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How to display images with custom paths relative to the project in QGIS Attribute Forms

Summary: You can use QGIS expressions to set the default path for an Attribute Form to be something like @project_folder || '/' || 'MapGraphs', which gives you a default path that is relative to the project folder but also has other custom subfolders in it.

I spent a while redrafting the title of this post to try and explain it well – and I don’t think I succeeded. Hopefully search engines will pick up what I’m talking about from the contents of this post.

So, you may be aware that in QGIS you can set up an Attribute Form which you can configure to pop up when you click on a feature. This can be used to display information about the feature (from its attribute table) in a nice way. See the QGIS docs for more information about this feature.

The bit I’m going to talk about specifically today is display images in these forms. There are various use cases for this, but in my case I had created a static PNG image of a graph for each feature, and I wanted this to be displayed in QGIS when I clicked on the feature. Note that here I’m focusing on displaying data that is already in the attribute table for my layer, rather than editing and adding new features to the layer – because of this, some of the labels for the settings may not sound quite right.

Anyway to set this up for a layer, go to a layer’s properties dialog and choose the Attribute Form tab on the left. You can then switch the mode at the top to ‘Drag and Drop Designer’ and then select the attribute field that has your image filenames in it. You’ll see a wide selection of options, looking a bit like the screenshot below (click to enlarge):

You need to choose a Widget Type of Attachment and a Storage Type of Select Existing file. We’ll come back to the Path selections in a moment. For now, scroll down a bit and choose a Type of Image under Integrated Document Viewer.

That will set up a basic image viewer – but the key part is telling QGIS where the files are located. Look at the dropdown labelled Store path as, in it you have three options:

• Absolute Path will expect to see the full absolute path to the image file. For example, on my Mac that could be /Users/robin/GIS/image.png or on a Windows machine that could be C:\Users\Robin\Documents\GIS\image.png. This is probably the easiest way to do this – just make sure that all the filenames in your attribute table are absolute paths – but it is by far the least flexible, and it won’t work if you send the QGIS project to anyone else, as /Users/robin/GIS probably won’t exist on their computer!
• Relative to Project Path is a pretty good choice – it allows you to specify relative paths, in this case relative to where your .qgz file is stored. For example, your attribute table could have entries like MapGraphs/image.png and as long as the MapGraphs folder was in the same folder as your QGIS project file then everything would work. However, this means that your attribute table has to have knowledge of what folder your images will be stored in. That’s why I prefer the last option…
• Relative to Default Path – this specifies that all of the paths are relative to whatever you’ve entered in the Default path field just above this dropdown. The standard way to do this would be to just select a folder for the Default path field, and all of your image paths will be relative to this folder. For example, if you select /Users/robin/GIS/MapGraphs as your default folder, then your attribute table values can just be image.png and image2.png and it will work.

However, there is a way to combine the best parts of Relative to Project Path and Relative to Default Path by using QGIS expressions. In this case, I wanted to make my QGIS project ‘portable’ – so it could be used on different computers. This meant that everything had to be relevant to the project path somehow (we couldn’t have any absolute paths). However, I didn’t want to store folder paths in my attribute table – I was writing the attribute table contents from a separate Python script and didn’t want to update that every time my folder structure changed. I wanted my attribute table to just contain image.png, image2.png etc (in my case, the filenames were the names of measurement sites for various environmental variables).

So, to give the final answer to this, you can click the button next to the Default path field and choose to enter a QGIS expression. You’ll need an expression like this:

@project_folder || '/' || 'MapGraphs'

This sets the default folder to whatever the current project folder is (that’s the folder that contains the .qgz file), then a / and then another folder called MapGraphs. This is giving you the best of both worlds – a path relative to the project folder, but also being able to specify the rest of the path.

I hope that’s been useful for you. My only question is whether there is any way to repeat fields in a QGIS form. I imagine there isn’t – as the forms are designed for entering data as well as viewing it, and it makes no sense to enter data for the same field twice – but it would be very useful if there was, as then I could have a single attribute called filename (which might contain Southampton.png or Worcester.png) and then have multiple image viewers, one with a default path set up to point to WaterGraphs/ and one to AirQualityGraphs/ etc. I’ve kind of done this at the moment, but I had to duplicate my attributes to have filename1, filename2 etc – which feels a little messy.

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This post originally appeared on Robin's Blog.

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Categorised as: Academic, Computing, GIS, How To