## Documentation Center |

Decompose Constructive Solid Geometry into minimal regions

`dl=decsg(gd,sf,ns)dl=decsg(gd)[dl,bt]=decsg(gd)[dl,bt]=decsg(gd,sf,ns)[dl,bt,dl1,bt1,msb]=decsg(gd)[dl,bt,dl1,bt1,msb]=decsg(gd,sf,ns)`

This function analyzes the *Constructive Solid Geometry
model* (CSG model) that you draw. It analyzes the CSG model,
constructs a set of disjoint minimal regions, bounded by boundary
segments and border segments, and optionally evaluates a set formula
in terms of the objects in the CSG model. We often refer to the set
of minimal regions as the *decomposed geometry.* The
decomposed geometry makes it possible for other Partial Differential Equation Toolbox™ functions
to "understand" the geometry you specify. For plotting
purposes a second set of minimal regions with a connected boundary
is constructed.

The PDE app uses `decsg` for many purposes.
Each time a new solid object is drawn or changed, the PDE app calls `decsg` to
be able to draw the solid objects and minimal regions correctly. The
Delaunay triangulation algorithm, `initmesh`, also
uses the output of `decsg` to generate an initial
mesh.

`dl=decsg(gd,sf,ns)` decomposes
the CSG model `gd` into the decomposed geometry `dl`.
The CSG model is represented by the Geometry Description matrix, and
the decomposed geometry is represented by the Decomposed Geometry
matrix. `decsg` returns the minimal regions that
evaluate to true for the set formula `sf`. The Name
Space matrix `ns` is a text matrix that relates the
columns in `gd` to variable names in `sf`.

`dl=decsg(gd)` returns all minimal
regions. (The same as letting `sf` correspond to
the union of all objects in `gd`.)

`[dl,bt]=decsg(gd)` and `[dl,bt]=decsg(gd,sf,ns)` additionally
return a *Boolean table *that relates the original
solid objects to the minimal regions. A column in `bt` corresponds
to the column with the same index in `gd`. A row
in `bt` corresponds to a minimal region index.

`[dl,bt,dl1,bt1,msb]=decsg(gd)` and `[dl,bt,dl1,bt1,msb]=decsg(gd,sf,ns)` return
a second set of minimal regions `dl1` with a corresponding
Boolean table `bt1`. This second set of minimal regions
all have a connected boundary. These minimal regions can be plotted
by using MATLAB^{®} patch objects. The second set of minimal regions
have borders that may not have been induced by the original solid
objects. This occurs when two or more groups of solid objects have
nonintersecting boundaries.

The calling sequences additionally return a sequence `msb` of
drawing commands for each second minimal region. The first row contains
the number of edge segment that bounds the minimal region. The additional
rows contain the sequence of edge segments from the Decomposed Geometry
matrix that constitutes the bound. If the index edge segment label
is greater than the total number of edge segments, it indicates that
the total number of edge segments should be subtracted from the contents
to get the edge segment label number and the drawing direction is
opposite to the one given by the Decomposed Geometry matrix.

The *Geometry Description matrix* `gd` describes
the CSG model that you draw using the PDE app. The current Geometry
Description matrix can be made available to the MATLAB workspace
by selecting the **Export Geometry Description, Set Formula,
Labels** option from the **Draw** menu
in the PDE app.

Each column in the Geometry Description matrix corresponds to
an object in the CSG model. Four types of *solid objects* are
supported. The object type is specified in row 1:

For the

*circle solid*, row one contains 1, and the second and third row contain the center*x*- and*y*-coordinates, respectively. Row four contains the radius of the circle.For a

*polygon solid,*row one contains 2, and the second row contains the number,*n*, of line segments in the boundary of the polygon. The following*n*rows contain the*x*-coordinates of the starting points of the edges, and the following*n*rows contain the*y*-coordinates of the starting points of the edges.For a

*rectangle solid*, row one contains 3. The format is otherwise identical to the polygon format.For an

*ellipse solid*, row one contains 4, the second and third row contains the center*x*- and*y*-coordinates, respectively. Rows four and five contain the semiaxes of the ellipse. The rotational angle (in radians) of the ellipse is stored in row six.

`sf` contains a *set formula* expressed
with the set of variables listed in `ns`. The operators
`+', `*', and `-' correspond to the set operations union, intersection,
and set difference, respectively. The precedence of the operators
`+' and `*' is the same. `-' has higher precedence. The precedence
can be controlled with parentheses.

The *Name Space matrix* `ns` relates
the columns in `gd` to variable names in `sf`.
Each column in `ns` contains a sequence of characters,
padded with spaces. Each such character column assigns a name to the
corresponding geometric object in `gd`. This way
we can refer to a specific object in `gd` in the
set formula `sf`.

The *Decomposed Geometry matrix* `dl` contains
a representation of the decomposed geometry in terms of disjointed *minimal
regions* that have been constructed by the `decsg` algorithm.
Each edge segment of the minimal regions corresponds to a column in `dl`.
We refer to edge segments between minimal regions as *border
segments* and outer boundaries as *boundary segments*.
In each such column rows two and three contain the starting and ending *x*-coordinate,
and rows four and five the corresponding *y*-coordinate.
Rows six and seven contain left and right minimal region labels with
respect to the direction induced by the start and end points (counter
clockwise direction on circle and ellipse segments). There are three
types of possible edge segments in a minimal region:

For circle edge segments row one is 1. Rows eight and nine contain the coordinates of the center of the circle. Row 10 contains the radius.

For line edge segments row one is 2.

For ellipse edge segments row one is 4. Rows eight and nine contain the coordinates of the center of the ellipse. Rows 10 and 11 contain the semiaxes of the ellipse, respectively. The rotational angle of the ellipse is stored in row 12.

The following command sequence starts the PDE app and draws a unit circle and a unit square.

pdecirc(0,0,1) pderect([0 1 0 1])

Insert the set formula `C1-SQ1`. Export the
Geometry Description matrix, set formula, and Name Space matrix to
the MATLAB workspace by selecting the **Export Geometry
Description** option from the **Draw** menu.
Then type

[dl,bt]=decsg(gd,sf,ns); dl = 2.0000 2.0000 1.0000 1.0000 1.0000 0 0 -1.0000 0.0000 0.0000 1.0000 0 0.0000 1.0000 -1.0000 0 1.0000 -0.0000 -1.0000 1.0000 0 0 -1.0000 0 -0.0000 0 0 1.0000 1.0000 1.0000 1.0000 1.0000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1.0000 1.0000 1.0000 bt = 1 0

There is one minimal region, with five edge segments, three circle edge segments, and two line edge segments.

`csgchk` | `csgdel` | `pdebound` | `pdecirc` | `pdeellip` | `pdegeom` | `pdepoly` | `pderect` | `pdetool` | `wbound` | `wgeom`

Was this topic helpful?