# GEOMTRANSFORM - Geometry Transformations¶

Author:

Håvard Tveite

Author:

Jeff McKenna

Contact:

jmckenna at gatewaygeomatics.com

Last Updated:

2022-08-14

Geometry transformations return a new geometry. The purpose of a geometry transformation can be to achieve special effects for symbol rendering and labeling.

Geometry transformation is available at the LAYER level and the STYLE level. At the LAYER level (since MapServer 6.4), the original vector geometry («real world» coordinates) is used. At the STYLE level (since MapServer 6.0), pixel coordinates are used.

It may be useful to apply pixel values also at the LAYER level, and that is possible. If UNITS is defined in the LAYER, the [map_cellsize] variable can be used to convert to pixel values at the LAYER level:

```GEOMTRANSFORM (simplify([shape], [map_cellsize]*10))
```

## Transformations for simple styling (CLASS STYLE only)¶

The following simple geometry transformations are available at the CLASS STYLE level:

• bbox

• centroid

• end

• start

• vertices

### bbox¶

• GEOMTRANSFORM bbox returns the bounding box of the geometry.

• GEOMTRANSFORM «bbox»

Nota

Only available for STYLE in the CLASS context.

Class definitions for the example:

```CLASS
STYLE
COLOR 0 0 0
WIDTH 6
END # STYLE
STYLE
GEOMTRANSFORM "bbox"
OUTLINECOLOR 255 0 0
WIDTH 1
PATTERN 1 2 END
END # STYLE
END # CLASS
```

### centroid¶

• GEOMTRANSFORM centroid returns the centroid of the geometry.

• GEOMTRANSFORM «centroid»

Nota

Only available for STYLE in the CLASS context.

Style definitions for the example:

```STYLE
GEOMTRANSFORM "centroid"
COLOR 255 0 0
SYMBOL circlef
SIZE 5
END # STYLE
```

Symbol definition for the circlef symbol:

```SYMBOL
NAME "circlef"
TYPE ellipse
FILLED true
POINTS
1 1
END # POINTS
END # SYMBOL
```

### end and start¶

• GEOMTRANSFORM end returns the end point of a line.

• GEOMTRANSFORM start returns the start point of a line.

• GEOMTRANSFORM «start»

• GEOMTRANSFORM «end» (since END is used to end objects in the map file, end must be embedded in quotes)

The direction of the line at the start / end point is available for rendering effects.

Nota

Only available for STYLE in the CLASS context.

Class definitions for the example.

Lower part of the figure:

```CLASS
STYLE
GEOMTRANSFORM "start"
SYMBOL "circlef"
COLOR 255 0 0
SIZE 20
END # STYLE
STYLE
COLOR 0 0 0
WIDTH 4
END # STYLE
STYLE
GEOMTRANSFORM "end"
SYMBOL "circlef"
COLOR 0 255 0
SIZE 20
END # STYLE
END # CLASS
```

Upper part of the figure:

```CLASS
STYLE
COLOR 0 0 0
WIDTH 4
END # STYLE
STYLE
GEOMTRANSFORM "start"
SYMBOL "startarrow"
COLOR 255 0 0
SIZE 20
ANGLE auto
END # STYLE
STYLE
GEOMTRANSFORM "start"
SYMBOL "circlef"
COLOR 0 0 255
SIZE 5
END # STYLE
STYLE
GEOMTRANSFORM "end"
SYMBOL "endarrow"
COLOR 0 255 0
SIZE 20
ANGLE auto
END # STYLE
STYLE
GEOMTRANSFORM "end"
SYMBOL "circlef"
COLOR 0 0 255
SIZE 5
END # STYLE
END # CLASS
```

The startarrow symbol definition (endarrow is the same, except for ANCHORPOINT (value for endarrow: 1 0.5):

```SYMBOL
NAME "startarrow"
TYPE vector
FILLED true
POINTS
0 0.4
3 0.4
3 0
5 0.8
3 1.6
3 1.2
0 1.2
0 0.4
END # POINTS
ANCHORPOINT 0 0.5
END # SYMBOL
```

### vertices¶

• GEOMTRANSFORM vertices produces the set of vertices of a line (with direction information).

