ImageLegend_widget: A QGIS layer tree (Legend) widget for displaying image legend in a raster layer   
V1.4 - 24 November 2019

This QGIS layer tree widget adds an "Image Legend" entry into the Legend tab of a raster layer properties.

Distributed under the GPL licence
 

This widget works only on QGIS version ≥ 3.4.

 

Please report any bug/problem/wish on GitHub or, not recommended, to the QGIS User mailing list [mailto: qgis - user at lists dot osgeo dot org].

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Contents

• Installation
• Preparing the image legend
• How to use
• Change log

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Installation

Be aware that this widget requires an extra pyton module: pillow.

pillow could require manual installation and can be "pip install" from QGIS command line (pip install pillow).

Minimum module version required to use this widget:

• pillow ≥ V5.3.0

Follow instructions here to install this widget.

Then enable it in Plugin Manager. Its name is: Layer Tree Image Legend Widget

Once successfully installed the widget is found in Properties -> Legend -> Embedded widgets in legend -> Available widgets.
For raster layers only!

 

Preparing the image legend

In order to work correctly this widget requires a specific sidecar file (specially named file) for the selected raster. Two cases are dealt with: 1) one legend sidecar file per raster or 2) a common legend file for all rasters in the directory that do not have their own legend sidecar file.

Case 1 - One legend sidecar file per raster file:

• Legend file must be either a PNG or JPG image file. No other format accepted!
• Legend file must be named as my_map.legend.png or my_map.legend.jpg where my_map is the name as the original raster file, without extension, and .legend.png or .legend.jpg is sidecar identity.
• Legend file must be in same directory as raster file.

Case 2 - Common legend for rasters in folder:

• Common legend file must be either a PNG or JPG image file. No other format accepted!
• Common legend file must be named as pref.legendcommon.png or pref.legendcommon.jpg where pref is a dummy name, and .legendcommon.png or .legendcommon.jpg must be as it is.
• Common legend file must be in same directory as raster files referencing it.
• One and only one .legendcommon.png or .legendcommon.jpg can exist in the given directory. If more than one file exist then the one displayed is undefined.

Example

Let's say we want to georeference a map. That map is probably an image showing map content and a legend associated with it. Would it be nice to georeference the map content only and have the legend as a separate item displayed in the layer tree?

Well, this is now possible and to do it we need to prepare our map/legend pair.

1. Open the image file (e.g. my_map.jpg) of the map with legend in an image editor
   
2. Save a copy of the file as a png (or jpg, from version 1.2) file named as follows: my_map.legend.png where my_map is the same name as the original file, without extension, and .legend.png (or .legend.jpg) must be like that (this is a sidecar to the original image. This convention is necessary on order for the widget to find the legend from the raster name)
3. In the image editor, crop the image to only show the legend, and save. Close the image
   
4. Re-open the original image file and crop the image to only show the map. Save and close
   
5. In QGIS, georeference the map
6. Then load the georeferenced image into the layer tree

 

How to use

• Select a raster layer in the layer tree (legend)
  
• Open its Properties (double-click the layer, or right-click it and select Properties...)
• Click Legend tab to open it
• Under Embedded widgets in legend -> Available widgets there should be an Image Legend item
  
• Select it and click right-arrow to move it to Used widgets pane
  
• Click OK
• In layer tree an image legend is displayed under selected raster name (for common legend, the same image is displaayed for all rasters in that directory)
  
• Click on image legend to open the legend full size into your default image viewer
  
• That's all!

Map taken from pinterest

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Download zip file here for manual installation.

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Change log

• V1.4: Change logic for common legend: only use common legend if raster has no legend sidecar on its own.
• V1.3: Add logic to deal with common legend between several rasters.
• V1.2: Add JPG as sidecar for image legend.
• V1.1: Code cleanup.
• V1.0: Initial public release.

Many thanks to Nyall Dawson at North Road for creating toggle_labels_widget on which this widget is based!

 

Published date: 25 Aug 2019.