E) drawings that I would like to
    digitize and discard. The first idea that came to mind
    was to do this by replacing the pointing device in an
    AutoCAD instance as this is most intuitive.

    But, I find it hard to believe that someone hasn't
    already done something to exploit this capability.

    Or, am I the only soul who still has pen plotters? :>

    I suppose I could also just write a little piece of code
    to gobble up incoming data and scribble it to a file -- which
    can later be imported to <whatever>. But, this separates the
    "data collection" from the *use* which can be tedious with
    lots of spatial data stripped of its *context*!

    (sigh) I'll try google, yet again...

  2. Winston

    Winston Guest

    Could you use a large format document scanner and
    capture your drawings to pdf?

  3. Don Y

    Don Y Guest

    Hi Winston,

    It would have to be a *really* big scanner (or, a photographic
    process) as E size drawings would humble my *B* size scanner!

    And, I suspect it would be hard to extract the geometric
    data from the PDF -- unless I wrote something to analyze the
    image contained therein, itself.

    (I.e., I want to be able to *use* the data represented
    in the drawings, not just "view" it)
  4. Winston

    Winston Guest

    Hey, Don.

    Large format scanners are available:

    ....or DAGS "E size scanner"

    Perhaps your local Kinko's can rent
    you one for reasonable money.

    From there you need a program to convert
    it's output to a DXF.

    And a *lot* of cleanup after that!


    There are service bureaus that will manually
    capture your print to a CAD file for $75.
    an hour!


    You can be sure there is a talented young
    person around who would leap at the chance
    to make $50 each to manually capture each
    of your prints, after a few free tutorial

  5. Rich Grise

    Rich Grise Guest

    It sounds like just making a bitmap isn't good enough; you want
    to maintain connectivity. If I've guessed right, then the only
    thing I can really think of is to shove your probe around by
    hand, and trace the wires; this would only work if your plotter
    can actually read back its pen's position, and you have some
    SW following it that understands connections.

    Other than that, I've got nothing but political propaganda, but
    that gets me shunned. ;-)

    Good Luck!
  6. Don Y

    Don Y Guest

    Hi Winston,

    Yes. E.g., the plans for the house would be helpful to have
    in CAD as it would make an easy job of determining how many
    sheets of wallboard I will need to hang, how many (floor)
    tiles to lay, etc. Much easier (and safer!) than trusting a
    tape rule :-/ (damn things *lie* every time I use one!)

    Also, there are things you can do with a dimensionally accurate
    drawing that are *really* tedious to do, otherwise. E.g.,
    deciding where to lay the *first* floor tile to yield the best
    "pattern" throughout the house!
    Gack! I'm already grumbling about having the damn plotters
    taking up space in the garage -- and you want me to add
    another dust catcher? :>
    Yes. And, never knowing if an error hasn't crept in.

    I was thinking about this on my evening walk and
    think I have come up with a solution -- when the
    problem defies solution, *change* the problem! :>

    I can, instead, write an *emulator* that allows the
    plotter to masquerade as a conventional digitizing
    tablet (e.g., a Kurta, Calcomp, Summagraphics, etc.).
    This allows me to use the plotter with a variety
    of software programs instead of just, e.g., AutoCAD.

    The issue then becomes one of finding the digitizer
    that is supported by the "most" tools that I am
    likely to use (so I don't have to write more than
    one "emulator")
  7. Don Y

    Don Y Guest

    Then, how do I get the *images* into, e.g., a CAD program?
    I.e., for the house plans, once in AutoCAD, I could lay a
    pattern of floor tiles onto it and use the "walls" to
    "trim" them. I could touch the corners of each room and
    have it report the area of the polygon. Etc.
  8. Winston

    Winston Guest

    I have *exactly* the same tapes.

    Better you than me. :)
    That is true for any approach one takes.
    One could find the disk files that one
    created initially.....
    Just a thought. :)


    My favoritest CAD program in the world, Rhino3D
    allows me to place a bitmap as 'wallpaper' on
    the screen.

    I have used that capability to capture parts
    I've scanned by tracing CAD lines over the
    lines I see in the bitmap.