• GEOMTRANSFORM «vertices»

Nota

Only available for STYLE in the CLASS context.

Class definitions for the example:

```CLASS
STYLE
COLOR 0 0 0
WIDTH 4
END # STYLE
STYLE
GEOMTRANSFORM "vertices"
SYMBOL "vertline"
COLOR 0 0 0
WIDTH 2
SIZE 20
ANGLE AUTO
END # STYLE
END # CLASS
```

The vertline symbol definition:

```SYMBOL
NAME "vertline"
TYPE vector
POINTS
0 0
0 1
END # POINTS
END # SYMBOL
```

## Labels (LABEL STYLE only)¶

The following simple geometry transformations are available at the LABEL STYLE level:

• labelpnt

• labelpoly

These are used for label styling (background colour, background shadow, background frame).

Nota

The result of using labelpnt is affected by the LAYER LABELCACHE setting. If LABELCACHE is ON (the default), the label will be shifted when a non-zero sized symbol is added using labelpnt.

### labelpnt and labelpoly¶

• GEOMTRANSFORM labelpnt produces the geographic position the label is attached to. This corresponds to the center of the label text only if the label is in position CC.

• GEOMTRANSFORM «labelpnt»

• GEOMTRANSFORM labelpoly produces a polygon that covers the label plus a 1 pixel padding.

• GEOMTRANSFORM «labelpoly»

Nota

Only available for STYLE in the LABEL context.

These transformations can be used to make background rectangles for labels and add symbols to the label points.

Class definitions for the example:

```CLASS
STYLE
OUTLINECOLOR 255 255 204
END # STYLE
LABEL
SIZE giant
POSITION UC
GEOMTRANSFORM "labelpoly"
COLOR 153 153 153
OFFSET 3 3
END # Style
STYLE # background
GEOMTRANSFORM "labelpoly"
COLOR 204 255 204
END # Style
STYLE # outline
GEOMTRANSFORM "labelpoly"
OUTLINECOLOR 0 0 255
WIDTH 1
END # Style
STYLE
GEOMTRANSFORM "labelpnt"
SYMBOL 'circlef'
COLOR 255 0 0
SIZE 15
END # Style
END # Label
END # Class
```

Symbol definition for the circlef symbol:

```SYMBOL
NAME "circlef"
TYPE ellipse
FILLED true
POINTS
1 1
END # POINTS
END # SYMBOL
```

## Expressions and advanced transformations (LAYER and CLASS STYLE)¶

### Combining / chaining expressions¶

A geometry transformation produces a geometry, and that geometry can be used as input to another geometry transformation. There are (at least) two ways to accomplish this. One is to combine basic geometry transformation expressions into more complex geometry transformation expressions, and another is to combine a geometry transformation expression at the LAYER level with a geometry transformation expressions or a simple geometry transformation at the CLASS STYLE level.

Combining geometry transformation expressions A geometry transformation expression contains a [shape] part. The [shape] part can be replaced by a geometry transformation expression.

For example:

```GEOMTRANSFORM (simplify(buffer([shape], 20),10))
```

In this transformation, buffer is first applied on the geometry ([shape]). The resulting geometry is then used as input to simplify.

A style that demonstrates this:

```STYLE
GEOMTRANSFORM (simplify(buffer([shape], 20),10))
OUTLINECOLOR 255 0 0
WIDTH 2
END # STYLE
```

The result of this transformation is shown at the top of the following figure (red line). The original polygon is shown with a full black line and the buffer with a dashed black line.

Combining expressions with simple geometry transformations Simple geometry transformations are only available for CLASS STYLE, but can be combined with geometry transformation expressions at the LAYER level.

Excerpts from a layer definitions that does this kind of combination:

```LAYER
...
GEOMTRANSFORM (simplify(buffer([shape], 10),5))
CLASS
...
STYLE
GEOMTRANSFORM "vertices"
COLOR 255 102 102
SYMBOL vertline
SIZE 20
WIDTH 2
ANGLE auto
END # STYLE
END # CLASS
END # LAYER
```

The result of this transformation is shown at the bottom of the following figure (the red lines). The result of the LAYER level geomtransform is shown with a full black line. The original polygon is the same as the one used at the top of the figure.

