How-to: Blending modes

People often ask me why I use QGIS so much more than another common GIS platform (cough), and for a long time there was one simple answer. Blending modes. The day I learned about blending modes was probably the day my cartographic skills took the biggest leap forwards.

Blending basically refers to how different colours in a map interact with each other, and blending modes allow you to control this. You can use them to better see a basemap under your polygons without having to lose your colours to transparency, or to highlight where features intersect and overlap. They will take your maps from *here* to *here* (imagine the gesturing I'm doing).

It's the difference between this and this.

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Or this and this.

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There are two main ways you can blend: feature blending and layer blending. In feature-based blending you control how colours interact within a layer, whereas in layer-based blending you control the interaction between layers.

Where do I find blending modes?

Blending modes aren't available in all GIS packages (cough ArcMap cough), and different GIS packages have different modes available. Below I'll share where you can find them in QGIS, ArcPro and Adobe Illustrator.

1 QGIS

You can find blending modes under Layer properties > Symbology tab > Layer rendering (right at the bottom of the window, normally you'll have to expand a drop-down arrow to access this). In QGIS blending modes can also be applied to most items, including labels, legends and graphics. Look for the 'rendering' option when styling these objects, and blending should be available there.

2 ArcPro

In ArcPro, select the layer you want to blend. Go into the Appearance tab that will appear, and blending modes will be in the "effects" section.

3 Adobe Illustrator

As a graphic design package, Adobe Illustrator probably offers the most flexibility in terms of blending options of the three software packages covered here. To alter the blending mode of a feature, open the transparency window when your item is selected - blending modes can be accessed under the drop-down with the text "Normal" in it (indicating that a normal blending mode is currently in use). This can also be accessed in the appearance window by clicking opacity.

To alter the blending mode of a layer or group, select them in their entirety and then repeat these steps. You can also alter blending modes of strokes and fill independently.

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So now you know where to find your blending modes - what do they actually do?

The graphic below should help you understand. It shows each type of blending mode (again, different packages may have different modes available or call them slightly different things - these are the options available in Adobe Illustrator), mixed against different colours and backgrounds. You can see different modes produce dramatically different effects by darkening, lightening or even completely changing the colour of a feature.

So those are the modes, but what should you use them for? There are no "rules" for this, and as always practice and experimentation are the best ways to get you where you want to be. Below are some examples of where I've found blending modes can help you solve common cartographic problems.

Problem: I want to see my base map better

Honestly, this is the problem that first made me embrace blending modes. It's a problem in most maps - how can I see my data, and see the geographic context to help me understand it? It's particularly a problem in choropleth maps where your data might cover the entirety of your map view.

Conventionally you might try to achieve this with transparency. Make your data more transparent, and you'll be able to see the basemap - solved, right? Ish. The more transparent your data is, the less contrast there is between both it and other layers. As well as being an accessibility nightmare, this also waters down the story of your data as well as any visual hierarchy you're hoping to achieve.

Use this blending mode: multiply (light base maps - for darker base maps try screen, soft light or hard light).

The multiply blend mode is one of the most commonly-used modes, and with good reason. If you're interested in the mechanics, it essentially multiplies the colour values of your layer or feature with those sitting beneath it, then divides it by 250. The impact of this is that darker areas of the basemap appear much darker, whilst lighter areas are less impacted. This means that if you're using - for example - a greyscale basemap where the labels are darker than their backdrop, they'll become much more darker under areas where your multiply mode is applied. This is relative to the colour of the blending mode object - they'll be much darker under a dark blue than under a pale yellow. It sounds complex - it isn't. Look at the impact it has on the map below, with multiply applied to the map on the right. You can see the base map much more clearly, whilst the contrast between the colours in the data layer remains strong.

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Problem: I want to see where my two layers overlap

Often the most important part of a map is the spatial relationship between two - or more - data layers, and how they overlap. This can be a difficult thing to convey, as often the colour that is formed in the overlap of two transparent layers is more muted than the original colours, or looks more like the colour of the layer which is on top. This is particularly a problem in complex bivariate maps, where the areas of overlap between two layers should be the most striking part of a map.

Use this blending mode: colour burn (sometimes referred to as just "burn").

Colour burn divides the inverted bottom layer by the top layer, and then inverts the result. The maths sounds a bit complex (or it does to me), but the effect is simple: the overlapping area becomes darker and more saturated. In the example below, the burn mode is appliedto the right-hand image. This means the overlap between the National Parks (purple) and Sites of Special Scientific Interest (blue) is more saturated and clear. Using this mode means the layer does not have to be as transparent, and so the colours can be much bolder and not as washed out.

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Handy tip: With all blending modes, it's also important to think about how changes to the mode will be impacted by - and impact - the base map, as well as any other layers on your map.

Problem: I want my map to have a cool "glowy" effect

Maps which look like they're glowing are a great way of communicating density and magnitude, but mostly they look GORGEOUS and we love them.

Use this blending mode: addition, screen or colour dodge.

I normally use addition for this effect, because it tends to have the boldest outcome. Addition simply adds the colour values of your items together, producing a lighter colour. The map above has a feature-level addition blend mode applied to the each of the green markers (a tree in the NYC street tree census) which is a great way of communicating the spatial pattern of them without having to do any actual analysis. And it looks sick, if I do say so myself.

I'd recommend using a darker basemap if you're aiming for this effect; with a light basemap the colour may become so light that the contrast with the basemap will be minimal and difficult to see.

Handy tip: With all blending modes, there's a maximum effect you can achieve. Using addition on two white layers will not have any impact as they're already as light as they can be; similarly multiply will not effect two black layers. When you're working with blending modes, it's really helpful to play around transparency levels to reach the optimal effect. For example, in the map below I've used the addition blending mode along with varying line thickness and transparency to highlight the greatest post-graduation flows of students.

Problem: I want my elevation to look epic

Yas you do.

Blending modes are so so helpful for pimping your elevation maps. Take the example below; a map of surface elevation in Central London (spot the landmarks!). It's alright but could do so much more to communicate the shape of the landscape to help users interpret the data.

Let's add some hillshade. Today I'm in QGIS so all I have to do is duplicate the layer and hit the hillshade symbology and we've got our hillshading.

Use this blending mode: multiply & addition.

To combine the hillshade with our elevation layer, we're going to use a combination of the tricks we've used before. First, I'll change the blending mode of my hillshade layer to multiply; this will place emphasis on the "shadows." With my hillshade layer on top of my elevation layer, we can see the elevation colour palette coming through.

Great - but pretty dark, right? We can tackle that in a few ways. First, let's drop the transparency of this hillshade layer down to 50%. Next, I'll duplicate the hillshade layer - I now have three layers sitting on top of each other: hillshade A (the one we've just created), hillshade B (the original multiply hillshade layer) and the elevation. I'm going to change the blend mode of hillshade A (the topmost one) from multiply to screen, and drop the transparency of that down to 20% to avoid a bit of a washout. This immediately brightens up the lighter areas of my hillshade without taking away from the darker areas I created with the other multiply layer.

You can create an even more dramatic effect by duplicating these layers multiple times or replicating the blending mode effect on layers like visible sky or shadow models.

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And there you have it - a fully-pimped elevation map!

The end!

I hope you enjoyed my run-through how to use blending modes in your mapping! I really look forward to seeing what you come up with, and please do let me know if you have any requests for future tutorials!