    You see where I'm headed with that? :)

  9. Have someone scan it.

    Get a quote and see what it costs.

  10. John S

    John S Guest

    Well, not yet. But we still have hope.
  11. Don Y

    Don Y Guest

    Hi Winston,

    I have learned that a length of *wire* (NOT "string" -- string
    stretches too much!) makes a better "ruler":
    "I need to cut the board to *this* length..."

    I wasn't the architect that drew the house plans! :>
    I wonder how reliable that would be trying to scale a
    50+ inch drawing to correspond with an on-screen image...

    I'm currently looking at various tablets with an
    eye towards the simplest/cleanest implementation
    (e.g., any "smarts" in the tablet would have to be
    emulated correctly in anything I write). OTOH,
    that might make the design easier to implement
    *if* I allow the transactions to drive the emulation
    instead of forcing the emulation to fit a specific

    E.g., scan the input file and let it define the
    "size" of the (virtual) tablet that I am defining.
  12. Winston

    Winston Guest

    And I have learned that sometimes I work faster and
    more accurately when I *don't* measure.

    Jig for cutting fence boards instead of measuring

    Holding a board up to where it needs to go and
    scribing a cut line against where it needs to stop.

    It is down to scanner resolution and careful

    Just now for larfs, I created a rectangle measuring
    100 feet by 100 feet so that I could zoom in to
    find out how much resolution I had for the minimum
    feature size for an underlying drawing.

    It ran out of gas at about 14 *pico inches*.
    (note that the underlying bitmap would look like
    one enormous pixel near x=0 Y=0 at this

    I *love* Rhino3D.
    At the end of the day, it will be you peering
    through crosshairs on a microscope attached to
    your pen holder, laboriously capturing vectors.
    You are made of stout stuff, Don. I couldn't
    do it. My back is sending me warning pains
    just thinking about it.

    I would cheat and make a scanner do the hard
    work. :)

  13. Don Y

    Don Y Guest

    Hi Winston,

    Yup. Though even this way you have to remember that
    the pen won't mark *exactly* where the cut should be.
    But you created that rectangle *in* the CAD program?
    I.e., using *it's* notion of what a "foot" might be.

    In my case, I have a drawing drawn to *some* scale
    which has to be *mapped* into a corresponding scale
    inside the CAD program.
    No, the "sight" is a ~1/4" diameter "pipe" made of lexan.
    All it really does is bring the image that is on the
    paper "under" the pen (sight) up to a point where it is
    visible to the user.

    Recall that architectural drawings are lots of right angles
    (typically... there are a few exceptions, here) so the
    points that you are digitizing tend to be offset along
    one axis -- or the other -- from each other (no diagonals).

    I have to think hard about how I want to handle the
    *schematics*, though. It may be simpler just to redraw
    *Big* plotter. It stands ~4 ft tall so the sight is
    pretty much where you can just look at it casually.
    There is a large Calcomp tablet in a local auction
    next week. If I can figure out an easy way to get
    it home, I may opt for that, instead.
  14. Guest

    Not to mention out-of-square (or plumb) walls. I doubt there is an "as built"
    drawing of many of these.
    When I do a room, I do add these to the CAD drawings. It helps later.
  15. Guest

    I hope it was a "modern" house. At least he saved a stud. ;-)
  16. Winston

    Winston Guest

    Hey there, Don!

    Yup. It will measure 'long' for 'outside' measurements.

    * For me that usually means a small overlap
    if I cut very close to the line. This is good.

    * The scribed line will echo any angular displacement
    in the joint. Sometimes a 90 degree cut will not
    rhyme very well against a twisted beam. :)

    Trivial. You can move and scale the bitmap in
    Rhino3D to match any reference dimension on your
    drawing. Ferinstance, I have a 70' long fence
    section on my property. I draw a "70'" long line
    in the correct orientation on my drawing and move
    (and scale) the 'plan' bitmap until the image of
    the fence section equals my "70'" reference line.

    Now I know my reference drawing and my CAD package
    agree what a 'foot' is, to within an acceptable error.

