Python MapScript Image Generation

Author:Sean Gillies
Last Updated:2008/07/15

Introduction

The MapScript HOWTO docs are intended to complement the API reference with examples of usage for specific subjects. All examples in this document refer to the mapfile and testing layers distributed with MapServer 4.2+ and found under mapserver/tests.

Pseudocode

All examples will use a pseudocode that is consistent with the language independent API reference. Each line is a statement. For object attributes and methods we use the dot, ‘.’, operator. Creation and deletion of objects will be indicated by ‘new’ and ‘del’ keywords. Other than that, the pseudocode looks a lot like Python.

Imagery Overview

The most common use of MapServer and MapScript is to create map imagery using the built-in GD format drivers: GD/GIF, GD/PNG, GD/PNG24, and GD/JPEG. This imagery might be saved to a file on disk or be streamed directly to another device.

The imageObj Class

Imagery is represented in MapScript by the imageObj class. Please see the API Reference (MapScript.txt) for class attribute and method details.

Creating imageObj from a mapObj

The mapObj class has two methods that return instances of imageObj: ‘draw’, and ‘prepareImage’. The first returns a full-fledged map image just as one would obtain from the mapserv CGI program

test_map = MapScript.mapObj('tests/test.map')
map_image = test_map.draw()

A properly sized and formatted blank image, without any layers, symbols, or labels, will be generated by ‘prepareImage’

blank_image = test_map.prepareImage()

Creating a new imageObj

The imageObj class constructor creates new instances without need of a map

format = MapScript.outputFormatObj('GD/JPEG')
image = MapScript.imageObj(300, 200, format)   # 300 wide, 200 high JPEG

and can even initialize from a file on disk

# First three args are overridden by attributes of the disk image file
disk_image = MapScript.imageObj(-1, -1, NULL, 'tests/test.png')

Image Output

Creating files on disk

Imagery is saved to disk by using the ‘save’ method. By accessing the ‘extension’ attribute of an image’s format, the proper file extension can be used without making any assumptions

filename = 'test.' + map_image.format.extension
map_image.save(filename)

If the image is using a GDAL/GTiff-based format, a GeoTIFF file can be created on disk by adding a mapObj as a second optional argument to ‘save’

map_image.save(filename, test_map)

Direct Output

An image can be dumped to an open filehandle using the ‘write’ method. By default, the filehandle is ‘stdout’

# Send an image to a web browser
print "Content-type: " + map_image.format.mimetype + "\n\n"
map_image.write()

This method is not fully functional for all SWIG MapScript languages. See the API Reference (MapScript.txt) for details. The ‘write’ method is new in 4.4.

Images and Symbols

The symbolObj::getImage() method will return an instance of imageObj for pixmap symbols

symbol = test_map.symbolset.getSymbolByName('home-png')
image = symbol.getImage()

There is a symmetric ‘setImage’ method which loads imagery into a symbol, allowing pixmap symbols to be created dynamically

new_symbol = MapScript.symbolObj('from_image')
new_symbol.type = MapScript.MS_SYMBOL_PIXMAP
new_symbol.setImage(image)
index = test_map.symbolset.appendSymbol(new_symbol)

The get/setImage methods are new in MapServer 4.4.