Extra expression functions for the QGIS expression engine that didn't make it into QGIS 2.0

Contributors welcome :)

Current functions so far ---------------------

ramp_color_rgb -> Return just rgb from the given color ramp

red()
green()
blue()
alpha()
hue()
saturation()

Functions used from this project will not be available in other QGIS installs without first installing and enabling this plugin.

Function Description and Usage ---------------------

jitter_geometry('max_offset_percent', 'segment_length_percent') +++++++++++++++++

This function creates a jittered geometry from a linestring or polygon geometry. Given the maximum offset and segment length as percentages, it first interpolates points on the linestring or polygon boundary at the segments. Then it displaces the point perpendicular to the linestring or polygon boundary by a random distance less than or equal to the maximum offset. Finally it joins all these displaced points and returns a jittered geometry.

To use this function:

1. Go to Layer Properties -> Style -> Click on the green Plus button to Add a Symbol layer.
2. Set the Symbol layer type as Geometry generator.
3. Set the Geometry Type as LineString/MultiLineString for linestrings or Polygon/Multipolygon for polygons.
4. Load the jitter_geometry.py file in the Function Editor.
5. In the Expression Tab, call the function as:

jitter_geometry(5, 20)

Where '5' and '20' are the maximum_offset_percent and segment_length_percent respectively. These values can range between 1 and 99.

6. On clicking 'OK' the jittered geometries will be displayed on the layer.

Note:

The maximum offset is calculated from the maximum_offset_percent as: :: maximum_offset = (max_offset_percent*length_of_line)/100

Similarly the segment length is calculated from the segment_length_percent as: :: segment_length = (segment_length_percent*length_of_line)/100

The reason is that if the user enters absolute values for the offset and segment lengths, they maybe too large or small compared to the length of the line or the polygon's perimeter, and we would not get the desired output. A smaller value for the max_offset_percent and segment_length_percent generates a better output, especially for polygons. Recommended range for max_offset_percent and segment_length_percent for polygons would be [1,5] and [1,10] respectively.

max_incremented('field_name') +++++++++++++++++

Given the name of the field (field_name), returns 1 + the maximum value for the field. Can be used to automatically generate primary keys (eg. id, fid) when adding features in QGIS.

To use this function:

1. Go to Layer Properties -> Fields -> Click on the Text Edit button in the Edit widget column to open up the Edit Widget Properties dialog box.
2. Click on the Expression symbol to the right of the Default value text box. This would open the Expression dialog.
3. Enter an expression like the following in the Expression tab.

max_incremented('id')

To avoid the user from editing the automatically generated id, uncheck the Editable checkbox in the Edit Widget Properties dialog.

hstore_get_value("field", 'key') +++++++++++++++++

Given the field containing the hstore tags and a key, this function returns the key's value from the hstore tags. If the key is not present, it returns null.

For example:

hstore_get_value("tags", 'amenity')

Note The name of the field must be surrounded with double quotes, while strings with single quotes. In the example above, the hstore tags are contained in the field "tags", and the key to be searched for is 'amenity'.

hstore_exist("field", 'key') +++++++++++++++++

Returns whether the given key exists in the hstore string (True or False). field is the name of the field containing the hstore tags.

For example:

hstore_exist("tags", 'amenity')

The expression above can be used to Select Features by Expression, and would select all features which have the amenity tag.

hstore_contains_key_value("field", 'key_value') +++++++++++++++++

Returns whether the given key value pair exists in the hstore string (True or False).

For example:

hstore_contains_key_value("tags", 'amenity=>restaurant')

The expression above can be used to Select Features by Expression, and would select all restaurants, i.e. all features which have the amenity=>restaurant tag.

hstore_contains_hstore("field", 'hstore_input') +++++++++++++++++

Returns whether the hstore tags field contains all the keys and values in the hstore_input (True or False).

hstore_contains_hstore("tags", 'amenity=>restaurant,cuisine=>swiss')

The query above will select all features which contain both the key-value pairs, amenity=>restaurant and cuisine=>swiss in their hstore tags.

nullif('argument_1', 'argument_2') +++++++++++++++++

Returns a None/NULL value if argument_1 is equal to argument_2, otherwise it returns argument_1 (SQL alike).

nullif('hello world','')

The expression function above will return 'hello world'. A good use case would be to use this function with the in-built coalesce in an expression like the following.

coalesce(nullif("name",''), nullif("name_en",''), 'unknown')

Where "name" and "name_en" are field names (note the double quotes). This would return the 'name' if it is not an empty string, 'name_en' if the 'name' is empty and 'unknown' if both the 'name' and 'name_en' are empty.