    That is better, but still not terribly fast, accurate
    and easy, IMHO.
    The error using the scanner at 600 dpi is about +- 0.003".
    Multiply that by the reduction scale to reveal the real-
    life error. Could you hold +-0.003" using the plotter
    digitizer jig? I am sure that I couldn't, though it
    is easy to do, all day using a scanned bitmap in Rhino3D.

    I concur, especially if you have a second-level data
    extraction process that creates a Bill Of Materials.

    Do you have a smaller digitizer tablet available?
    It would be highly interesting to do a 'trial run'
    using hardware on hand.

    I can move a sandpile with only a teaspoon, but I can do
    it much faster and more easily with a shovel.
    Discovering that early in the process can save me a
    lot of time and sweat.


  17. Winston

    Winston Guest

    Fred Bloggs wrote:

    This assumes that everything on the source drawing is dimensioned
    completely and there is no need to scale the drawing to extract
    the missing data.

    I don't believe that is the case here.

  18. Don Y

    Don Y Guest

    Hard to be much closer than the "local auction" (probably
    6 miles, as the crow flies) :>
    I've already got three large-format plotters. The Calcomp *tablet*
    would allow me to "not solve" the "plotter as digitizer" problem.
    Hmmm... I thought all HP's were serial/HPIB (?). I don't know how
    you could provide the digitizer functionality using a (conventional)
    parallel port...
  19. Don Y

    Don Y Guest

    Hi Michael,

    Correct. They represent the architects *intent*. They don't
    represent the inevitable screw ups!

    In our case, the plans aren't even for this *exact* house but,
    rather, for another "identical floorplan" (different elevation)
    in the neighborhood. E.g., they don't show the living room
    as sunken, don't show the "frontier style" roof, don't show
    the fireplace, etc.

    There are also obvious errors *in* the original drawings -- like
    the RO for the kitchen window extending *below* the height of
    the adjoining countertop! (oops!)
    Yes, and dimensions alone won't help you decide *exactly* how
    many 8, 10, 12 or 16 ft sheets you'll need. It's an
    *estimating* tool.

    Until I saw the "professional" plans (i.e., more than just my
    *sketches* of the floorplan), I had never considered how significant
    *where* I laid the first tile would be! Having seen it, now, I
    will spend a good deal more time evaluating potential layouts
    before placing that tile as it has a significant impact on the
    final appearance of the tile in total.

    This is *much* easier to do with a scaled CAD rendering than
  20. Don Y

    Don Y Guest

    Hi Winston,

    Yes, I've learned many of these lessons the hard way :<
    I installed blocking in the kitchen walls off of which
    to hang the new cabinets. Nice square cuts with the
    chop saw. Yet, few of them "fit" properly due to
    the twists and "non-square" placements of the studs.
    Recall these are just *walls*. Walls that I am intimately
    familiar with! :> Find a corner. "ENTER" it. Slew the
    pen along the wall to the next "corner". ENTER that.
    Lather, rinse, repeat.

    You're just walking along the surfaces of the house interior
    (then exterior) so its really just a couple of bizarre

    Once all the points are ENTERed, go back and figure out
    if you missed any "features". E.g., we have one partial
    wall that separates the living room from dining room.
    Not load bearing but, rather, present for "artistic purposes"
    to break the sight line (we visited another house with
    similar floorplan where this wall had been removed and the
    "feel" of the house was decidedly less friendly/intimate).
    I think the bigger problem would be finding a symbol library
    that was consistent with the symbols that I used in the
    original schematics. Note that it not only has to draw the
    components the same way but, also, the relative placements
    of the various connections to those components must be
    I have a couple of 12x12 and 12x18 tablets on hand.
    These will work well in AutoCAD, for example, -- but,
    only for drawings of those sizes (I regularly use
    the 12x12 with autocad for this sort of thing).

    The Calcomp tablet in question is at least D size
    (this is what would make fetching it home difficult)
    so it would allow me to just mount the drawings and
    go to it! It would take up a sh*tload of room, here,
    but could double as a drafting table when not in
    use as a digitizer.
    Just cover it up with *dirt* and buy *new* sand for the
    "relocated pile"! :>
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