### buffer¶

• GEOMTRANSFORM buffer returns the buffer of the original geometry. The result is always a polygon geometry.

• GEOMTRANSFORM (buffer ([shape], buffersize))

Nota

Negative values for buffersize (setback) is not supported.

Nota

Can be used at the LAYER level and for STYLE in the CLASS context.

Nota

Buffer does not seem to work for point geometries.

Some class definitions for the example.

Lower part (polygon with buffers):

```CLASS
STYLE
OUTLINECOLOR 0 255 0
GEOMTRANSFORM (buffer([shape], 20))
WIDTH 1
END # STYLE
STYLE
OUTLINECOLOR 0 0 255
GEOMTRANSFORM (buffer([shape], 10))  #
WIDTH 1
END # STYLE
STYLE
COLOR 255 0 0
GEOMTRANSFORM (buffer([shape], 5))  #
END # STYLE
STYLE
COLOR 0 0 0
END # STYLE
END # CLASS
```

Upper right part (layer level geomtransform):

```LAYER  # line buffer layer
STATUS DEFAULT
TYPE LINE
FEATURE
POINTS
80 70
80 75
END # Points
END # Feature
GEOMTRANSFORM (buffer([shape], 10))
CLASS
STYLE
COLOR 0 0 255
END # STYLE
END # CLASS
END # LAYER
```

### centerline¶

Nuovo nella versione 8.0: centerline()

• (centerline([shape]): Useful for labeling polygons, creates a centered line ([shape]) using a Voronoi diagram generated by GEOS and then additional simplification. Requires GEOS >= 3.5 Centerlines can only be computed for polygon shapes.

• an example showing a generated red centerline that is used to label lakes, from a polygon datasource lakes.db (and uses densify() to add more verticies to the generated red line, so it follows the lake’s shape) :

```LAYER
NAME "red-line"
TYPE LINE
CONNECTIONTYPE OGR
CONNECTION "lakes.db"
DATA "lakes"
GEOMTRANSFORM (centerline(densify([shape], 0.1)))
...
CLASS
NAME "red"
STYLE
COLOR 255 0 0
END #style
LABEL
COLOR  0 0 0
FONT sans
TYPE truetype
SIZE 7
POSITION cc
FORCE TRUE
ANGLE FOLLOW
END #label
END #class
...
END #layer
```

### densify¶

Nuovo nella versione 8.0: densify()

• (densify([shape],tolerance): Add more verticies to the feature ([shape]). Requires a positive tolerance value. Might be useful in combination with centerline() to add more verticies to the generated labeled line.

### generalize ([shape], tolerance)¶

• GEOMTRANSFORM generalize simplifies a geometry ([shape]) in a way comparable to FME’s ThinNoPoint algorithm. See http://trac.osgeo.org/gdal/ticket/966 for more information.

• GEOMTRANSFORM (generalize([shape], tolerance))

tolerance is mandatory, and is a specification of the maximum deviation allowed for the generalized line compared to the original line. A higher value for tolerance will give a more generalised / simplified line.

Nota

Can be used at the LAYER level and for STYLE in the CLASS context.

Nota

Depends on GEOS.

The figure below shows the result of applying generalize at the STYLE level with increasing values for tolerance (10 - green, 20 - blue and 40 - red).

One of the STYLE definitions for the example (tolerance 40):

```STYLE
GEOMTRANSFORM (generalize([shape], 40))
COLOR 255 0 0
WIDTH 1
PATTERN 3 3 END
END # STYLE
```

### inner¶

Nuovo nella versione 8.0: inner()

• (inner([shape]): creates an inner corresponding ring from an input polygon, as a new [shape]

### simplify([shape], tolerance)¶

• GEOMTRANSFORM simplify simplifies a geometry ([shape]) using the standard Douglas-Peucker algorithm.

• GEOMTRANSFORM (simplify([shape], tolerance))

tolerance is mandatory, and is a specification of the maximum deviation allowed for the generalized line compared to the original line. A higher value for tolerance will give a more generalised / simplified line.

Nota

Can be used at the LAYER level and for STYLE in the CLASS context.

