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Problems making mirror images
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* June 19, 2009, 08:07:38 PM
When I attempt to make mirror images, such as bathrooms in architectural drawings, I do it by entering a "-1" in the scale field of the appropriate coordinate. Most of the time, the mirror images come out scrambled. What tricks do I need to know about mirror images?

Thanks for your help, guys.

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* June 20, 2009, 09:01:17 AM
#1
What's the purpose of the "-1"?

When I mirror, I set the workplane if necessary, select the object to be mirrored, pick my mirror axis, select Ortho, and let 'er rip.  No numbers needed.

The most difficult thing is picking the right point for the mirror axis so the new object lands where I want it.

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* June 20, 2009, 09:40:29 AM
#2
What's the purpose of the "-1"?

It's a traditional method of making a mirror copy, but it has pitfalls. One problem is that when applied to a 3D object, the result is NOT a mirror copy (i.e., with chirality opposite the original), but simply a rotated copy. The attached .jpg illustrates the difference between mirror copying and negative scaling.

Another quirk is that using a negative value of not exactly 1 often produces a negative  object which responds weirdly to Boolean ops and thus can damage your drawing. To see one aspect of this behavior, open the attached NegativeObject.tcw and try 3D-slicing the object. (It was created in v10.5 and should open OK in that version or any later version.) After slicing, zoom way out to see what really happened.

Henry H

[attachment deleted by admin]

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* June 20, 2009, 02:15:18 PM
#3
Wow, wierd.  (NegativeObject sliced)

Is that because the object is inside out and all the faces are back to front?

Looks like the slice routine is not aware as to the extents of the (broken) object.

P.


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* June 20, 2009, 03:05:40 PM
#4
Thanks, guys. Once again, you've gotten me out of trouble. I tried the suggested method, and it works. What's the "-1" for? So help me, I remember reading somewhere that entering a minus sign in the Scale value in the inspector bar was the way to make a mirror copy. Maybe that was in the Designer version that I had previously. I now have Deluxe 15.1, which by the way wasn't capable of opening the NEGATIVEOBJECT.TCW file.

Thanks again. All is well.

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* June 20, 2009, 10:17:10 PM
#5
I now have Deluxe 15.1, which by the way wasn't capable of opening the NEGATIVEOBJECT.TCW file.

I shoulda thought of that. It's an ACIS Solid, or at least purports to be, an object type which Deluxe cannot handle.

Henry H

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* June 21, 2009, 11:03:32 AM
#6
Is that because the object is inside out and all the faces are back to front?

Don't really know, Paul. But if you'd care to play with that drawing a little longer, you'll see why I call the thing a "negative" object: When you Boolean-add it to a normal object, or Boolean-subtract to or from another object, or Boolean-intersect with another object, the result is completely different from the same operation between two normal objects.

BTW, to create a negative object, start with an ACIS Solid and rescale by negative 1.01 in the X direction. Other numbers and other directions work, too, but I'm not sure the results are consistent.

Henry H

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