Draw polyline around a block

Hi,
I am trying to draw a LWPolyline around any given block. GetBoundingBox returns a rectangle that doesn't represent the actual shape of the block. I need to capture the exact shape of the block, so that, say, a triangle is shown as a triangle. Unfortunately, blocks can be more complicated than a triangle. They may consist of lines, arcs and polylines. All entities are connected, that is there is no entity in a block that lies away from the others.
This is a very challenging task, and I would greatly appreciate any help.
Thank you in advance,
Mike

 

Complicated indeed.
But how about copying all the entities in the block and converting them into
regions, then do a boolean add ??


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returns a rectangle that doesn't represent the actual shape of the block. I
need to capture the exact shape of the block, so that, say, a triangle is
shown as a triangle. Unfortunately, blocks can be more complicated than a
triangle. They may consist of lines, arcs and polylines. All entities are
connected, that is there is no entity in a block that lies away from the
others.

 

Could you offset all the nested items and union together the results? I
haven't tried this myself, but I think it would work.

I guess for nested text you would still use bounding box, and for non-closed
items you would have to close them (I could see this part getting tricky).
Once you've gotten that far use offset, addregion, and boolean(acUnion).

I have thought about this before but never actually done anything with it.
Maybe someone else has already done it?

HTH,

 

Cheat: insert a new block at the same point with a slightly larger scale.

 

This sounds like a great idea. Closing entities will definitely be tricky, though. If we were just talking about lines.. When arcs are involved the final shape could become kinda unpredictable.

 

I am sorry, but I can't understand how inserting a larger block can help.

 

Try this algorithm:

Start with an imaginary circle that shares a center with the center of
the bounding box. Save the coordinate for this circle's center.

Iterating through all of the block's primtives one by one.

Find the intersections for each primitive that also lie on the
primitive. Break the primitive into fragments having ends at the
intersections. Store the data for these fragments in a way that you can
detect their orginal source. (Know they came from the same original
primitive.) The end result will be line and arc fragments.

Find the outermost original primitive. Start from its outer end. Search
through the fragments belonging to it picking the correct on to use.
This becomes the first part of you shape. Use its end closer to the
center. Find the other fragments not originaly from the same primitive
that share this coordinate. These are the intersecting fragments. Pick
the one with the most outer direction from the center as defined by its
other end. Add this to you shape. Remove it from the fragment list.

Repeat the above until you reach the starting point. If no other
fragment shares an end, then double back on the previous fragment since
this must be an outer spike.

 

Maybe you should recheck your strategy (approach) toward detecting if
something is in or out of the block. Sometimes it's not easy to distinguish
between the block itself and it's attributes when they're all rotated and
possibly non-uniformly scaled. If you are programming a system that consists
of the nodes (blocks) and polylines (links) interconnecting them, then
redefine global strategy.

Regards,
Maksim Sestic
www.geoinova.com

 

myvar = block.explode

'returns an array of entities in the block ie, three lines for a triangle
poly

myregion = ThisDrawing.ModelSpace.AddRegion(myvar)
'creates a region from the object array.

Thought it might help.

Paul

 

You could offset each sub both ways and connect the ends. This would trace
around each sub individually, potentially leaving 'islands' after the union.
But I think it would take care of closing the loops.

In AutoCAD I would then explode the region, pedit/join the results back to
plines, and erase the islands, but I'm not sure how to do the pedit/join in
vba w/o sendcommand. Or how to tell programmatically which are the islands.

James

 

How about using the bounding box to draw a rectangle around the entire
block.
Then somehow use bpoly and select just inside that rectangle.
Erase the rectangle made with boundingbox and the one made with bpoly.
Then you could offset the resulting polyline which should be a profile of
your block.

returns a rectangle that doesn't represent the actual shape of the block. I
need to capture the exact shape of the block, so that, say, a triangle is
shown as a triangle. Unfortunately, blocks can be more complicated than a
triangle. They may consist of lines, arcs and polylines. All entities are
connected, that is there is no entity in a block that lies away from the
others.

 

Hi,

Do you have sample code for this algorithm?

Thanks,

Paul

 

This sounds like another good idea. Although, how about if the block is devided into many small pieces inside? Using bpoly will return not the whole block but one small closed entity inside it.

 

Good question.
If the block is made up of separated pieces bpoly will wrap each piece
individually.
I suppose what you want to do could still be done but since we're talking
about using SendCommand it's going to be very messy.

By the way if you connected each separate polyline (boundary) with a line
then ran bpoly again the result would be one polyline instead of many small
ones.

Mark

devided into many small pieces inside? Using bpoly will return not the whole
block but one small closed entity inside it.

 

Mike,

I just Xplode the block, which leaves the block in place and creates a copy.
ie closed 3 sided poly turned into a block become a closed 3sided poly when
..xplode is issued.

Am I missing something?

Paul

returns a rectangle that doesn't represent the actual shape of the block. I
need to capture the exact shape of the block, so that, say, a triangle is
shown as a triangle. Unfortunately, blocks can be more complicated than a
triangle. They may consist of lines, arcs and polylines. All entities are
connected, that is there is no entity in a block that lies away from the
others.

 

Mike, You could also use GetSubEntity to find the entities that make up the
block and their coordinates and then draw a pline.
pr

returns a rectangle that doesn't represent the actual shape of the block. I
need to capture the exact shape of the block, so that, say, a triangle is
shown as a triangle. Unfortunately, blocks can be more complicated than a
triangle. They may consist of lines, arcs and polylines. All entities are
connected, that is there is no entity in a block that lies away from the
others.

 

Im not going to try to put any code to this but this is what I would do::

Isolate the block
Get the bounding box and draw a rectangle slightly bigger
draw a "boundary" polyline, this will create 2 polylines, one for the
bounding box
delete the outer rectangles
you are left with a tight fit polyline

Lloyd Rubidge