DXF processing: how to calculate absolute coordinates in blocks

I have a software that needs to process dxf files and I am having a
nightmare trying to find the absolute corrdinates of LINEs inside
blocks.

This is what I know about BLOCKS:

Each BLOCK (in the BLOCKS section) has a block name that is related to
an INSERT entity (In the ENTITIES section).
The INSERT entity defines the transformation parameters such as origin,
scale and rotation, to transform the BLOCK coordinates from relative to
absolute coordinates.
All coordinates in the BLOCK may be transformed using the parameters
defined in INSERT.
Each BLOCK ends with ENDBLK.

This is what I need to do:

I need to transform the relative coordinates of LINEs inside blocks (in
the BLOCKS section) to absolute coordinates.
What I do is search (using the block name) the corresponding INSERT
entity (in the ENTITIES section) which gives me the coordinates of the
block, so by adding the relative + block corrdinates I can obtain the
absolute coordinates for LINEs.

SO FAR THE EXPLANATION, NOW THE PROBLEM TO SOLVE:

What can I do if I find more than one INSERT entity with the same block
name ?
How do I know which one applies to each LINE, since for a given block
name there is only one BLOCK structure in the blocks SECTION but in
some cases there is more than one INSERT entity in the ENTITIES
section.

Thanks for your help
Jose Maria

 

a écrit :

In the BLOCKS section, you've got the BLOCK definition. A dxf can
contain a BLOCK definition, but none INSERT occurrence
The entities that are defining the block are described only in BLOCK
section. All their coordinates are relative to the specified insertion
point of the bloc.

The Block and insert system is designed to compress files and make the
drawing reactive :
One BLOCK definition, and for each occurence, only a insertion point,
and optional, x,y,z scale factor and rotation angle.
The way you've got to think is that for each occurence of the Block
(Insert) you will have to perform a geodesic projection change, to
calculate the real position of all entities of your insert.

You must take care of the fact that DXF code are optional :
By default, only 10,20,30 code for insertion point.
If the scale is not 1 : 41,42,43 appear
If rotation : 50 code
If 3d_rotation : 210 220 230

If you wish to have a syntaxic coloration of DXF files see this page :
http://www.g-eaux.com/powerclic/explorer/bonusedit_eng.html

If your program is is in C, you will find open source library for
projection change, that may be easely adapted to that case ?
http://proj.maptools.org/

 

Thanks for your help, but the I do not understand. Once I know which
INSERT corresponds to each line, there is no problem, I know how to
calculate.

The problem I have is that I need to know which of the INSERTs in the
entities secion WITH THE SAME BLOCK NAME I must take to calculate the
absolute coordinates form each LINE in the BLOCKS section.

The problem is that for ONE block name there is only ONE block in the
blocks sections with many LINEs and there may be one or MORE THAN ONE
INSERT in the entities section. If there is only one INSERT with the
corresponding block name, there is no problem. But if there are several
inserts, I need to know for each LINE which of the INSERTS I need to
use to calculate the absolute coordinates.

Thanks

 

Every block defined in the block section can be inserted more than one
time!
So you have to transform the complete block for ANY insert you find.

regards
Dirk

 

Thank you very much, that makes sense.

Another question: there are cases where the BLOCK in the blocks section
does not correspond to any INSERT in the entities section, so this
block is "garbage". As I read the dxf file from beginning to end, I
read the block first, later the insert. Do I have anyway to know when I
read the BLOCK in the blocks section that this block is useless before
reaching the entities section and realizing that there is no INSERT
with the same name ?

 

No you, have to check the INSERT's to see if a block is used or not.
And yes INSERT's can be within block's, too and not only in the
entities
section.

regards
Dirk

 

And yes INSERT's can be within block's, too and not only in the

So this means nested blocks ? So this makes absolute coordinates more
difficult to calculate.

 

yes, and don't forget that block's can be scaled and rotated...

regards
Dirk

 

Thank you for all your help. I will try to do my best.