The figure below shows the result of applying simplify at the STYLE level with increasing values for tolerance (10 - green, 20 - blue and 40 - red).

One of the STYLE definitions for the example (tolerance 40):

```STYLE
GEOMTRANSFORM (simplify([shape], 40))
COLOR 255 0 0
WIDTH 1
PATTERN 3 3 END
END # STYLE
```
• an example showing a generated red centerline that is used to label a province, from a polygon datasource ns.db (and uses simplify() to reduce the verticies in the generated line) :

```LAYER
NAME "red-line"
TYPE LINE
STATUS ON
CONNECTIONTYPE OGR
CONNECTION "ns.db"
DATA "ns"
...
GEOMTRANSFORM (simplify((centerline([shape])), 10000))
CLASS
NAME "red"
STYLE
COLOR 255 0 0
END #style
LABEL
COLOR  0 0 0
FONT sans
TYPE truetype
SIZE 10
POSITION AUTO
MINFEATURESIZE AUTO
ANGLE FOLLOW
END #label
END #class
...
END #layer
```

### simplifypt([shape], tolerance)¶

• GEOMTRANSFORM simplifypt simplifies a geometry ([shape]), ensuring that the result is a valid geometry having the same dimension and number of components as the input. tolerance must be non-negative.

• GEOMTRANSFORM (simplifypt([shape], tolerance))

tolerance is mandatory, and is a specification of the maximum deviation allowed for the generalized line compared to the original line. A higher value for tolerance will give a more generalised / simplified line.

Nota

Can be used at the LAYER level and for STYLE in the CLASS context.

The figure below shows the result of applying simplifypt at the STYLE level with increasing values for tolerance (10 - green, 20 - blue and 40 - red).

One of the STYLE definitions for the example (tolerance 40):

```STYLE
GEOMTRANSFORM (simplifypt([shape], 40))
COLOR 255 0 0
WIDTH 1
PATTERN 3 3 END
END # STYLE
```

### smoothsia ( [shape], smoothing_size, smoothing_iterations, preprocessing )¶

• GEOMTRANSFORM smoothsia returns a smoothed version of a line.

• GEOMTRANSFORM (smoothsia ( [shape], smoothing_size, smoothing_iterations, preprocessing ))

The following parameters are used:

• shape (mandatory). Specify the geometry to be used

• smoothing_size (optional). The window size (number of points) used by the algorithm. The default is 3.

• smoothing_iterations (optional). The number of iterations of the algorithm. The default is 1.

• preprocessing (optional). Preprocessing method to add more vertices to the geometry prior to smoothing, described below. There are two possible preprocessing methods:

• all Adds two intermediate vertices on each side of each original vertex. This is useful to preserve the general shape of the line with low resolution data.

• angle Add vertices at some specific places based on angle detection.

Nota

Can be used at the LAYER level and for STYLE in the CLASS context.

Example of a simple layer definition:

```LAYER NAME "my_layer"
TYPE LINE
STATUS DEFAULT
GEOMTRANSFORM (smoothsia([shape], 3, 1, 'angle'))
CLASS
STYLE
WIDTH 2
COLOR 255 0 0
END
END
```

Here are some examples showing results with different parameter values.

#### Tuning the behaviour of smoothsia¶

smoothsia has several parameters that can be used to tune its behaviour. The following sections describe some cases / possibilities.

#### Dataset resolution is too high¶

If you are trying to smooth a line that has a very high resolution (high density of vertices at the current view scale), you may not get the expected result because the vertices are too dense for the smoothing window size. In this case you might want to simplify the geometries before the smoothing. You can combine smoothing and simplification in a single geomtransform for that:

```GEOMTRANSFORM (smoothsia(simplifypt([shape], 10)))
```

See RFC 89: Layer Geomtransform for more info. Here’s a visualization of the issue:

#### Dataset resolution is too low¶

If you are trying to smooth a long line that has a low density of vertices, you may not get the expected result in some situations. You may lose some important parts of the geometry during the smoothing, for instance around acute angles. You can improve the result by enabling a preprocessing step to add intermediate vertices along the line prior to smoothing.

