Interpreting DXF BLOCK and INSERT

Hi. I'm not an AutoCAD expert but am trying to write a DXF reader.

I'm having trouble interpreting the information in DXF's BLOCK and INSERT.

What's the algorithm for taking the location in a BLOCK, the Xscale factor,
the Yscale factor, the rotation, the INSERT point, and determining where
something
goes? Let's assume we're in 2D, no Z values need to be considered.

Here's a concrete example:

The BLOCK has a line from, say, (3, 5) to (7, 13).
The INSERT says to Xscale by -1.5, to Yscale by 1.5, to rotate 270 degrees,
and
to insert at (15, 17). Where should this line end up?

Thanks

 

Let me follow up by describing what my code is doing now.



This code gets the right answer almost all the time,



but there must be something wrong, because I have one



test case on which it draws the result in the wrong place.



 



What's wrong?



 



   do_one_insert() {



      X = the insert's X - the block's X basepoint



      Y = the insert's Y - the block's Y basepoint



 



      if (the insert's X scale < 0 and the insert's Y scale > 0)



         add 180 degrees to the insert's rotation



 



      Take all the points from the entities in the block and



      rotate them "rotation" amount



 



      Take all the points from the entities in the block and



      multiply them by X scale for all the X coordinates or by



      Y scale for all the Y coordinates and then



      add X or Y as appropriate



   }



 



-----Original Message-----



From: Biederman, Steve



Sent: Friday, September 12, 2003 4:23 PM



To: Biederman, Steve



Subject: Interpreting DXF BLOCK and INSERT



 



Hi. I'm not an AutoCAD expert but am trying to write a DXF reader.



I'm having trouble interpreting the information in DXF's BLOCK and INSERT.



What's the algorithm for taking the location in a BLOCK, the Xscale factor,



the Yscale factor, the rotation, the INSERT point, and determining where



something



goes? Let's assume we're in 2D, no Z values need to be considered.



Here's a concrete example:



The BLOCK has a line from, say, (3, 5) to (7, 13).



The INSERT says to Xscale by -1.5, to Yscale by 1.5, to rotate 270 degrees,



and



to insert at (15, 17). Where should this line end up?



Thanks

 

Steve,

You should use a transformation matrix. There is plenty of info in this ng,
and on the web regarding these.

--
R. Robert Bell, MCSE
www.AcadX.com


| Let me follow up by describing what my code is doing now.
| This code gets the right answer almost all the time,
| but there must be something wrong, because I have one
| test case on which it draws the result in the wrong place.
|
| What's wrong?
|
| do_one_insert() {
| X = the insert's X - the block's X basepoint
| Y = the insert's Y - the block's Y basepoint
|
| if (the insert's X scale < 0 and the insert's Y scale > 0)
| add 180 degrees to the insert's rotation
|
| Take all the points from the entities in the block and
| rotate them "rotation" amount
|
| Take all the points from the entities in the block and
| multiply them by X scale for all the X coordinates or by
| Y scale for all the Y coordinates and then
| add X or Y as appropriate
| }
|
| -----Original Message-----
| From: Biederman, Steve
| Sent: Friday, September 12, 2003 4:23 PM
| To: Biederman, Steve
| Subject: Interpreting DXF BLOCK and INSERT
|
| Hi. I'm not an AutoCAD expert but am trying to write a DXF reader.
| I'm having trouble interpreting the information in DXF's BLOCK and INSERT.
| What's the algorithm for taking the location in a BLOCK, the Xscale
factor,
| the Yscale factor, the rotation, the INSERT point, and determining where
| something
| goes? Let's assume we're in 2D, no Z values need to be considered.
| Here's a concrete example:
| The BLOCK has a line from, say, (3, 5) to (7, 13).
| The INSERT says to Xscale by -1.5, to Yscale by 1.5, to rotate 270
degrees,
| and
| to insert at (15, 17). Where should this line end up?
| Thanks