This behavior is controlled using the all value in the preprocessing argument of the smoothsia geomtransform:

```GEOMTRANSFORM (smoothsia([shape], 3, 1, 'all'))
```

This preprocessing will be performed before the smoothing. It adds 2 intermediate vertices on each side of each original vertex. This is useful if we really need to preserve the general shape of the low resolution line. Note that this might have an impact on the rendering since there will be more vertices in the output.

Here’s a visualization of the issue:

#### Curves¶

The preprocessing step might not be appropriate for all cases since it can impact the smoothing result significantly. However, without it, you might notice bad smoothing for curved lines with large distances between the line vertices. See this example:

You can improve that by enabling another type of preprocessing: angle. This one will add points at some specific places based on angle detection to recognize the curves. Here’s how you can enable it:

```GEOMTRANSFORM (smoothsia([shape], 3, 1, 'angle'))
```

### outer¶

Nuovo nella versione 8.0: outer()

• (outer([shape]): creates an outer corresponding ring from an input polygon, as a new [shape]. This might be useful when you want to ignore islands when labeling a lake.

## Javascript transformation¶

Author:

Alan Boudreault

Contact:

aboudreault at mapgears.com

Last Updated:

2013-12-16

### Introduction¶

Using GEOMTRANSFORM this way makes it possible to modify the geometry programmatically in addition to the built-in geomtransform functions.

### Usage¶

Simply declare the javascript plugin this way:

```MAP
...
LAYER
...
GEOMTRANSFORM "javascript://transform.js" # relative path
CLASS
END
END
END
```

The path can also be absolute.

```MAP
...
LAYER
...
GEOMTRANSFORM "javascript:///home/user/transform.js" # absolute path
CLASS
END
END
END
```

The javascript plugin has to implement a function named geomtransform that will be automatically called. This function has to return a new shape. Note that only the geometry of this new shape will be used, so your original shape attributes will be preserved.

The following javascript functions are available:

print some text in MapServer logs

• print(str1, str2, …, str)

print some text in MapServer logs

• require(path_to_lib1, path_to_lib2, …, path_to_lib)

include one or more javascript lib

#### Example 1. Simple Geomtransform¶

This example does a simple vertical translation …

```function geomtransform() {
new_shape = new shapeObj();
new_point = new pointObj();
new_point.x = shape.line(0).point(0).x;
new_point.y = shape.line(0).point(0).y+30000;
return new_shape;
}
```

#### Example 2. Printing logs in MapServer logs¶

Extends example 1 by printing information to the MapServer log.

```MAP
...
CONFIG  "MS_ERRORFILE" "/tmp/mapserver.log"
DEBUG 1
LAYER
...
GEOMTRANSFORM "javascript://transform.js"
...
END
END
```
```function geomtransform() {
new_shape = new shapeObj();
new_point = new pointObj();
new_point.x = shape.line(0).point(0).x;
new_point.y = shape.line(0).point(0).y+30000;
print('Modified y value from: ' + shape.line(0).point(0).y + ' to: '+new_point.y);
return new_shape;
}
```

### Basic API¶

A minimal API is currently available to create a new shape.

#### pointObj¶

##### Constructor¶
```new pointObj()
```

Type

Name

Note

double

x

double

y

double

z

double

m

##### Methods¶
void setXY(double x, double y[, double m])

Set the x,y[,m] values.

void setXYZ(double x, double y, double Z[, double m])

Set the x,y,z[,m] values.

#### lineObj¶

##### Constructor¶
```new lineObj()
```

Type

Name

Note

int

numpoints

##### Methods¶
pointObj point(int index)

Returns the point at the index position.

Add a point to the line.

void addXY(double x, double y[, double m])

Add point to the line from an x,y[,m] values.

void addXYZ(double x, double y, double Z[, double m])

Add point to the line from an x,y,z[,m] values.

#### shapeObj¶

##### Constructor¶
```new shapeObj(int type)
```

“type” is one of shapeObj.Point, shapeObj.Line, shapeObj.Polygon or shapeObj.Null

Type

Name

Note

int

numvalues

int

numlines

int

index

int

type

int

tileindex

int

classindex

text

text

object

attributes

##### Methods¶
shapeObj clone()

Returns a clone of the shape.

lineObj line(int index)

Returns the line at the index position.